diff --git a/crates/haku-cli/src/main.rs b/crates/haku-cli/src/main.rs
index 5c506c0..d1c39d9 100644
--- a/crates/haku-cli/src/main.rs
+++ b/crates/haku-cli/src/main.rs
@@ -1,71 +1,31 @@
-// NOTE: This is a very bad CLI.
-// Sorry!
+// NOTE: This is a very bad CLI. I only use it for debugging haku with LLDB.
+// Sorry that it doesn't actually do anything!
 
 use std::{error::Error, fmt::Display, io::BufRead};
 
 use haku::{
-    bytecode::{Chunk, Defs},
-    compiler::{compile_expr, Compiler, Source},
-    sexp::{parse_toplevel, Ast, Parser, SourceCode},
-    system::System,
-    value::{BytecodeLoc, Closure, FunctionName, Ref, Value},
-    vm::{Vm, VmLimits},
+    ast::{dump::dump, Ast},
+    lexer::{lex, Lexer},
+    parser::{expr, Parser, ParserLimits},
+    source::SourceCode,
+    token::Lexis,
+    value::Value,
 };
 
 fn eval(code: &str) -> Result<Value, Box<dyn Error>> {
-    let mut system = System::new(1);
-
-    let ast = Ast::new(1024);
     let code = SourceCode::unlimited_len(code);
-    let mut parser = Parser::new(ast, code);
-    let root = parse_toplevel(&mut parser);
-    let ast = parser.ast;
-    let src = Source {
-        code,
-        ast: &ast,
-        system: &system,
-    };
+    let mut lexer = Lexer::new(Lexis::new(1024), code);
+    lex(&mut lexer).expect("too many tokens");
 
-    let mut defs = Defs::new(256);
-    let mut chunk = Chunk::new(65536).unwrap();
-    let mut compiler = Compiler::new(&mut defs, &mut chunk);
-    compile_expr(&mut compiler, &src, root)?;
-    let diagnostics = compiler.diagnostics;
-    let defs = compiler.defs;
-    println!("{chunk:?}");
+    let mut parser = Parser::new(&lexer.lexis, &ParserLimits { max_events: 1024 });
+    expr(&mut parser);
 
-    for diagnostic in &diagnostics {
-        eprintln!(
-            "{}..{}: {}",
-            diagnostic.span.start, diagnostic.span.end, diagnostic.message
-        );
-    }
+    let mut ast = Ast::new(1024);
+    let (root, _) = parser.into_ast(&mut ast).unwrap();
 
-    if !diagnostics.is_empty() {
-        return Err(Box::new(DiagnosticsEmitted));
-    }
+    eprintln!("{}", dump(&ast, root, Some(code)));
 
-    let mut vm = Vm::new(
-        defs,
-        &VmLimits {
-            stack_capacity: 256,
-            call_stack_capacity: 256,
-            ref_capacity: 256,
-            fuel: 32768,
-            memory: 1024,
-        },
-    );
-    let chunk_id = system.add_chunk(chunk)?;
-    let closure = vm.create_ref(Ref::Closure(Closure {
-        start: BytecodeLoc {
-            chunk_id,
-            offset: 0,
-        },
-        name: FunctionName::Anonymous,
-        param_count: 0,
-        captures: Vec::new(),
-    }))?;
-    Ok(vm.run(&system, closure)?)
+    Ok(Value::Nil)
 }
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
diff --git a/crates/haku-wasm/src/lib.rs b/crates/haku-wasm/src/lib.rs
index 5f37ab5..4562f94 100644
--- a/crates/haku-wasm/src/lib.rs
+++ b/crates/haku-wasm/src/lib.rs
@@ -2,18 +2,23 @@
 
 extern crate alloc;
 
-use core::{alloc::Layout, slice};
+use core::{alloc::Layout, num::Saturating, slice};
 
 use alloc::{boxed::Box, vec::Vec};
 use haku::{
+    ast::Ast,
     bytecode::{Chunk, Defs, DefsImage},
-    compiler::{compile_expr, CompileError, Compiler, Diagnostic, Source},
+    compiler::{compile_expr, CompileError, Compiler, Source},
+    diagnostic::Diagnostic,
+    lexer::{lex, Lexer},
+    parser::{self, Parser},
     render::{
         tiny_skia::{Pixmap, PremultipliedColorU8},
         Renderer, RendererLimits,
     },
-    sexp::{parse_toplevel, Ast, Parser, SourceCode},
+    source::SourceCode,
     system::{ChunkId, System, SystemImage},
+    token::Lexis,
     value::{BytecodeLoc, Closure, FunctionName, Ref, Value},
     vm::{Exception, Vm, VmImage, VmLimits},
 };
@@ -41,6 +46,8 @@ struct Limits {
     max_source_code_len: usize,
     max_chunks: usize,
     max_defs: usize,
+    max_tokens: usize,
+    max_parser_events: usize,
     ast_capacity: usize,
     chunk_capacity: usize,
     stack_capacity: usize,
@@ -58,6 +65,8 @@ impl Default for Limits {
             max_source_code_len: 65536,
             max_chunks: 2,
             max_defs: 256,
+            max_tokens: 1024,
+            max_parser_events: 1024,
             ast_capacity: 1024,
             chunk_capacity: 65536,
             stack_capacity: 1024,
@@ -101,6 +110,8 @@ macro_rules! limit_setter {
 limit_setter!(max_source_code_len);
 limit_setter!(max_chunks);
 limit_setter!(max_defs);
+limit_setter!(max_tokens);
+limit_setter!(max_parser_events);
 limit_setter!(ast_capacity);
 limit_setter!(chunk_capacity);
 limit_setter!(stack_capacity);
@@ -207,6 +218,8 @@ unsafe extern "C" fn haku_exception_message_len(instance: *const Instance) -> u3
 enum StatusCode {
     Ok,
     SourceCodeTooLong,
+    TooManyTokens,
+    TooManyAstNodes,
     ChunkTooBig,
     DiagnosticsEmitted,
     TooManyChunks,
@@ -238,6 +251,8 @@ extern "C" fn haku_status_string(code: StatusCode) -> *const i8 {
     match code {
         StatusCode::Ok => c"ok",
         StatusCode::SourceCodeTooLong => c"source code is too long",
+        StatusCode::TooManyTokens => c"source code has too many tokens",
+        StatusCode::TooManyAstNodes => c"source code has too many AST nodes",
         StatusCode::ChunkTooBig => c"compiled bytecode is too large",
         StatusCode::DiagnosticsEmitted => c"diagnostics were emitted",
         StatusCode::TooManyChunks => c"too many registered bytecode chunks",
@@ -281,22 +296,22 @@ unsafe extern "C" fn haku_num_diagnostics(brush: *const Brush) -> u32 {
 
 #[no_mangle]
 unsafe extern "C" fn haku_diagnostic_start(brush: *const Brush, index: u32) -> u32 {
-    (*brush).diagnostics[index as usize].span.start as u32
+    (*brush).diagnostics[index as usize].span().start
 }
 
 #[no_mangle]
 unsafe extern "C" fn haku_diagnostic_end(brush: *const Brush, index: u32) -> u32 {
-    (*brush).diagnostics[index as usize].span.end as u32
+    (*brush).diagnostics[index as usize].span().end
 }
 
 #[no_mangle]
 unsafe extern "C" fn haku_diagnostic_message(brush: *const Brush, index: u32) -> *const u8 {
-    (*brush).diagnostics[index as usize].message.as_ptr()
+    (*brush).diagnostics[index as usize].message().as_ptr()
 }
 
 #[no_mangle]
 unsafe extern "C" fn haku_diagnostic_message_len(brush: *const Brush, index: u32) -> u32 {
-    (*brush).diagnostics[index as usize].message.len() as u32
+    (*brush).diagnostics[index as usize].message().len() as u32
 }
 
 #[no_mangle]
@@ -315,15 +330,27 @@ unsafe extern "C" fn haku_compile_brush(
 
     let code = core::str::from_utf8(slice::from_raw_parts(code, code_len as usize))
         .expect("invalid UTF-8");
-    let code = match SourceCode::limited_len(code, instance.limits.max_source_code_len) {
-        Some(code) => code,
-        None => return StatusCode::SourceCodeTooLong,
+    let Some(code) = SourceCode::limited_len(code, instance.limits.max_source_code_len as u32)
+    else {
+        return StatusCode::SourceCodeTooLong;
     };
 
-    let ast = Ast::new(instance.limits.ast_capacity);
-    let mut parser = Parser::new(ast, code);
-    let root = parse_toplevel(&mut parser);
-    let ast = parser.ast;
+    let mut lexer = Lexer::new(Lexis::new(instance.limits.max_tokens), code);
+    if lex(&mut lexer).is_err() {
+        return StatusCode::TooManyTokens;
+    };
+
+    let mut ast = Ast::new(instance.limits.ast_capacity);
+    let mut parser = Parser::new(
+        &lexer.lexis,
+        &haku::parser::ParserLimits {
+            max_events: instance.limits.max_parser_events,
+        },
+    );
+    parser::toplevel(&mut parser);
+    let Ok((root, mut parser_diagnostics)) = parser.into_ast(&mut ast) else {
+        return StatusCode::TooManyAstNodes;
+    };
 
     let src = Source {
         code,
@@ -339,8 +366,11 @@ unsafe extern "C" fn haku_compile_brush(
         }
     }
 
-    if !compiler.diagnostics.is_empty() {
-        brush.diagnostics = compiler.diagnostics;
+    let mut diagnostics = lexer.diagnostics;
+    diagnostics.append(&mut parser_diagnostics);
+    diagnostics.append(&mut compiler.diagnostics);
+    if !diagnostics.is_empty() {
+        brush.diagnostics = diagnostics;
         return StatusCode::DiagnosticsEmitted;
     }
 
diff --git a/crates/haku/src/ast.rs b/crates/haku/src/ast.rs
new file mode 100644
index 0000000..843708c
--- /dev/null
+++ b/crates/haku/src/ast.rs
@@ -0,0 +1,125 @@
+use core::{error::Error, fmt::Display};
+
+use alloc::vec::Vec;
+
+use crate::source::Span;
+
+pub mod dump;
+pub mod walk;
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub struct NodeId(u32);
+
+impl NodeId {
+    pub const NIL: NodeId = NodeId(0);
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum NodeKind {
+    Nil,
+
+    Token,
+
+    Ident,
+    Tag,
+    Number,
+    Color,
+    List,
+
+    Op,
+    Unary,
+    Binary,
+    Call,
+    ParenEmpty,
+    Paren,
+    Lambda,
+    Params,
+    Param,
+    If,
+    Let,
+
+    Toplevel,
+
+    Error,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct Node {
+    pub span: Span,
+    pub kind: NodeKind,
+}
+
+#[derive(Debug, Clone)]
+pub struct Ast {
+    kinds: Vec<NodeKind>,
+    spans: Vec<Span>,
+    children_spans: Vec<(u32, u32)>,
+    children: Vec<NodeId>,
+}
+
+impl Ast {
+    pub fn new(capacity: usize) -> Self {
+        assert!(capacity >= 1, "there must be space for at least a nil node");
+        assert!(capacity <= u32::MAX as usize);
+
+        let mut ast = Self {
+            kinds: Vec::with_capacity(capacity),
+            spans: Vec::with_capacity(capacity),
+            children_spans: Vec::with_capacity(capacity),
+            children: Vec::new(),
+        };
+
+        ast.alloc(NodeKind::Nil, Span::new(0, 0)).unwrap();
+
+        ast
+    }
+
+    pub fn alloc(&mut self, kind: NodeKind, span: Span) -> Result<NodeId, NodeAllocError> {
+        if self.kinds.len() >= self.kinds.capacity() {
+            return Err(NodeAllocError);
+        }
+
+        let index = self.kinds.len() as u32;
+        self.kinds.push(kind);
+        self.spans.push(span);
+        self.children_spans.push((0, 0));
+        Ok(NodeId(index))
+    }
+
+    // NOTE: This never produces a NodeAllocError, because there can more or less only ever be as many children for
+    // nodes as there are nodes.
+    pub fn alloc_children(&mut self, for_node: NodeId, children: &[NodeId]) {
+        let start = self.children.len();
+        self.children.extend_from_slice(children);
+        let end = self.children.len();
+        self.children_spans[for_node.0 as usize] = (start as u32, end as u32);
+    }
+
+    pub fn extend_span(&mut self, in_node: NodeId, end: u32) {
+        self.spans[in_node.0 as usize].end = end;
+    }
+
+    pub fn kind(&self, id: NodeId) -> NodeKind {
+        self.kinds[id.0 as usize]
+    }
+
+    pub fn span(&self, id: NodeId) -> Span {
+        self.spans[id.0 as usize]
+    }
+
+    pub fn children(&self, id: NodeId) -> &[NodeId] {
+        let (start, end) = self.children_spans[id.0 as usize];
+        &self.children[start as usize..end as usize]
+    }
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub struct NodeAllocError;
+
+impl Display for NodeAllocError {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        f.write_str("too many nodes")
+    }
+}
+
+impl Error for NodeAllocError {}
diff --git a/crates/haku/src/ast/dump.rs b/crates/haku/src/ast/dump.rs
new file mode 100644
index 0000000..32ed89a
--- /dev/null
+++ b/crates/haku/src/ast/dump.rs
@@ -0,0 +1,34 @@
+use alloc::string::String;
+use core::fmt::Write;
+
+use crate::{ast::NodeKind, source::SourceCode};
+
+use super::{Ast, NodeId};
+
+pub fn dump(ast: &Ast, node: NodeId, code: Option<&SourceCode>) -> String {
+    let mut result = String::new();
+
+    fn rec(ast: &Ast, node: NodeId, code: Option<&SourceCode>, result: &mut String, depth: usize) {
+        for _ in 0..depth {
+            result.push_str("    ");
+        }
+
+        write!(result, "{:?} @ {:?}", ast.kind(node), ast.span(node)).unwrap();
+        if let Some(code) = code {
+            if ast.kind(node) == NodeKind::Token {
+                write!(result, " {:?}", ast.span(node).slice(code)).unwrap();
+            }
+        }
+        writeln!(result).unwrap();
+        for &child in ast.children(node) {
+            rec(ast, child, code, result, depth + 1);
+        }
+    }
+
+    rec(ast, node, code, &mut result, 0);
+
+    // Remove the trailing newline.
+    result.pop();
+
+    result
+}
diff --git a/crates/haku/src/ast/walk.rs b/crates/haku/src/ast/walk.rs
new file mode 100644
index 0000000..d76623b
--- /dev/null
+++ b/crates/haku/src/ast/walk.rs
@@ -0,0 +1,73 @@
+use super::{Ast, NodeId, NodeKind};
+
+impl Ast {
+    pub fn child(&self, parent: NodeId, kind: NodeKind) -> Option<NodeId> {
+        self.children(parent)
+            .iter()
+            .find(|&&child| self.kind(child) == kind)
+            .copied()
+    }
+
+    pub fn walk(&self, parent: NodeId) -> Walk<'_> {
+        Walk {
+            ast: self,
+            parent,
+            index: 0,
+        }
+    }
+}
+
+/// An iterator over a node's children, with convenience methods for accessing those children.
+#[derive(Clone)]
+pub struct Walk<'a> {
+    ast: &'a Ast,
+    parent: NodeId,
+    index: usize,
+}
+
+impl<'a> Walk<'a> {
+    /// Walk to the first non-Nil, non-Error, non-Token node.
+    pub fn node(&mut self) -> Option<NodeId> {
+        while let Some(id) = self.next() {
+            if !matches!(
+                self.ast.kind(id),
+                NodeKind::Nil | NodeKind::Token | NodeKind::Error
+            ) {
+                return Some(id);
+            }
+        }
+
+        None
+    }
+
+    /// Walk to the next [`node`][`Self::node`] of the given kind.
+    pub fn node_of(&mut self, kind: NodeKind) -> Option<NodeId> {
+        while let Some(id) = self.node() {
+            if self.ast.kind(id) == kind {
+                return Some(id);
+            }
+        }
+
+        None
+    }
+
+    /// Find the first node of the given kind. This does not advance the iterator.
+    pub fn get(&self, kind: NodeKind) -> Option<NodeId> {
+        self.clone().find(|&id| self.ast.kind(id) == kind)
+    }
+}
+
+impl<'a> Iterator for Walk<'a> {
+    type Item = NodeId;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let children = self.ast.children(self.parent);
+        if self.index < children.len() {
+            let index = self.index;
+            self.index += 1;
+            Some(children[index])
+        } else {
+            None
+        }
+    }
+}
diff --git a/crates/haku/src/bytecode.rs b/crates/haku/src/bytecode.rs
index 82932bc..a98285b 100644
--- a/crates/haku/src/bytecode.rs
+++ b/crates/haku/src/bytecode.rs
@@ -17,6 +17,8 @@ pub enum Opcode {
     // Duplicate existing values.
     /// Push a value relative to the bottom of the current stack window.
     Local, // (index: u8)
+    /// Set the value of a value relative to the bottom of the current stack window.
+    SetLocal, // (index: u8)
     /// Push a captured value.
     Capture, // (index: u8)
     /// Get the value of a definition.
@@ -24,12 +26,8 @@ pub enum Opcode {
     /// Set the value of a definition.
     SetDef, // (index: u16)
 
-    /// Drop `number` values from the stack.
-    /// <!-- OwO -->
-    DropLet, // (number: u8)
-
     // Create literal functions.
-    Function, // (params: u8, then: u16), at `then`: (capture_count: u8, captures: [(source: u8, index: u8); capture_count])
+    Function, // (params: u8, then: u16), at `then`: (local_count: u8, capture_count: u8, captures: [(source: u8, index: u8); capture_count])
 
     // Control flow.
     Jump,      // (offset: u16)
diff --git a/crates/haku/src/compiler.rs b/crates/haku/src/compiler.rs
index a6966f3..a3b82e2 100644
--- a/crates/haku/src/compiler.rs
+++ b/crates/haku/src/compiler.rs
@@ -6,9 +6,11 @@ use core::{
 use alloc::vec::Vec;
 
 use crate::{
+    ast::{Ast, NodeId, NodeKind},
     bytecode::{Chunk, DefError, Defs, EmitError, Opcode, CAPTURE_CAPTURE, CAPTURE_LOCAL},
-    sexp::{Ast, NodeId, NodeKind, SourceCode, Span},
-    system::System,
+    diagnostic::Diagnostic,
+    source::SourceCode,
+    system::{System, SystemFnArity},
 };
 
 pub struct Source<'a> {
@@ -17,12 +19,6 @@ pub struct Source<'a> {
     pub system: &'a System,
 }
 
-#[derive(Debug, Clone, Copy)]
-pub struct Diagnostic {
-    pub span: Span,
-    pub message: &'static str,
-}
-
 #[derive(Debug, Clone, Copy)]
 struct Local<'a> {
     name: &'a str,
@@ -46,6 +42,11 @@ pub struct Compiler<'a, 'b> {
     scopes: Vec<Scope<'a>>,
 }
 
+#[derive(Debug, Clone, Copy)]
+pub struct ClosureSpec {
+    pub(crate) local_count: u8,
+}
+
 impl<'a, 'b> Compiler<'a, 'b> {
     pub fn new(defs: &'a mut Defs, chunk: &'b mut Chunk) -> Self {
         Self {
@@ -59,20 +60,24 @@ impl<'a, 'b> Compiler<'a, 'b> {
         }
     }
 
-    pub fn diagnose(&mut self, diagnostic: Diagnostic) {
-        if self.diagnostics.len() >= self.diagnostics.capacity() {
-            return;
-        }
-
-        if self.diagnostics.len() == self.diagnostics.capacity() - 1 {
-            self.diagnostics.push(Diagnostic {
-                span: Span::new(0, 0),
-                message: "too many diagnostics emitted, stopping", // hello clangd!
-            })
-        } else {
+    fn emit(&mut self, diagnostic: Diagnostic) {
+        if self.diagnostics.len() < self.diagnostics.capacity() {
             self.diagnostics.push(diagnostic);
         }
     }
+
+    pub fn closure_spec(&self) -> ClosureSpec {
+        ClosureSpec {
+            local_count: self
+                .scopes
+                .last()
+                .unwrap()
+                .locals
+                .len()
+                .try_into()
+                .unwrap_or_default(),
+        }
+    }
 }
 
 type CompileResult<T = ()> = Result<T, CompileError>;
@@ -82,27 +87,51 @@ pub fn compile_expr<'a>(
     src: &Source<'a>,
     node_id: NodeId,
 ) -> CompileResult {
-    let node = src.ast.get(node_id);
-    match node.kind {
-        NodeKind::Eof => unreachable!("eof node should never be emitted"),
+    match src.ast.kind(node_id) {
+        // The nil node is special, as it inhabits node ID 0.
+        NodeKind::Nil => {
+            unreachable!("Nil node should never be emitted (ParenEmpty is used for nil literals)")
+        }
+        // Tokens are trivia and should never be emitted---they're only useful for error reporting.
+        NodeKind::Token => unreachable!("Token node should never be emitted"),
+        // Op nodes are only used to provide a searching anchor for the operator in Unary and Binary.
+        NodeKind::Op => unreachable!("Op node should never be emitted"),
+        // Params nodes are only used to provide a searching anchor for Lambda parameters.
+        NodeKind::Params => unreachable!("Param node should never be emitted"),
+        // Param nodes are only used to provide a searching anchor for identifiers in Params nodes,
+        // as they may also contain commas and other trivia.
+        NodeKind::Param => unreachable!("Param node should never be emitted"),
+
+        NodeKind::Color => unsupported(c, src, node_id, "color literals are not implemented yet"),
 
-        NodeKind::Nil => compile_nil(c),
         NodeKind::Ident => compile_ident(c, src, node_id),
         NodeKind::Number => compile_number(c, src, node_id),
-        NodeKind::List(_, _) => compile_list(c, src, node_id),
-        NodeKind::Toplevel(_) => compile_toplevel(c, src, node_id),
+        NodeKind::Tag => compile_tag(c, src, node_id),
+        NodeKind::List => unsupported(c, src, node_id, "list literals are not implemented yet"),
 
-        NodeKind::Error(message) => {
-            c.diagnose(Diagnostic {
-                span: node.span,
-                message,
-            });
-            Ok(())
-        }
+        NodeKind::Unary => compile_unary(c, src, node_id),
+        NodeKind::Binary => compile_binary(c, src, node_id),
+        NodeKind::Call => compile_call(c, src, node_id),
+        NodeKind::Paren => compile_paren(c, src, node_id),
+        NodeKind::ParenEmpty => compile_nil(c),
+        NodeKind::Lambda => compile_lambda(c, src, node_id),
+        NodeKind::If => compile_if(c, src, node_id),
+        NodeKind::Let => compile_let(c, src, node_id),
+
+        NodeKind::Toplevel => compile_toplevel(c, src, node_id),
+
+        // Error nodes are ignored, because for each error node an appropriate parser
+        // diagnostic is emitted anyways.
+        NodeKind::Error => Ok(()),
     }
 }
 
-fn compile_nil(c: &mut Compiler<'_, '_>) -> CompileResult {
+fn unsupported(c: &mut Compiler, src: &Source, node_id: NodeId, message: &str) -> CompileResult {
+    c.emit(Diagnostic::error(src.ast.span(node_id), message));
+    Ok(())
+}
+
+fn compile_nil(c: &mut Compiler) -> CompileResult {
     c.chunk.emit_opcode(Opcode::Nil)?;
 
     Ok(())
@@ -144,48 +173,39 @@ fn find_variable(
 }
 
 fn compile_ident<'a>(c: &mut Compiler<'a, '_>, src: &Source<'a>, node_id: NodeId) -> CompileResult {
-    let ident = src.ast.get(node_id);
-    let name = ident.span.slice(src.code);
+    let span = src.ast.span(node_id);
+    let name = span.slice(src.code);
 
-    match name {
-        "false" => _ = c.chunk.emit_opcode(Opcode::False)?,
-        "true" => _ = c.chunk.emit_opcode(Opcode::True)?,
-        _ => match find_variable(c, name, c.scopes.len() - 1) {
-            Ok(Some(Variable::Local(index))) => {
-                c.chunk.emit_opcode(Opcode::Local)?;
-                c.chunk.emit_u8(index)?;
+    match find_variable(c, name, c.scopes.len() - 1) {
+        Ok(Some(Variable::Local(index))) => {
+            c.chunk.emit_opcode(Opcode::Local)?;
+            c.chunk.emit_u8(index)?;
+        }
+        Ok(Some(Variable::Captured(index))) => {
+            c.chunk.emit_opcode(Opcode::Capture)?;
+            c.chunk.emit_u8(index)?;
+        }
+        Ok(None) => {
+            if let Some(def_id) = c.defs.get(name) {
+                c.chunk.emit_opcode(Opcode::Def)?;
+                c.chunk.emit_u16(def_id.to_u16())?;
+            } else {
+                c.emit(Diagnostic::error(span, "undefined variable"));
             }
-            Ok(Some(Variable::Captured(index))) => {
-                c.chunk.emit_opcode(Opcode::Capture)?;
-                c.chunk.emit_u8(index)?;
-            }
-            Ok(None) => {
-                if let Some(def_id) = c.defs.get(name) {
-                    c.chunk.emit_opcode(Opcode::Def)?;
-                    c.chunk.emit_u16(def_id.to_u16())?;
-                } else {
-                    c.diagnose(Diagnostic {
-                        span: ident.span,
-                        message: "undefined variable",
-                    });
-                }
-            }
-            Err(CaptureError) => {
-                c.diagnose(Diagnostic {
-                    span: ident.span,
-                    message: "too many variables captured from outer functions in this scope",
-                });
-            }
-        },
-    }
+        }
+        Err(CaptureError) => {
+            c.emit(Diagnostic::error(
+                span,
+                "too many variables captured from outer functions in this scope",
+            ));
+        }
+    };
 
     Ok(())
 }
 
 fn compile_number(c: &mut Compiler<'_, '_>, src: &Source<'_>, node_id: NodeId) -> CompileResult {
-    let node = src.ast.get(node_id);
-
-    let literal = node.span.slice(src.code);
+    let literal = src.ast.span(node_id).slice(src.code);
     let float: f32 = literal
         .parse()
         .expect("the parser should've gotten us a string parsable by the stdlib");
@@ -196,48 +216,130 @@ fn compile_number(c: &mut Compiler<'_, '_>, src: &Source<'_>, node_id: NodeId) -
     Ok(())
 }
 
-fn compile_list<'a>(c: &mut Compiler<'a, '_>, src: &Source<'a>, node_id: NodeId) -> CompileResult {
-    let NodeKind::List(function_id, args) = src.ast.get(node_id).kind else {
-        unreachable!("compile_list expects a List");
-    };
+fn compile_tag(c: &mut Compiler<'_, '_>, src: &Source, node_id: NodeId) -> CompileResult {
+    let tag = src.ast.span(node_id).slice(src.code);
 
-    let function = src.ast.get(function_id);
-    let name = function.span.slice(src.code);
-
-    if function.kind == NodeKind::Ident {
-        match name {
-            "fn" => return compile_fn(c, src, args),
-            "if" => return compile_if(c, src, args),
-            "let" => return compile_let(c, src, args),
-            _ => (),
-        };
+    match tag {
+        "False" => {
+            c.chunk.emit_opcode(Opcode::False)?;
+        }
+        "True" => {
+            c.chunk.emit_opcode(Opcode::True)?;
+        }
+        _ => {
+            c.emit(Diagnostic::error(src.ast.span(node_id), "uppercased identifiers are reserved for future use; please start your identifiers with a lowercase letter instead"));
+        }
     }
 
+    Ok(())
+}
+
+fn compile_unary<'a>(c: &mut Compiler<'a, '_>, src: &Source<'a>, node_id: NodeId) -> CompileResult {
+    let mut walk = src.ast.walk(node_id);
+    let Some(op) = walk.node() else { return Ok(()) };
+    let Some(expr) = walk.node() else {
+        return Ok(());
+    };
+
+    if src.ast.kind(op) != NodeKind::Op {
+        return Ok(());
+    }
+    let name = src.ast.span(op).slice(src.code);
+
+    compile_expr(c, src, expr)?;
+    if let Some(index) = (src.system.resolve_fn)(SystemFnArity::Unary, name) {
+        let argument_count = 1;
+        c.chunk.emit_opcode(Opcode::System)?;
+        c.chunk.emit_u8(index)?;
+        c.chunk.emit_u8(argument_count)?;
+    } else {
+        c.emit(Diagnostic::error(
+            src.ast.span(op),
+            "this unary operator is currently unimplemented",
+        ));
+    }
+
+    Ok(())
+}
+
+fn compile_binary<'a>(
+    c: &mut Compiler<'a, '_>,
+    src: &Source<'a>,
+    node_id: NodeId,
+) -> CompileResult {
+    let mut walk = src.ast.walk(node_id);
+    let Some(left) = walk.node() else {
+        return Ok(());
+    };
+    let Some(op) = walk.node() else { return Ok(()) };
+    let Some(right) = walk.node() else {
+        return Ok(());
+    };
+
+    if src.ast.kind(op) != NodeKind::Op {
+        return Ok(());
+    }
+    let name = src.ast.span(op).slice(src.code);
+
+    if name == "=" {
+        c.emit(Diagnostic::error(
+            src.ast.span(op),
+            "defs `a = b` may only appear at the top level",
+        ));
+        return Ok(());
+    }
+
+    compile_expr(c, src, left)?;
+    compile_expr(c, src, right)?;
+    if let Some(index) = (src.system.resolve_fn)(SystemFnArity::Binary, name) {
+        let argument_count = 2;
+        c.chunk.emit_opcode(Opcode::System)?;
+        c.chunk.emit_u8(index)?;
+        c.chunk.emit_u8(argument_count)?;
+    } else {
+        c.emit(Diagnostic::error(
+            src.ast.span(op),
+            "this unary operator is currently unimplemented",
+        ));
+    }
+
+    Ok(())
+}
+
+fn compile_call<'a>(c: &mut Compiler<'a, '_>, src: &Source<'a>, node_id: NodeId) -> CompileResult {
+    let mut walk = src.ast.walk(node_id);
+    let Some(func) = walk.node() else {
+        return Ok(());
+    };
+    let name = src.ast.span(func).slice(src.code);
+
     let mut argument_count = 0;
-    let mut args = args;
-    while let NodeKind::List(head, tail) = src.ast.get(args).kind {
-        compile_expr(c, src, head)?;
+    while let Some(arg) = walk.node() {
+        compile_expr(c, src, arg)?;
         argument_count += 1;
-        args = tail;
     }
 
     let argument_count = u8::try_from(argument_count).unwrap_or_else(|_| {
-        c.diagnose(Diagnostic {
-            span: src.ast.get(args).span,
-            message: "function call has too many arguments",
-        });
+        c.emit(Diagnostic::error(
+            src.ast.span(node_id),
+            "function call has too many arguments",
+        ));
         0
     });
 
-    if let (NodeKind::Ident, Some(index)) = (function.kind, (src.system.resolve_fn)(name)) {
+    if let (NodeKind::Ident, Some(index)) = (
+        src.ast.kind(func),
+        (src.system.resolve_fn)(SystemFnArity::Nary, name),
+    ) {
         c.chunk.emit_opcode(Opcode::System)?;
         c.chunk.emit_u8(index)?;
         c.chunk.emit_u8(argument_count)?;
     } else {
         // This is a bit of an oddity: we only emit the function expression _after_ the arguments,
         // but since the language is effectless this doesn't matter in practice.
-        // It makes for less code in the compiler and the VM.
-        compile_expr(c, src, function_id)?;
+        // It makes for a bit less code in the VM, since there's no need to find the function
+        // down the stack - it's always on top.
+        compile_expr(c, src, func)?;
         c.chunk.emit_opcode(Opcode::Call)?;
         c.chunk.emit_u8(argument_count)?;
     }
@@ -245,67 +347,28 @@ fn compile_list<'a>(c: &mut Compiler<'a, '_>, src: &Source<'a>, node_id: NodeId)
     Ok(())
 }
 
-struct WalkList {
-    current: NodeId,
-    ok: bool,
-}
-
-impl WalkList {
-    fn new(start: NodeId) -> Self {
-        Self {
-            current: start,
-            ok: true,
-        }
-    }
-
-    fn expect_arg(
-        &mut self,
-        c: &mut Compiler<'_, '_>,
-        src: &Source<'_>,
-        message: &'static str,
-    ) -> NodeId {
-        if !self.ok {
-            return NodeId::NIL;
-        }
-
-        if let NodeKind::List(expr, tail) = src.ast.get(self.current).kind {
-            self.current = tail;
-            expr
-        } else {
-            c.diagnose(Diagnostic {
-                span: src.ast.get(self.current).span,
-                message,
-            });
-            self.ok = false;
-            NodeId::NIL
-        }
-    }
-
-    fn expect_nil(&mut self, c: &mut Compiler<'_, '_>, src: &Source<'_>, message: &'static str) {
-        if src.ast.get(self.current).kind != NodeKind::Nil {
-            c.diagnose(Diagnostic {
-                span: src.ast.get(self.current).span,
-                message,
-            });
-            // NOTE: Don't set self.ok to false, since this is not a fatal error.
-            // The nodes returned previously are valid and therefore it's safe to operate on them.
-            // Just having extra arguments shouldn't inhibit emitting additional diagnostics in
-            // the expression.
-        }
-    }
-}
-
-fn compile_if<'a>(c: &mut Compiler<'a, '_>, src: &Source<'a>, args: NodeId) -> CompileResult {
-    let mut list = WalkList::new(args);
-
-    let condition = list.expect_arg(c, src, "missing `if` condition");
-    let if_true = list.expect_arg(c, src, "missing `if` true branch");
-    let if_false = list.expect_arg(c, src, "missing `if` false branch");
-    list.expect_nil(c, src, "extra arguments after `if` false branch");
-
-    if !list.ok {
+fn compile_paren<'a>(c: &mut Compiler<'a, '_>, src: &Source<'a>, node_id: NodeId) -> CompileResult {
+    let Some(inner) = src.ast.walk(node_id).node() else {
         return Ok(());
-    }
+    };
+
+    compile_expr(c, src, inner)?;
+
+    Ok(())
+}
+
+fn compile_if<'a>(c: &mut Compiler<'a, '_>, src: &Source<'a>, node_id: NodeId) -> CompileResult {
+    let mut walk = src.ast.walk(node_id);
+
+    let Some(condition) = walk.node() else {
+        return Ok(());
+    };
+    let Some(if_true) = walk.node() else {
+        return Ok(());
+    };
+    let Some(if_false) = walk.node() else {
+        return Ok(());
+    };
 
     compile_expr(c, src, condition)?;
 
@@ -328,113 +391,70 @@ fn compile_if<'a>(c: &mut Compiler<'a, '_>, src: &Source<'a>, args: NodeId) -> C
     Ok(())
 }
 
-fn compile_let<'a>(c: &mut Compiler<'a, '_>, src: &Source<'a>, args: NodeId) -> CompileResult {
-    let mut list = WalkList::new(args);
+fn compile_let<'a>(c: &mut Compiler<'a, '_>, src: &Source<'a>, node_id: NodeId) -> CompileResult {
+    let mut walk = src.ast.walk(node_id);
 
-    let binding_list = list.expect_arg(c, src, "missing `let` binding list ((x 1) (y 2) ...)");
-    let expr = list.expect_arg(c, src, "missing expression to `let` names into");
-    list.expect_nil(c, src, "extra arguments after `let` expression");
-
-    if !list.ok {
+    let Some(ident) = walk.node() else {
         return Ok(());
-    }
-
-    // NOTE: Our `let` behaves like `let*` from Lisps.
-    // This is because this is generally the more intuitive behaviour with how variable declarations
-    // work in traditional imperative languages.
-    // We do not offer an alternative to Lisp `let` to be as minimal as possible.
-
-    let mut current = binding_list;
-    let mut local_count: usize = 0;
-    while let NodeKind::List(head, tail) = src.ast.get(current).kind {
-        if !matches!(src.ast.get(head).kind, NodeKind::List(_, _)) {
-            c.diagnose(Diagnostic {
-                span: src.ast.get(head).span,
-                message: "`let` binding expected, like (x 1)",
-            });
-            current = tail;
-            continue;
-        }
-
-        let mut list = WalkList::new(head);
-        let ident = list.expect_arg(c, src, "binding name expected");
-        let value = list.expect_arg(c, src, "binding value expected");
-        list.expect_nil(c, src, "extra expressions after `let` binding value");
-
-        if src.ast.get(ident).kind != NodeKind::Ident {
-            c.diagnose(Diagnostic {
-                span: src.ast.get(ident).span,
-                message: "binding name must be an identifier",
-            });
-        }
-
-        // NOTE: Compile expression _before_ putting the value into scope.
-        // This is so that the variable cannot refer to itself, as it is yet to be declared.
-        compile_expr(c, src, value)?;
-
-        let name = src.ast.get(ident).span.slice(src.code);
-        let scope = c.scopes.last_mut().unwrap();
-        if scope.locals.len() >= u8::MAX as usize {
-            c.diagnose(Diagnostic {
-                span: src.ast.get(ident).span,
-                message: "too many names bound in this function at a single time",
-            });
-        } else {
-            scope.locals.push(Local { name });
-        }
-
-        local_count += 1;
-        current = tail;
-    }
+    };
+    let Some(expr) = walk.node() else {
+        return Ok(());
+    };
+    let Some(then) = walk.node() else {
+        return Ok(());
+    };
 
     compile_expr(c, src, expr)?;
-
+    let name = src.ast.span(ident).slice(src.code);
     let scope = c.scopes.last_mut().unwrap();
-    scope
-        .locals
-        .resize_with(scope.locals.len() - local_count, || unreachable!());
+    let index = if scope.locals.len() >= u8::MAX as usize {
+        c.emit(Diagnostic::error(
+            src.ast.span(ident),
+            "too many names bound in this function at a single time",
+        ));
 
-    // NOTE: If we reach more than 255 locals declared in our `let`, we should've gotten
-    // a diagnostic emitted in the `while` loop beforehand.
-    let local_count = u8::try_from(local_count).unwrap_or(0);
-    c.chunk.emit_opcode(Opcode::DropLet)?;
-    c.chunk.emit_u8(local_count)?;
+        // Don't emit the expression, because it will most likely contain errors due to this
+        // `let` failing.
+        return Ok(());
+    } else {
+        let index = scope.locals.len();
+        scope.locals.push(Local { name });
+        index as u8
+    };
+    c.chunk.emit_opcode(Opcode::SetLocal)?;
+    c.chunk.emit_u8(index)?;
+
+    compile_expr(c, src, then)?;
 
     Ok(())
 }
 
-fn compile_fn<'a>(c: &mut Compiler<'a, '_>, src: &Source<'a>, args: NodeId) -> CompileResult {
-    let mut list = WalkList::new(args);
-
-    let param_list = list.expect_arg(c, src, "missing function parameters");
-    let body = list.expect_arg(c, src, "missing function body");
-    list.expect_nil(c, src, "extra arguments after function body");
-
-    if !list.ok {
+fn compile_lambda<'a>(
+    c: &mut Compiler<'a, '_>,
+    src: &Source<'a>,
+    node_id: NodeId,
+) -> CompileResult {
+    let mut walk = src.ast.walk(node_id);
+    let Some(params) = walk.node() else {
         return Ok(());
-    }
+    };
+    let Some(body) = walk.node() else {
+        return Ok(());
+    };
 
     let mut locals = Vec::new();
-    let mut current = param_list;
-    while let NodeKind::List(ident, tail) = src.ast.get(current).kind {
-        if let NodeKind::Ident = src.ast.get(ident).kind {
-            locals.push(Local {
-                name: src.ast.get(ident).span.slice(src.code),
-            })
-        } else {
-            c.diagnose(Diagnostic {
-                span: src.ast.get(ident).span,
-                message: "function parameters must be identifiers",
-            })
-        }
-        current = tail;
+    let mut params_walk = src.ast.walk(params);
+    while let Some(param) = params_walk.node() {
+        locals.push(Local {
+            name: src.ast.span(param).slice(src.code),
+        });
     }
 
     let param_count = u8::try_from(locals.len()).unwrap_or_else(|_| {
-        c.diagnose(Diagnostic {
-            span: src.ast.get(param_list).span,
-            message: "too many function parameters",
-        });
+        c.emit(Diagnostic::error(
+            src.ast.span(params),
+            "too many function parameters",
+        ));
         0
     });
 
@@ -453,13 +473,21 @@ fn compile_fn<'a>(c: &mut Compiler<'a, '_>, src: &Source<'a>, args: NodeId) -> C
     c.chunk.patch_u16(after_offset, after);
 
     let scope = c.scopes.pop().unwrap();
-    let capture_count = u8::try_from(scope.captures.len()).unwrap_or_else(|_| {
-        c.diagnose(Diagnostic {
-            span: src.ast.get(body).span,
-            message: "function refers to too many variables from the outer function",
-        });
+    let local_count = u8::try_from(scope.locals.len()).unwrap_or_else(|_| {
+        c.emit(Diagnostic::error(
+            src.ast.span(body),
+            "function contains too many local variables",
+        ));
         0
     });
+    let capture_count = u8::try_from(scope.captures.len()).unwrap_or_else(|_| {
+        c.emit(Diagnostic::error(
+            src.ast.span(body),
+            "function refers to too many variables from its outer functions",
+        ));
+        0
+    });
+    c.chunk.emit_u8(local_count)?;
     c.chunk.emit_u8(capture_count)?;
     for capture in scope.captures {
         match capture {
@@ -484,31 +512,27 @@ fn compile_toplevel<'a>(
     src: &Source<'a>,
     node_id: NodeId,
 ) -> CompileResult {
-    let NodeKind::Toplevel(mut current) = src.ast.get(node_id).kind else {
-        unreachable!("compile_toplevel expects a Toplevel");
-    };
+    def_prepass(c, src, node_id)?;
 
-    def_prepass(c, src, current)?;
+    let mut walk = src.ast.walk(node_id);
+    let mut result_expr = None;
+    while let Some(toplevel_expr) = walk.node() {
+        if let Some(result_expr) = result_expr {
+            // TODO: This diagnostic should show you the expression after the result.
+            c.emit(Diagnostic::error(
+                src.ast.span(result_expr),
+                "the result value must be the last thing in the program",
+            ));
+        }
 
-    let mut had_result = false;
-    while let NodeKind::List(expr, tail) = src.ast.get(current).kind {
-        match compile_toplevel_expr(c, src, expr)? {
+        match compile_toplevel_expr(c, src, toplevel_expr)? {
             ToplevelExpr::Def => (),
-            ToplevelExpr::Result => had_result = true,
+            ToplevelExpr::Result if result_expr.is_none() => result_expr = Some(toplevel_expr),
+            ToplevelExpr::Result => (),
         }
-
-        if had_result && src.ast.get(tail).kind != NodeKind::Nil {
-            c.diagnose(Diagnostic {
-                span: src.ast.get(tail).span,
-                message: "result value may not be followed by anything else",
-            });
-            break;
-        }
-
-        current = tail;
     }
 
-    if !had_result {
+    if result_expr.is_none() {
         c.chunk.emit_opcode(Opcode::Nil)?;
     }
     c.chunk.emit_opcode(Opcode::Return)?;
@@ -516,36 +540,28 @@ fn compile_toplevel<'a>(
     Ok(())
 }
 
-fn def_prepass<'a>(c: &mut Compiler<'a, '_>, src: &Source<'a>, node_id: NodeId) -> CompileResult {
+fn def_prepass<'a>(c: &mut Compiler<'a, '_>, src: &Source<'a>, toplevel: NodeId) -> CompileResult {
+    let mut walk = src.ast.walk(toplevel);
+
     // This is a bit of a pattern matching tapeworm, but Rust unfortunately doesn't have `if let`
     // chains yet to make this more readable.
-    let mut current = node_id;
-    while let NodeKind::List(expr, tail) = src.ast.get(current).kind {
-        if let NodeKind::List(head_id, tail_id) = src.ast.get(expr).kind {
-            let head = src.ast.get(head_id);
-            let name = head.span.slice(src.code);
-            if head.kind == NodeKind::Ident && name == "def" {
-                if let NodeKind::List(ident_id, _) = src.ast.get(tail_id).kind {
-                    let ident = src.ast.get(ident_id);
-                    if ident.kind == NodeKind::Ident {
-                        let name = ident.span.slice(src.code);
-                        match c.defs.add(name) {
-                            Ok(_) => (),
-                            Err(DefError::Exists) => c.diagnose(Diagnostic {
-                                span: ident.span,
-                                message: "redefinitions of defs are not allowed",
-                            }),
-                            Err(DefError::OutOfSpace) => c.diagnose(Diagnostic {
-                                span: ident.span,
-                                message: "too many defs",
-                            }),
-                        }
+    while let Some(binary) = walk.node_of(NodeKind::Binary) {
+        let mut binary_walk = src.ast.walk(binary);
+        if let (Some(ident), Some(op)) = (binary_walk.node(), binary_walk.get(NodeKind::Op)) {
+            if src.ast.span(op).slice(src.code) == "=" {
+                let name = src.ast.span(ident).slice(src.code);
+                match c.defs.add(name) {
+                    Ok(_) => (),
+                    Err(DefError::Exists) => c.emit(Diagnostic::error(
+                        src.ast.span(ident),
+                        "a def with this name already exists",
+                    )),
+                    Err(DefError::OutOfSpace) => {
+                        c.emit(Diagnostic::error(src.ast.span(binary), "too many defs"))
                     }
                 }
             }
         }
-
-        current = tail;
     }
 
     Ok(())
@@ -562,14 +578,10 @@ fn compile_toplevel_expr<'a>(
     src: &Source<'a>,
     node_id: NodeId,
 ) -> CompileResult<ToplevelExpr> {
-    let node = src.ast.get(node_id);
-
-    if let NodeKind::List(head_id, tail_id) = node.kind {
-        let head = src.ast.get(head_id);
-        if head.kind == NodeKind::Ident {
-            let name = head.span.slice(src.code);
-            if name == "def" {
-                compile_def(c, src, tail_id)?;
+    if src.ast.kind(node_id) == NodeKind::Binary {
+        if let Some(op) = src.ast.walk(node_id).get(NodeKind::Op) {
+            if src.ast.span(op).slice(src.code) == "=" {
+                compile_def(c, src, node_id)?;
                 return Ok(ToplevelExpr::Def);
             }
         }
@@ -579,24 +591,32 @@ fn compile_toplevel_expr<'a>(
     Ok(ToplevelExpr::Result)
 }
 
-fn compile_def<'a>(c: &mut Compiler<'a, '_>, src: &Source<'a>, args: NodeId) -> CompileResult {
-    let mut list = WalkList::new(args);
-
-    let ident = list.expect_arg(c, src, "missing definition name");
-    let value = list.expect_arg(c, src, "missing definition value");
-    list.expect_nil(c, src, "extra arguments after definition");
-
-    if !list.ok {
+fn compile_def<'a>(c: &mut Compiler<'a, '_>, src: &Source<'a>, node_id: NodeId) -> CompileResult {
+    let mut walk = src.ast.walk(node_id);
+    let Some(left) = walk.node() else {
         return Ok(());
+    };
+    let Some(_op) = walk.node() else {
+        return Ok(());
+    };
+    let Some(right) = walk.node() else {
+        return Ok(());
+    };
+
+    if src.ast.kind(left) != NodeKind::Ident {
+        c.emit(Diagnostic::error(
+            src.ast.span(left),
+            "def name (identifier) expected",
+        ));
     }
 
-    let name = src.ast.get(ident).span.slice(src.code);
+    let name = src.ast.span(left).slice(src.code);
     // NOTE: def_prepass collects all definitions beforehand.
     // In case a def ends up not existing, that means we ran out of space for defs - so emit a
     // zero def instead.
     let def_id = c.defs.get(name).unwrap_or_default();
 
-    compile_expr(c, src, value)?;
+    compile_expr(c, src, right)?;
     c.chunk.emit_opcode(Opcode::SetDef)?;
     c.chunk.emit_u16(def_id.to_u16())?;
 
diff --git a/crates/haku/src/diagnostic.rs b/crates/haku/src/diagnostic.rs
new file mode 100644
index 0000000..3ce6917
--- /dev/null
+++ b/crates/haku/src/diagnostic.rs
@@ -0,0 +1,26 @@
+use alloc::string::String;
+
+use crate::source::Span;
+
+#[derive(Debug, Clone)]
+pub struct Diagnostic {
+    span: Span,
+    message: String,
+}
+
+impl Diagnostic {
+    pub fn error(span: Span, message: impl Into<String>) -> Self {
+        Self {
+            span,
+            message: message.into(),
+        }
+    }
+
+    pub fn span(&self) -> Span {
+        self.span
+    }
+
+    pub fn message(&self) -> &str {
+        &self.message
+    }
+}
diff --git a/crates/haku/src/lexer.rs b/crates/haku/src/lexer.rs
new file mode 100644
index 0000000..32aa8a5
--- /dev/null
+++ b/crates/haku/src/lexer.rs
@@ -0,0 +1,237 @@
+use alloc::vec::Vec;
+
+use crate::{
+    diagnostic::Diagnostic,
+    source::{SourceCode, Span},
+    token::{Lexis, TokenAllocError, TokenKind},
+};
+
+pub struct Lexer<'a> {
+    pub lexis: Lexis,
+    pub diagnostics: Vec<Diagnostic>,
+    input: &'a SourceCode,
+    position: u32,
+}
+
+impl<'a> Lexer<'a> {
+    pub fn new(lexis: Lexis, input: &'a SourceCode) -> Self {
+        Self {
+            lexis,
+            diagnostics: Vec::new(),
+            input,
+            position: 0,
+        }
+    }
+
+    fn current(&self) -> char {
+        self.input[self.position as usize..]
+            .chars()
+            .next()
+            .unwrap_or('\0')
+    }
+
+    fn advance(&mut self) {
+        self.position += self.current().len_utf8() as u32;
+    }
+
+    fn emit(&mut self, diagnostic: Diagnostic) {
+        if self.diagnostics.len() < self.diagnostics.capacity() {
+            self.diagnostics.push(diagnostic);
+        }
+    }
+}
+
+fn one(l: &mut Lexer<'_>, kind: TokenKind) -> TokenKind {
+    l.advance();
+    kind
+}
+
+fn one_or_two(l: &mut Lexer<'_>, kind1: TokenKind, c2: char, kind2: TokenKind) -> TokenKind {
+    l.advance();
+    if l.current() == c2 {
+        l.advance();
+        kind2
+    } else {
+        kind1
+    }
+}
+
+fn is_ident_char(c: char) -> bool {
+    matches!(c, 'a'..='z' | 'A'..='Z' | '0'..='9' | '_')
+}
+
+fn ident(l: &mut Lexer<'_>) -> TokenKind {
+    let start = l.position;
+    while is_ident_char(l.current()) {
+        l.advance();
+    }
+    let end = l.position;
+
+    match Span::new(start, end).slice(l.input) {
+        "_" => TokenKind::Underscore,
+        "and" => TokenKind::And,
+        "or" => TokenKind::Or,
+        "if" => TokenKind::If,
+        "else" => TokenKind::Else,
+        "let" => TokenKind::Let,
+        _ => TokenKind::Ident,
+    }
+}
+
+fn tag(l: &mut Lexer<'_>) -> TokenKind {
+    while is_ident_char(l.current()) {
+        l.advance();
+    }
+    TokenKind::Tag
+}
+
+// NOTE: You shouldn't expect that the numbers produced by the lexer are parsable.
+fn number(l: &mut Lexer<'_>) -> TokenKind {
+    while l.current().is_ascii_digit() {
+        l.advance();
+    }
+
+    if l.current() == '.' {
+        let dot = l.position;
+        l.advance();
+        if !l.current().is_ascii_digit() {
+            l.emit(Diagnostic::error(
+                Span::new(dot, l.position),
+                "there must be at least a single digit after the decimal point",
+            ));
+        }
+        while l.current().is_ascii_digit() {
+            l.advance();
+        }
+    }
+
+    TokenKind::Number
+}
+
+// NOTE: You shouldn't expect that the color literals produced by the lexer are parsable.
+fn color(l: &mut Lexer<'_>) -> TokenKind {
+    let hash = l.position;
+    l.advance(); // #
+
+    if !l.current().is_ascii_hexdigit() {
+        l.emit(Diagnostic::error(
+            Span::new(hash, l.position),
+            "hex digits expected after `#` (color literal)",
+        ));
+    }
+
+    let start = l.position;
+    while l.current().is_ascii_hexdigit() {
+        l.advance();
+    }
+    let len = l.position - start;
+
+    if !matches!(len, 3 | 4 | 6 | 8) {
+        l.emit(Diagnostic::error(Span::new(hash, l.position), "incorrect number of digits in color literal (must be #RGB, #RGBA, #RRGGBB, or #RRGGBBAA)"));
+    }
+
+    TokenKind::Color
+}
+
+fn whitespace_and_comments(l: &mut Lexer<'_>) {
+    loop {
+        match l.current() {
+            '-' => {
+                let position = l.position;
+                l.advance();
+                if l.current() == '-' {
+                    while l.current() != '\n' {
+                        l.advance();
+                    }
+                } else {
+                    // An unfortunate little bit of backtracking here;
+                    // This seems like the simplest possible solution though.
+                    // We don't treat comments as a separate token to simplify the parsing phase,
+                    // and because of this, handling this at the "real" token level would complicate
+                    // things quite a bit.
+                    l.position = position;
+                    break;
+                }
+            }
+
+            ' ' | '\r' | '\t' => l.advance(),
+
+            _ => break,
+        }
+    }
+}
+
+fn newline(l: &mut Lexer<'_>) -> (TokenKind, Span) {
+    let start = l.position;
+    l.advance(); // skip the initial newline
+    let end = l.position;
+
+    // Skip additional newlines after this one, to only produce one token.
+    // These do not count into this newline's span though.
+    loop {
+        whitespace_and_comments(l);
+        if l.current() == '\n' {
+            l.advance();
+            continue;
+        } else {
+            break;
+        }
+    }
+
+    (TokenKind::Newline, Span::new(start, end))
+}
+
+fn token(l: &mut Lexer<'_>) -> (TokenKind, Span) {
+    whitespace_and_comments(l);
+
+    let start = l.position;
+    let kind = match l.current() {
+        '\0' => TokenKind::Eof,
+
+        // NOTE: Order matters here. Numbers and tags take priority over identifers.
+        c if c.is_ascii_uppercase() => tag(l),
+        c if c.is_ascii_digit() => number(l),
+        c if is_ident_char(c) => ident(l),
+
+        '#' => color(l),
+
+        '+' => one(l, TokenKind::Plus),
+        '-' => one_or_two(l, TokenKind::Minus, '>', TokenKind::RArrow),
+        '*' => one(l, TokenKind::Star),
+        '/' => one(l, TokenKind::Slash),
+        '=' => one_or_two(l, TokenKind::Equal, '=', TokenKind::EqualEqual),
+        '!' => one_or_two(l, TokenKind::Not, '=', TokenKind::NotEqual),
+        '<' => one_or_two(l, TokenKind::Less, '=', TokenKind::LessEqual),
+        '>' => one_or_two(l, TokenKind::Greater, '=', TokenKind::GreaterEqual),
+
+        '\n' => return newline(l),
+        '(' => one(l, TokenKind::LParen),
+        ')' => one(l, TokenKind::RParen),
+        '[' => one(l, TokenKind::LBrack),
+        ']' => one(l, TokenKind::RBrack),
+        ',' => one(l, TokenKind::Comma),
+        '\\' => one(l, TokenKind::Backslash),
+
+        _ => {
+            l.advance();
+            l.emit(Diagnostic::error(
+                Span::new(start, l.position),
+                "unexpected character",
+            ));
+            TokenKind::Error
+        }
+    };
+    let end = l.position;
+    (kind, Span::new(start, end))
+}
+
+pub fn lex(l: &mut Lexer<'_>) -> Result<(), TokenAllocError> {
+    loop {
+        let (kind, span) = token(l);
+        l.lexis.push(kind, span)?;
+        if kind == TokenKind::Eof {
+            break;
+        }
+    }
+    Ok(())
+}
diff --git a/crates/haku/src/lib.rs b/crates/haku/src/lib.rs
index 22aecc8..81e69f2 100644
--- a/crates/haku/src/lib.rs
+++ b/crates/haku/src/lib.rs
@@ -2,10 +2,15 @@
 
 extern crate alloc;
 
+pub mod ast;
 pub mod bytecode;
 pub mod compiler;
+pub mod diagnostic;
+pub mod lexer;
+pub mod parser;
 pub mod render;
-pub mod sexp;
+pub mod source;
 pub mod system;
+pub mod token;
 pub mod value;
 pub mod vm;
diff --git a/crates/haku/src/parser.rs b/crates/haku/src/parser.rs
new file mode 100644
index 0000000..df487ee
--- /dev/null
+++ b/crates/haku/src/parser.rs
@@ -0,0 +1,607 @@
+use core::cell::Cell;
+
+use alloc::vec::Vec;
+
+use crate::{
+    ast::{Ast, NodeAllocError, NodeId, NodeKind},
+    diagnostic::Diagnostic,
+    source::Span,
+    token::{Lexis, TokenKind, TokenKindSet},
+};
+
+#[derive(Debug, Clone, Copy)]
+pub struct ParserLimits {
+    pub max_events: usize,
+}
+
+pub struct Parser<'a> {
+    tokens: &'a Lexis,
+    events: Vec<Event>,
+    position: u32,
+    fuel: Cell<u32>,
+    pub diagnostics: Vec<Diagnostic>,
+}
+
+#[derive(Debug)]
+enum Event {
+    Open { kind: NodeKind },
+    Close,
+    Advance,
+}
+
+struct Open {
+    index: Option<usize>,
+}
+
+struct Closed {
+    index: Option<usize>,
+}
+
+impl<'a> Parser<'a> {
+    const FUEL: u32 = 256;
+
+    pub fn new(input: &'a Lexis, limits: &ParserLimits) -> Self {
+        assert!(limits.max_events < u32::MAX as usize);
+
+        Self {
+            tokens: input,
+            events: Vec::with_capacity(limits.max_events),
+            position: 0,
+            diagnostics: Vec::with_capacity(16),
+            fuel: Cell::new(Self::FUEL),
+        }
+    }
+
+    fn event(&mut self, event: Event) -> Option<usize> {
+        if self.events.len() < self.events.capacity() {
+            let index = self.events.len();
+            self.events.push(event);
+            Some(index)
+        } else {
+            None
+        }
+    }
+
+    fn open(&mut self) -> Open {
+        Open {
+            index: self.event(Event::Open {
+                kind: NodeKind::Error,
+            }),
+        }
+    }
+
+    fn open_before(&mut self, closed: Closed) -> Open {
+        if let Some(index) = closed.index {
+            if self.events.len() < self.events.capacity() {
+                self.events.insert(
+                    index,
+                    Event::Open {
+                        kind: NodeKind::Error,
+                    },
+                );
+                return Open { index: Some(index) };
+            }
+        }
+        Open { index: None }
+    }
+
+    fn close(&mut self, open: Open, kind: NodeKind) -> Closed {
+        if let Some(index) = open.index {
+            self.events[index] = Event::Open { kind };
+            self.event(Event::Close);
+            Closed { index: Some(index) }
+        } else {
+            Closed { index: None }
+        }
+    }
+
+    fn is_eof(&self) -> bool {
+        self.peek() == TokenKind::Eof
+    }
+
+    fn advance(&mut self) {
+        if !self.is_eof() {
+            self.position += 1;
+            self.event(Event::Advance);
+            self.fuel.set(Self::FUEL);
+        }
+    }
+
+    #[track_caller]
+    fn peek(&self) -> TokenKind {
+        assert_ne!(self.fuel.get(), 0, "parser is stuck");
+        self.fuel.set(self.fuel.get() - 1);
+
+        self.tokens.kind(self.position)
+    }
+
+    fn span(&self) -> Span {
+        self.tokens.span(self.position)
+    }
+
+    fn emit(&mut self, diagnostic: Diagnostic) {
+        if self.diagnostics.len() < self.diagnostics.capacity() {
+            self.diagnostics.push(diagnostic);
+        }
+    }
+
+    fn advance_with_error(&mut self) -> Closed {
+        let opened = self.open();
+        self.advance();
+        self.close(opened, NodeKind::Error)
+    }
+
+    fn optional_newline(&mut self) -> bool {
+        if self.peek() == TokenKind::Newline {
+            self.advance();
+            true
+        } else {
+            false
+        }
+    }
+
+    pub fn into_ast(self, ast: &mut Ast) -> Result<(NodeId, Vec<Diagnostic>), NodeAllocError> {
+        let mut token = 0;
+        let mut events = self.events;
+        let mut stack = Vec::new();
+
+        struct StackEntry {
+            node_id: NodeId,
+            // TODO: This should probably be optimized to use a shared stack.
+            children: Vec<NodeId>,
+        }
+
+        // Remove the last Close to keep a single node on the stack.
+        assert!(matches!(events.pop(), Some(Event::Close)));
+
+        for event in events {
+            match event {
+                Event::Open { kind } => {
+                    stack.push(StackEntry {
+                        node_id: ast.alloc(kind, self.tokens.span(token))?,
+                        children: Vec::new(),
+                    });
+                }
+                Event::Close => {
+                    let end_span = self.tokens.span(token.saturating_sub(1));
+                    let stack_entry = stack.pop().unwrap();
+                    ast.alloc_children(stack_entry.node_id, &stack_entry.children);
+                    ast.extend_span(stack_entry.node_id, end_span.end);
+                    stack.last_mut().unwrap().children.push(stack_entry.node_id);
+                }
+                Event::Advance => {
+                    let span = self.tokens.span(token);
+                    let node_id = ast.alloc(NodeKind::Token, span)?;
+                    stack
+                        .last_mut()
+                        .expect("advance() may only be used in an open node")
+                        .children
+                        .push(node_id);
+                    token += 1;
+                }
+            }
+        }
+
+        if stack.len() != 1 {
+            // This means we had too many events emitted and they are no longer balanced.
+            return Err(NodeAllocError);
+        }
+        // assert_eq!(token, self.tokens.len());
+
+        let end_span = self.tokens.span(token.saturating_sub(1));
+        let stack_entry = stack.pop().unwrap();
+        ast.alloc_children(stack_entry.node_id, &stack_entry.children);
+        ast.extend_span(stack_entry.node_id, end_span.end);
+
+        Ok((stack_entry.node_id, self.diagnostics))
+    }
+}
+
+impl<'a> core::fmt::Debug for Parser<'a> {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        f.debug_struct("Parser")
+            .field("events", &self.events)
+            .finish_non_exhaustive()
+    }
+}
+
+enum Tighter {
+    Left,
+    Right,
+}
+
+fn tighter(left: TokenKind, right: TokenKind) -> Tighter {
+    fn tightness(kind: TokenKind) -> Option<usize> {
+        match kind {
+            TokenKind::Equal => Some(0),
+            TokenKind::EqualEqual
+            | TokenKind::NotEqual
+            | TokenKind::Less
+            | TokenKind::LessEqual
+            | TokenKind::Greater
+            | TokenKind::GreaterEqual => Some(1),
+            TokenKind::Plus | TokenKind::Minus => Some(2),
+            TokenKind::Star | TokenKind::Slash => Some(3),
+            _ if PREFIX_TOKENS.contains(kind) => Some(4),
+            _ => None,
+        }
+    }
+
+    let Some(right_tightness) = tightness(right) else {
+        return Tighter::Left;
+    };
+    let Some(left_tightness) = tightness(left) else {
+        assert!(left == TokenKind::Eof);
+        return Tighter::Right;
+    };
+
+    if right_tightness > left_tightness {
+        Tighter::Right
+    } else {
+        Tighter::Left
+    }
+}
+
+fn precedence_parse(p: &mut Parser, left: TokenKind) {
+    let mut lhs = prefix(p);
+
+    loop {
+        let right = p.peek();
+        match tighter(left, right) {
+            Tighter::Left => break,
+            Tighter::Right => {
+                let o = p.open_before(lhs);
+                let kind = infix(p, right);
+                lhs = p.close(o, kind);
+            }
+        }
+    }
+}
+
+fn one(p: &mut Parser, kind: NodeKind) -> Closed {
+    let o = p.open();
+    p.advance();
+    p.close(o, kind)
+}
+
+fn list(p: &mut Parser) -> Closed {
+    let o = p.open();
+    let lspan = p.span();
+    p.advance(); // [
+    p.optional_newline();
+
+    loop {
+        match p.peek() {
+            TokenKind::Eof => {
+                p.emit(Diagnostic::error(lspan, "missing `]` to close this list"));
+                break;
+            }
+
+            TokenKind::RBrack => {
+                p.advance();
+                break;
+            }
+
+            _ => (),
+        }
+
+        expr(p);
+
+        match p.peek() {
+            TokenKind::Comma | TokenKind::Newline => {
+                p.advance();
+                continue;
+            }
+
+            TokenKind::RBrack => {
+                p.advance();
+                break;
+            }
+
+            _ => {
+                let span = p.span();
+                p.emit(Diagnostic::error(
+                    span,
+                    "comma `,` or new line expected after list element",
+                ));
+                p.advance_with_error();
+            }
+        }
+    }
+
+    p.close(o, NodeKind::List)
+}
+
+fn unary(p: &mut Parser) -> Closed {
+    let o = p.open();
+
+    let op = p.open();
+    p.advance();
+    p.close(op, NodeKind::Op);
+
+    prefix(p);
+
+    p.close(o, NodeKind::Unary)
+}
+
+fn paren(p: &mut Parser) -> Closed {
+    let o = p.open();
+    let lspan = p.span();
+    p.advance(); // (
+    if p.peek() == TokenKind::RParen {
+        p.advance(); // )
+        p.close(o, NodeKind::ParenEmpty)
+    } else {
+        p.optional_newline();
+        expr(p);
+        p.optional_newline();
+        if p.peek() != TokenKind::RParen {
+            p.emit(Diagnostic::error(lspan, "missing closing parenthesis `)`"));
+            p.advance_with_error()
+        } else {
+            p.advance();
+            p.close(o, NodeKind::Paren)
+        }
+    }
+}
+
+fn param(p: &mut Parser) {
+    let o = p.open();
+
+    if let TokenKind::Ident | TokenKind::Underscore = p.peek() {
+        p.advance();
+    } else {
+        let span = p.span();
+        p.emit(Diagnostic::error(
+            span,
+            "parameter names must be identifiers or `_`",
+        ));
+        p.advance_with_error();
+    }
+
+    p.close(o, NodeKind::Param);
+}
+
+fn lambda(p: &mut Parser) -> Closed {
+    let o = p.open();
+    p.advance(); // backslash
+
+    let params = p.open();
+    loop {
+        param(p);
+        match p.peek() {
+            TokenKind::Comma => {
+                p.advance();
+                continue;
+            }
+
+            TokenKind::RArrow => break,
+
+            _ => {
+                let span = p.span();
+                p.emit(Diagnostic::error(
+                    span,
+                    "`,` or `->` expected after function parameter",
+                ));
+                p.advance_with_error();
+                break;
+            }
+        }
+    }
+    p.close(params, NodeKind::Params);
+
+    // NOTE: Can be false if there are some stray tokens.
+    // We prefer to bail early and let the rest of the program parse.
+    if p.peek() == TokenKind::RArrow {
+        p.advance();
+        p.optional_newline();
+        expr(p);
+    }
+
+    p.close(o, NodeKind::Lambda)
+}
+
+fn if_expr(p: &mut Parser) -> Closed {
+    let o = p.open();
+
+    p.advance(); // if
+    if p.peek() != TokenKind::LParen {
+        let span = p.span();
+        p.emit(Diagnostic::error(
+            span,
+            "the condition in an `if` expression must be surrounded with parentheses",
+        ));
+        // NOTE: Don't advance, it's more likely the programmer expected no parentheses to be needed.
+    }
+    p.advance();
+    expr(p); // Condition
+    if p.peek() != TokenKind::RParen {
+        let span = p.span();
+        p.emit(Diagnostic::error(
+            span,
+            "missing closing parenthesis after `if` condition",
+        ));
+    }
+    p.advance();
+    p.optional_newline();
+
+    expr(p); // True branch
+    p.optional_newline();
+
+    if p.peek() != TokenKind::Else {
+        let span = p.span();
+        p.emit(Diagnostic::error(
+            span,
+            "`if` expression is missing an `else` clause",
+        ));
+    }
+    p.advance();
+    p.optional_newline();
+
+    expr(p); // False branch
+
+    p.close(o, NodeKind::If)
+}
+
+fn let_expr(p: &mut Parser) -> Closed {
+    let o = p.open();
+
+    p.advance(); // let
+
+    if p.peek() == TokenKind::Ident {
+        let ident = p.open();
+        p.advance();
+        p.close(ident, NodeKind::Ident);
+    } else {
+        let span = p.span();
+        p.emit(Diagnostic::error(span, "`let` variable name expected"));
+        p.advance_with_error();
+    }
+
+    if p.peek() == TokenKind::Equal {
+        p.advance();
+    } else {
+        let span = p.span();
+        p.emit(Diagnostic::error(span, "`=` expected after variable name"));
+        p.advance_with_error();
+    }
+
+    expr(p);
+
+    if p.peek() == TokenKind::Newline {
+        p.advance();
+    } else {
+        let span = p.span();
+        p.emit(Diagnostic::error(
+            span,
+            "new line expected after `let` expression",
+        ));
+        p.advance_with_error();
+    }
+
+    expr(p);
+
+    p.close(o, NodeKind::Let)
+}
+
+const PREFIX_TOKENS: TokenKindSet = TokenKindSet::new(&[
+    TokenKind::Ident,
+    TokenKind::Tag,
+    TokenKind::Number,
+    TokenKind::Color,
+    // NOTE: This is ambiguous in function calls.
+    // In that case, the infix operator takes precedence (because the `match` arms for the infix op
+    // come first.)
+    TokenKind::Minus,
+    TokenKind::Not,
+    TokenKind::LParen,
+    TokenKind::Backslash,
+    TokenKind::If,
+    TokenKind::Let,
+    TokenKind::LBrack,
+]);
+
+fn prefix(p: &mut Parser) -> Closed {
+    match p.peek() {
+        TokenKind::Ident => one(p, NodeKind::Ident),
+        TokenKind::Tag => one(p, NodeKind::Tag),
+        TokenKind::Number => one(p, NodeKind::Number),
+        TokenKind::Color => one(p, NodeKind::Color),
+        TokenKind::LBrack => list(p),
+
+        TokenKind::Minus | TokenKind::Not => unary(p),
+        TokenKind::LParen => paren(p),
+        TokenKind::Backslash => lambda(p),
+        TokenKind::If => if_expr(p),
+        TokenKind::Let => let_expr(p),
+
+        _ => {
+            assert!(
+                !PREFIX_TOKENS.contains(p.peek()),
+                "{:?} found in PREFIX_TOKENS",
+                p.peek()
+            );
+
+            let span = p.span();
+            p.emit(Diagnostic::error(
+                span,
+                "an expression was expected, but this token does not start one",
+            ));
+            p.advance_with_error()
+        }
+    }
+}
+
+fn infix(p: &mut Parser, op: TokenKind) -> NodeKind {
+    match op {
+        TokenKind::Plus
+        | TokenKind::Minus
+        | TokenKind::Star
+        | TokenKind::Slash
+        | TokenKind::EqualEqual
+        | TokenKind::NotEqual
+        | TokenKind::Less
+        | TokenKind::LessEqual
+        | TokenKind::Greater
+        | TokenKind::GreaterEqual
+        | TokenKind::Equal => infix_binary(p, op),
+
+        _ if PREFIX_TOKENS.contains(op) => infix_call(p),
+
+        _ => panic!("unhandled infix operator {op:?}"),
+    }
+}
+
+fn infix_binary(p: &mut Parser, op: TokenKind) -> NodeKind {
+    let o = p.open();
+    p.advance();
+    p.close(o, NodeKind::Op);
+
+    if p.peek() == TokenKind::Newline {
+        p.advance();
+    }
+
+    precedence_parse(p, op);
+    NodeKind::Binary
+}
+
+fn infix_call(p: &mut Parser) -> NodeKind {
+    while PREFIX_TOKENS.contains(p.peek()) {
+        prefix(p);
+    }
+
+    NodeKind::Call
+}
+
+pub fn expr(p: &mut Parser) {
+    precedence_parse(p, TokenKind::Eof)
+}
+
+pub fn toplevel(p: &mut Parser) {
+    let o = p.open();
+    p.optional_newline();
+    while p.peek() != TokenKind::Eof {
+        expr(p);
+
+        match p.peek() {
+            TokenKind::Newline => {
+                p.advance();
+                continue;
+            }
+
+            TokenKind::Eof => break,
+
+            _ => {
+                let span = p.span();
+                p.emit(Diagnostic::error(
+                    span,
+                    "newline expected after toplevel expression",
+                ))
+            }
+        }
+    }
+    p.close(o, NodeKind::Toplevel);
+}
+
+#[cfg(test)]
+mod tests;
diff --git a/crates/haku/src/parser/tests.rs b/crates/haku/src/parser/tests.rs
new file mode 100644
index 0000000..381bc1a
--- /dev/null
+++ b/crates/haku/src/parser/tests.rs
@@ -0,0 +1,912 @@
+use alloc::{format, string::String};
+
+use crate::{
+    ast::{dump::dump, Ast, NodeId},
+    lexer::{lex, Lexer},
+    parser::expr,
+    source::SourceCode,
+    token::Lexis,
+};
+
+use super::{toplevel, Parser, ParserLimits};
+
+fn parse(s: &str, f: fn(&mut Parser)) -> (Ast, NodeId) {
+    let mut lexer = Lexer::new(Lexis::new(1024), SourceCode::unlimited_len(s));
+    lex(&mut lexer).expect("too many tokens");
+
+    let mut parser = Parser::new(&lexer.lexis, &ParserLimits { max_events: 1024 });
+    f(&mut parser);
+
+    if !parser.diagnostics.is_empty() {
+        panic!("parser emitted diagnostics: {:#?}", parser.diagnostics);
+    }
+
+    let mut ast = Ast::new(1024);
+    let (root, _) = parser.into_ast(&mut ast).unwrap();
+    (ast, root)
+}
+
+fn ast(s: &str, f: fn(&mut Parser)) -> String {
+    let (ast, root) = parse(s, f);
+    // The extra newline is mostly so that it's easier to make the string literals look nice.
+    format!("\n{}", dump(&ast, root, None))
+}
+
+#[track_caller]
+fn assert_ast_eq(s: &str, f: fn(&mut Parser), ast_s: &str) {
+    let got = ast(s, f);
+    if ast_s != got {
+        panic!("AST mismatch. expected:\n{ast_s}\n\ngot:\n{got}\n");
+    }
+}
+
+#[test]
+fn one_literals() {
+    assert_ast_eq(
+        "1",
+        expr,
+        "
+Number @ 0..1
+    Token @ 0..1",
+    );
+
+    assert_ast_eq(
+        "ExampleTag123",
+        expr,
+        "
+Tag @ 0..13
+    Token @ 0..13",
+    );
+
+    assert_ast_eq(
+        "example_ident123",
+        expr,
+        "
+Ident @ 0..16
+    Token @ 0..16",
+    );
+
+    assert_ast_eq(
+        "#000",
+        expr,
+        "
+Color @ 0..4
+    Token @ 0..4",
+    );
+
+    assert_ast_eq(
+        "#000F",
+        expr,
+        "
+Color @ 0..5
+    Token @ 0..5",
+    );
+
+    assert_ast_eq(
+        "#058EF0",
+        expr,
+        "
+Color @ 0..7
+    Token @ 0..7",
+    );
+
+    assert_ast_eq(
+        "#058EF0FF",
+        expr,
+        "
+Color @ 0..9
+    Token @ 0..9",
+    );
+}
+
+#[test]
+fn list() {
+    assert_ast_eq(
+        "[]",
+        expr,
+        "
+List @ 0..2
+    Token @ 0..1
+    Token @ 1..2",
+    );
+
+    assert_ast_eq(
+        "[1]",
+        expr,
+        "
+List @ 0..3
+    Token @ 0..1
+    Number @ 1..2
+        Token @ 1..2
+    Token @ 2..3",
+    );
+
+    assert_ast_eq(
+        "[1, 2]",
+        expr,
+        "
+List @ 0..6
+    Token @ 0..1
+    Number @ 1..2
+        Token @ 1..2
+    Token @ 2..3
+    Number @ 4..5
+        Token @ 4..5
+    Token @ 5..6",
+    );
+
+    assert_ast_eq(
+        "[
+             1
+             2
+         ]",
+        expr,
+        "
+List @ 0..42
+    Token @ 0..1
+    Token @ 1..2
+    Number @ 15..16
+        Token @ 15..16
+    Token @ 16..17
+    Number @ 30..31
+        Token @ 30..31
+    Token @ 31..32
+    Token @ 41..42",
+    );
+}
+
+#[test]
+fn unary() {
+    assert_ast_eq(
+        "-1",
+        expr,
+        "
+Unary @ 0..2
+    Op @ 0..1
+        Token @ 0..1
+    Number @ 1..2
+        Token @ 1..2",
+    );
+
+    assert_ast_eq(
+        "!1",
+        expr,
+        "
+Unary @ 0..2
+    Op @ 0..1
+        Token @ 0..1
+    Number @ 1..2
+        Token @ 1..2",
+    );
+}
+
+#[test]
+fn binary_single() {
+    assert_ast_eq(
+        "1 + 1",
+        expr,
+        "
+Binary @ 0..5
+    Number @ 0..1
+        Token @ 0..1
+    Op @ 2..3
+        Token @ 2..3
+    Number @ 4..5
+        Token @ 4..5",
+    );
+
+    assert_ast_eq(
+        "1 - 1",
+        expr,
+        "
+Binary @ 0..5
+    Number @ 0..1
+        Token @ 0..1
+    Op @ 2..3
+        Token @ 2..3
+    Number @ 4..5
+        Token @ 4..5",
+    );
+
+    assert_ast_eq(
+        "1 * 1",
+        expr,
+        "
+Binary @ 0..5
+    Number @ 0..1
+        Token @ 0..1
+    Op @ 2..3
+        Token @ 2..3
+    Number @ 4..5
+        Token @ 4..5",
+    );
+
+    assert_ast_eq(
+        "1 / 1",
+        expr,
+        "
+Binary @ 0..5
+    Number @ 0..1
+        Token @ 0..1
+    Op @ 2..3
+        Token @ 2..3
+    Number @ 4..5
+        Token @ 4..5",
+    );
+
+    assert_ast_eq(
+        "1 < 1",
+        expr,
+        "
+Binary @ 0..5
+    Number @ 0..1
+        Token @ 0..1
+    Op @ 2..3
+        Token @ 2..3
+    Number @ 4..5
+        Token @ 4..5",
+    );
+
+    assert_ast_eq(
+        "1 > 1",
+        expr,
+        "
+Binary @ 0..5
+    Number @ 0..1
+        Token @ 0..1
+    Op @ 2..3
+        Token @ 2..3
+    Number @ 4..5
+        Token @ 4..5",
+    );
+
+    assert_ast_eq(
+        "1 == 1",
+        expr,
+        "
+Binary @ 0..6
+    Number @ 0..1
+        Token @ 0..1
+    Op @ 2..4
+        Token @ 2..4
+    Number @ 5..6
+        Token @ 5..6",
+    );
+
+    assert_ast_eq(
+        "1 != 1",
+        expr,
+        "
+Binary @ 0..6
+    Number @ 0..1
+        Token @ 0..1
+    Op @ 2..4
+        Token @ 2..4
+    Number @ 5..6
+        Token @ 5..6",
+    );
+
+    assert_ast_eq(
+        "1 <= 1",
+        expr,
+        "
+Binary @ 0..6
+    Number @ 0..1
+        Token @ 0..1
+    Op @ 2..4
+        Token @ 2..4
+    Number @ 5..6
+        Token @ 5..6",
+    );
+
+    assert_ast_eq(
+        "1 >= 1",
+        expr,
+        "
+Binary @ 0..6
+    Number @ 0..1
+        Token @ 0..1
+    Op @ 2..4
+        Token @ 2..4
+    Number @ 5..6
+        Token @ 5..6",
+    );
+
+    assert_ast_eq(
+        "1 = 1",
+        expr,
+        "
+Binary @ 0..5
+    Number @ 0..1
+        Token @ 0..1
+    Op @ 2..3
+        Token @ 2..3
+    Number @ 4..5
+        Token @ 4..5",
+    );
+}
+
+#[test]
+fn binary_precedence() {
+    assert_ast_eq(
+        "1 + 1 + 1",
+        expr,
+        "
+Binary @ 0..9
+    Binary @ 0..5
+        Number @ 0..1
+            Token @ 0..1
+        Op @ 2..3
+            Token @ 2..3
+        Number @ 4..5
+            Token @ 4..5
+    Op @ 6..7
+        Token @ 6..7
+    Number @ 8..9
+        Token @ 8..9",
+    );
+
+    assert_ast_eq(
+        "1 * 1 + 1",
+        expr,
+        "
+Binary @ 0..9
+    Binary @ 0..5
+        Number @ 0..1
+            Token @ 0..1
+        Op @ 2..3
+            Token @ 2..3
+        Number @ 4..5
+            Token @ 4..5
+    Op @ 6..7
+        Token @ 6..7
+    Number @ 8..9
+        Token @ 8..9",
+    );
+
+    assert_ast_eq(
+        "1 + 1 * 1",
+        expr,
+        "
+Binary @ 0..9
+    Number @ 0..1
+        Token @ 0..1
+    Op @ 2..3
+        Token @ 2..3
+    Binary @ 4..9
+        Number @ 4..5
+            Token @ 4..5
+        Op @ 6..7
+            Token @ 6..7
+        Number @ 8..9
+            Token @ 8..9",
+    );
+
+    assert_ast_eq(
+        "1 < 1 + 1",
+        expr,
+        "
+Binary @ 0..9
+    Number @ 0..1
+        Token @ 0..1
+    Op @ 2..3
+        Token @ 2..3
+    Binary @ 4..9
+        Number @ 4..5
+            Token @ 4..5
+        Op @ 6..7
+            Token @ 6..7
+        Number @ 8..9
+            Token @ 8..9",
+    );
+
+    assert_ast_eq(
+        "1 + 1 < 1",
+        expr,
+        "
+Binary @ 0..9
+    Binary @ 0..5
+        Number @ 0..1
+            Token @ 0..1
+        Op @ 2..3
+            Token @ 2..3
+        Number @ 4..5
+            Token @ 4..5
+    Op @ 6..7
+        Token @ 6..7
+    Number @ 8..9
+        Token @ 8..9",
+    );
+
+    assert_ast_eq(
+        "1 + 1 * 1 < 1",
+        expr,
+        "
+Binary @ 0..13
+    Binary @ 0..9
+        Number @ 0..1
+            Token @ 0..1
+        Op @ 2..3
+            Token @ 2..3
+        Binary @ 4..9
+            Number @ 4..5
+                Token @ 4..5
+            Op @ 6..7
+                Token @ 6..7
+            Number @ 8..9
+                Token @ 8..9
+    Op @ 10..11
+        Token @ 10..11
+    Number @ 12..13
+        Token @ 12..13",
+    );
+
+    assert_ast_eq(
+        "1 * 1 + 1 < 1",
+        expr,
+        "
+Binary @ 0..13
+    Binary @ 0..9
+        Binary @ 0..5
+            Number @ 0..1
+                Token @ 0..1
+            Op @ 2..3
+                Token @ 2..3
+            Number @ 4..5
+                Token @ 4..5
+        Op @ 6..7
+            Token @ 6..7
+        Number @ 8..9
+            Token @ 8..9
+    Op @ 10..11
+        Token @ 10..11
+    Number @ 12..13
+        Token @ 12..13",
+    );
+}
+
+#[test]
+fn binary_cont() {
+    assert_ast_eq(
+        "1 +
+           1",
+        expr,
+        "
+Binary @ 0..16
+    Number @ 0..1
+        Token @ 0..1
+    Op @ 2..3
+        Token @ 2..3
+    Token @ 3..4
+    Number @ 15..16
+        Token @ 15..16",
+    );
+
+    assert_ast_eq(
+        "1 +
+
+           1",
+        expr,
+        "
+Binary @ 0..17
+    Number @ 0..1
+        Token @ 0..1
+    Op @ 2..3
+        Token @ 2..3
+    Token @ 3..4
+    Number @ 16..17
+        Token @ 16..17",
+    );
+}
+
+#[test]
+fn paren_empty() {
+    assert_ast_eq(
+        "()",
+        expr,
+        "
+ParenEmpty @ 0..2
+    Token @ 0..1
+    Token @ 1..2",
+    );
+}
+
+#[test]
+fn paren() {
+    assert_ast_eq(
+        "(1)",
+        expr,
+        "
+Paren @ 0..3
+    Token @ 0..1
+    Number @ 1..2
+        Token @ 1..2
+    Token @ 2..3",
+    );
+
+    assert_ast_eq(
+        "(1 + 1) * 1",
+        expr,
+        "
+Binary @ 0..11
+    Paren @ 0..7
+        Token @ 0..1
+        Binary @ 1..6
+            Number @ 1..2
+                Token @ 1..2
+            Op @ 3..4
+                Token @ 3..4
+            Number @ 5..6
+                Token @ 5..6
+        Token @ 6..7
+    Op @ 8..9
+        Token @ 8..9
+    Number @ 10..11
+        Token @ 10..11",
+    );
+
+    assert_ast_eq(
+        "1 * (1 + 1)",
+        expr,
+        "
+Binary @ 0..11
+    Number @ 0..1
+        Token @ 0..1
+    Op @ 2..3
+        Token @ 2..3
+    Paren @ 4..11
+        Token @ 4..5
+        Binary @ 5..10
+            Number @ 5..6
+                Token @ 5..6
+            Op @ 7..8
+                Token @ 7..8
+            Number @ 9..10
+                Token @ 9..10
+        Token @ 10..11",
+    );
+
+    assert_ast_eq(
+        "(
+             1 +
+             1   
+         )",
+        expr,
+        "
+Paren @ 0..47
+    Token @ 0..1
+    Token @ 1..2
+    Binary @ 15..33
+        Number @ 15..16
+            Token @ 15..16
+        Op @ 17..18
+            Token @ 17..18
+        Token @ 18..19
+        Number @ 32..33
+            Token @ 32..33
+    Token @ 36..37
+    Token @ 46..47",
+    );
+}
+
+#[test]
+fn infix_call() {
+    assert_ast_eq(
+        "f x y",
+        toplevel,
+        "
+Toplevel @ 0..5
+    Call @ 0..5
+        Ident @ 0..1
+            Token @ 0..1
+        Ident @ 2..3
+            Token @ 2..3
+        Ident @ 4..5
+            Token @ 4..5",
+    );
+
+    assert_ast_eq(
+        "sin 1 + cos 2",
+        toplevel,
+        "
+Toplevel @ 0..13
+    Binary @ 0..13
+        Call @ 0..5
+            Ident @ 0..3
+                Token @ 0..3
+            Number @ 4..5
+                Token @ 4..5
+        Op @ 6..7
+            Token @ 6..7
+        Call @ 8..13
+            Ident @ 8..11
+                Token @ 8..11
+            Number @ 12..13
+                Token @ 12..13",
+    );
+}
+
+#[test]
+fn infix_call_unary_arg() {
+    assert_ast_eq(
+        // NOTE: The whitespace here is misleading.
+        // This is a binary `-`.
+        "f -1",
+        toplevel,
+        "
+Toplevel @ 0..4
+    Binary @ 0..4
+        Ident @ 0..1
+            Token @ 0..1
+        Op @ 2..3
+            Token @ 2..3
+        Number @ 3..4
+            Token @ 3..4",
+    );
+
+    assert_ast_eq(
+        "f (-1)",
+        toplevel,
+        "
+Toplevel @ 0..6
+    Call @ 0..6
+        Ident @ 0..1
+            Token @ 0..1
+        Paren @ 2..6
+            Token @ 2..3
+            Unary @ 3..5
+                Op @ 3..4
+                    Token @ 3..4
+                Number @ 4..5
+                    Token @ 4..5
+            Token @ 5..6",
+    );
+}
+
+#[test]
+fn lambda() {
+    assert_ast_eq(
+        r#" \_ -> () "#,
+        toplevel,
+        "
+Toplevel @ 1..9
+    Lambda @ 1..9
+        Token @ 1..2
+        Params @ 2..3
+            Param @ 2..3
+                Token @ 2..3
+        Token @ 4..6
+        ParenEmpty @ 7..9
+            Token @ 7..8
+            Token @ 8..9",
+    );
+
+    assert_ast_eq(
+        r#" \x -> x "#,
+        toplevel,
+        "
+Toplevel @ 1..8
+    Lambda @ 1..8
+        Token @ 1..2
+        Params @ 2..3
+            Param @ 2..3
+                Token @ 2..3
+        Token @ 4..6
+        Ident @ 7..8
+            Token @ 7..8",
+    );
+
+    assert_ast_eq(
+        r#" \x, y -> x + y "#,
+        toplevel,
+        "
+Toplevel @ 1..15
+    Lambda @ 1..15
+        Token @ 1..2
+        Params @ 2..6
+            Param @ 2..3
+                Token @ 2..3
+            Token @ 3..4
+            Param @ 5..6
+                Token @ 5..6
+        Token @ 7..9
+        Binary @ 10..15
+            Ident @ 10..11
+                Token @ 10..11
+            Op @ 12..13
+                Token @ 12..13
+            Ident @ 14..15
+                Token @ 14..15",
+    );
+
+    assert_ast_eq(
+        r#" \x, y ->
+              x + y "#,
+        toplevel,
+        "
+Toplevel @ 1..29
+    Lambda @ 1..29
+        Token @ 1..2
+        Params @ 2..6
+            Param @ 2..3
+                Token @ 2..3
+            Token @ 3..4
+            Param @ 5..6
+                Token @ 5..6
+        Token @ 7..9
+        Token @ 9..10
+        Binary @ 24..29
+            Ident @ 24..25
+                Token @ 24..25
+            Op @ 26..27
+                Token @ 26..27
+            Ident @ 28..29
+                Token @ 28..29",
+    );
+
+    assert_ast_eq(
+        r#" f \x -> g \y -> x + y "#,
+        toplevel,
+        "
+Toplevel @ 1..22
+    Call @ 1..22
+        Ident @ 1..2
+            Token @ 1..2
+        Lambda @ 3..22
+            Token @ 3..4
+            Params @ 4..5
+                Param @ 4..5
+                    Token @ 4..5
+            Token @ 6..8
+            Call @ 9..22
+                Ident @ 9..10
+                    Token @ 9..10
+                Lambda @ 11..22
+                    Token @ 11..12
+                    Params @ 12..13
+                        Param @ 12..13
+                            Token @ 12..13
+                    Token @ 14..16
+                    Binary @ 17..22
+                        Ident @ 17..18
+                            Token @ 17..18
+                        Op @ 19..20
+                            Token @ 19..20
+                        Ident @ 21..22
+                            Token @ 21..22",
+    );
+
+    assert_ast_eq(
+        r#" f \x ->
+            g \y ->
+              x + y "#,
+        toplevel,
+        "
+Toplevel @ 1..48
+    Call @ 1..48
+        Ident @ 1..2
+            Token @ 1..2
+        Lambda @ 3..48
+            Token @ 3..4
+            Params @ 4..5
+                Param @ 4..5
+                    Token @ 4..5
+            Token @ 6..8
+            Token @ 8..9
+            Call @ 21..48
+                Ident @ 21..22
+                    Token @ 21..22
+                Lambda @ 23..48
+                    Token @ 23..24
+                    Params @ 24..25
+                        Param @ 24..25
+                            Token @ 24..25
+                    Token @ 26..28
+                    Token @ 28..29
+                    Binary @ 43..48
+                        Ident @ 43..44
+                            Token @ 43..44
+                        Op @ 45..46
+                            Token @ 45..46
+                        Ident @ 47..48
+                            Token @ 47..48",
+    );
+}
+
+#[test]
+fn if_expr() {
+    assert_ast_eq(
+        r#" if (true) 1 else 2 "#,
+        toplevel,
+        "
+Toplevel @ 1..19
+    If @ 1..19
+        Token @ 1..3
+        Token @ 4..5
+        Ident @ 5..9
+            Token @ 5..9
+        Token @ 9..10
+        Number @ 11..12
+            Token @ 11..12
+        Token @ 13..17
+        Number @ 18..19
+            Token @ 18..19",
+    );
+
+    assert_ast_eq(
+        r#" if (true)
+                1
+            else
+                2 "#,
+        toplevel,
+        "
+Toplevel @ 1..63
+    If @ 1..63
+        Token @ 1..3
+        Token @ 4..5
+        Ident @ 5..9
+            Token @ 5..9
+        Token @ 9..10
+        Token @ 10..11
+        Number @ 27..28
+            Token @ 27..28
+        Token @ 28..29
+        Token @ 41..45
+        Token @ 45..46
+        Number @ 62..63
+            Token @ 62..63",
+    );
+}
+
+#[test]
+fn let_expr() {
+    assert_ast_eq(
+        r#" let x = 1
+            x "#,
+        toplevel,
+        "
+Toplevel @ 1..24
+    Let @ 1..24
+        Token @ 1..4
+        Ident @ 5..6
+            Token @ 5..6
+        Token @ 7..8
+        Number @ 9..10
+            Token @ 9..10
+        Token @ 10..11
+        Ident @ 23..24
+            Token @ 23..24",
+    );
+
+    assert_ast_eq(
+        r#" let x = 1
+            let y = 2
+            x + y "#,
+        toplevel,
+        "
+Toplevel @ 1..50
+    Let @ 1..50
+        Token @ 1..4
+        Ident @ 5..6
+            Token @ 5..6
+        Token @ 7..8
+        Number @ 9..10
+            Token @ 9..10
+        Token @ 10..11
+        Let @ 23..50
+            Token @ 23..26
+            Ident @ 27..28
+                Token @ 27..28
+            Token @ 29..30
+            Number @ 31..32
+                Token @ 31..32
+            Token @ 32..33
+            Binary @ 45..50
+                Ident @ 45..46
+                    Token @ 45..46
+                Op @ 47..48
+                    Token @ 47..48
+                Ident @ 49..50
+                    Token @ 49..50",
+    )
+}
diff --git a/crates/haku/src/sexp.rs b/crates/haku/src/sexp.rs
deleted file mode 100644
index 9a2b8b2..0000000
--- a/crates/haku/src/sexp.rs
+++ /dev/null
@@ -1,510 +0,0 @@
-use core::{cell::Cell, fmt, ops::Deref};
-
-use alloc::vec::Vec;
-
-#[derive(Debug, Clone, Copy, PartialEq, Eq)]
-pub struct Span {
-    pub start: usize,
-    pub end: usize,
-}
-
-impl Span {
-    pub fn new(start: usize, end: usize) -> Self {
-        Self { start, end }
-    }
-
-    pub fn slice<'a>(&self, source: &'a SourceCode) -> &'a str {
-        &source.code[self.start..self.end]
-    }
-}
-
-/// Source code string with a verified size limit.
-/// An exact size limit is not enforced by this type - it only ensures the string isn't longer than
-/// intended, to not stall the parser for an unexpected amount of time.
-#[derive(Debug, PartialEq, Eq)]
-#[repr(transparent)]
-pub struct SourceCode {
-    code: str,
-}
-
-impl SourceCode {
-    pub fn limited_len(code: &str, max_len: usize) -> Option<&Self> {
-        if code.len() <= max_len {
-            Some(Self::unlimited_len(code))
-        } else {
-            None
-        }
-    }
-
-    pub fn unlimited_len(code: &str) -> &Self {
-        // SAFETY: SourceCode is a transparent wrapper around str, so converting between them is safe.
-        unsafe { core::mem::transmute(code) }
-    }
-}
-
-impl Deref for SourceCode {
-    type Target = str;
-
-    fn deref(&self) -> &Self::Target {
-        &self.code
-    }
-}
-
-#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
-pub struct NodeId(usize);
-
-impl NodeId {
-    pub const NIL: NodeId = NodeId(0);
-}
-
-#[derive(Debug, Clone, Copy, PartialEq, Eq)]
-pub enum NodeKind {
-    Nil,
-    Eof,
-
-    // Atoms
-    Ident,
-    Number,
-
-    List(NodeId, NodeId),
-    Toplevel(NodeId),
-
-    Error(&'static str),
-}
-
-#[derive(Debug, Clone, Copy, PartialEq, Eq)]
-pub struct Node {
-    pub span: Span,
-    pub kind: NodeKind,
-}
-
-#[derive(Debug, Clone, PartialEq, Eq)]
-pub struct Ast {
-    pub nodes: Vec<Node>,
-}
-
-#[derive(Debug, Clone, Copy, PartialEq, Eq)]
-pub enum AstWriteMode {
-    Compact,
-    Spans,
-}
-
-impl Ast {
-    pub fn new(capacity: usize) -> Self {
-        assert!(capacity >= 1, "there must be space for at least a nil node");
-
-        let mut ast = Self {
-            nodes: Vec::with_capacity(capacity),
-        };
-
-        ast.alloc(Node {
-            span: Span::new(0, 0),
-            kind: NodeKind::Nil,
-        })
-        .unwrap();
-
-        ast
-    }
-
-    pub fn alloc(&mut self, node: Node) -> Result<NodeId, NodeAllocError> {
-        if self.nodes.len() >= self.nodes.capacity() {
-            return Err(NodeAllocError);
-        }
-
-        let index = self.nodes.len();
-        self.nodes.push(node);
-        Ok(NodeId(index))
-    }
-
-    pub fn get(&self, node_id: NodeId) -> &Node {
-        &self.nodes[node_id.0]
-    }
-
-    pub fn get_mut(&mut self, node_id: NodeId) -> &mut Node {
-        &mut self.nodes[node_id.0]
-    }
-
-    pub fn write(
-        &self,
-        source: &SourceCode,
-        node_id: NodeId,
-        w: &mut dyn fmt::Write,
-        mode: AstWriteMode,
-    ) -> fmt::Result {
-        #[allow(clippy::too_many_arguments)]
-        fn write_list(
-            ast: &Ast,
-            source: &SourceCode,
-            w: &mut dyn fmt::Write,
-            mode: AstWriteMode,
-            mut head: NodeId,
-            mut tail: NodeId,
-            sep_element: &str,
-            sep_tail: &str,
-        ) -> fmt::Result {
-            loop {
-                write_rec(ast, source, w, mode, head)?;
-                match ast.get(tail).kind {
-                    NodeKind::Nil => break,
-                    NodeKind::List(head2, tail2) => {
-                        w.write_str(sep_element)?;
-                        (head, tail) = (head2, tail2);
-                    }
-                    _ => {
-                        w.write_str(sep_tail)?;
-                        write_rec(ast, source, w, mode, tail)?;
-                        break;
-                    }
-                }
-            }
-            Ok(())
-        }
-
-        // NOTE: Separated out to a separate function in case we ever want to introduce auto-indentation.
-        fn write_rec(
-            ast: &Ast,
-            source: &SourceCode,
-            w: &mut dyn fmt::Write,
-            mode: AstWriteMode,
-            node_id: NodeId,
-        ) -> fmt::Result {
-            let node = ast.get(node_id);
-            match &node.kind {
-                NodeKind::Nil => write!(w, "()")?,
-                NodeKind::Eof => write!(w, "<eof>")?,
-                NodeKind::Ident | NodeKind::Number => write!(w, "{}", node.span.slice(source))?,
-
-                NodeKind::List(head, tail) => {
-                    w.write_char('(')?;
-                    write_list(ast, source, w, mode, *head, *tail, " ", " . ")?;
-                    w.write_char(')')?;
-                }
-
-                NodeKind::Toplevel(list) => {
-                    let NodeKind::List(head, tail) = ast.get(*list).kind else {
-                        unreachable!("child of Toplevel must be a List");
-                    };
-
-                    write_list(ast, source, w, mode, head, tail, "\n", " . ")?;
-                }
-
-                NodeKind::Error(message) => write!(w, "#error({message})")?,
-            }
-
-            if mode == AstWriteMode::Spans {
-                write!(w, "@{}..{}", node.span.start, node.span.end)?;
-            }
-
-            Ok(())
-        }
-
-        write_rec(self, source, w, mode, node_id)?;
-
-        Ok(())
-    }
-}
-
-#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
-pub struct NodeAllocError;
-
-pub struct Parser<'a> {
-    pub ast: Ast,
-    input: &'a SourceCode,
-    position: usize,
-    fuel: Cell<usize>,
-    alloc_error: NodeId,
-}
-
-impl<'a> Parser<'a> {
-    const FUEL: usize = 256;
-
-    pub fn new(mut ast: Ast, input: &'a SourceCode) -> Self {
-        let alloc_error = ast
-            .alloc(Node {
-                span: Span::new(0, 0),
-                kind: NodeKind::Error("program is too big"),
-            })
-            .expect("there is not enough space in the arena for an error node");
-
-        Self {
-            ast,
-            input,
-            position: 0,
-            fuel: Cell::new(Self::FUEL),
-            alloc_error,
-        }
-    }
-
-    #[track_caller]
-    pub fn current(&self) -> char {
-        assert_ne!(self.fuel.get(), 0, "parser is stuck");
-        self.fuel.set(self.fuel.get() - 1);
-
-        self.input[self.position..].chars().next().unwrap_or('\0')
-    }
-
-    pub fn advance(&mut self) {
-        self.position += self.current().len_utf8();
-        self.fuel.set(Self::FUEL);
-    }
-
-    pub fn alloc(&mut self, expr: Node) -> NodeId {
-        self.ast.alloc(expr).unwrap_or(self.alloc_error)
-    }
-}
-
-pub fn skip_whitespace_and_comments(p: &mut Parser<'_>) {
-    loop {
-        match p.current() {
-            ' ' | '\t' | '\n' => {
-                p.advance();
-                continue;
-            }
-            ';' => {
-                while p.current() != '\n' && p.current() != '\0' {
-                    p.advance();
-                }
-            }
-            _ => break,
-        }
-    }
-}
-
-fn is_decimal_digit(c: char) -> bool {
-    c.is_ascii_digit()
-}
-
-pub fn parse_number(p: &mut Parser<'_>) -> NodeKind {
-    while is_decimal_digit(p.current()) {
-        p.advance();
-    }
-    if p.current() == '.' {
-        p.advance();
-        if !is_decimal_digit(p.current()) {
-            return NodeKind::Error("missing digits after decimal point '.' in number literal");
-        }
-        while is_decimal_digit(p.current()) {
-            p.advance();
-        }
-    }
-
-    NodeKind::Number
-}
-
-fn is_ident(c: char) -> bool {
-    // The identifier character set is quite limited to help with easy expansion in the future.
-    // Rationale:
-    // - alphabet and digits are pretty obvious
-    // - '-' and '_' can be used for identifier separators, whichever you prefer.
-    // - '+', '-', '*', '/', '^' are for arithmetic.
-    // - '=', '!', '<', '>' are fore comparison.
-    // - '\' is for builtin string constants, such as \n.
-    // For other operators, it's generally clearer to use words (such as `and` and `or`.)
-    matches!(c, 'a'..='z' | 'A'..='Z' | '0'..='9' | '-' | '_' | '+' | '*' | '/' | '\\' | '^' | '!' | '=' | '<' | '>')
-}
-
-pub fn parse_ident(p: &mut Parser<'_>) -> NodeKind {
-    while is_ident(p.current()) {
-        p.advance();
-    }
-
-    NodeKind::Ident
-}
-
-struct List {
-    head: NodeId,
-    tail: NodeId,
-}
-
-impl List {
-    fn new() -> Self {
-        Self {
-            head: NodeId::NIL,
-            tail: NodeId::NIL,
-        }
-    }
-
-    fn append(&mut self, p: &mut Parser<'_>, node: NodeId) {
-        let node_span = p.ast.get(node).span;
-
-        let new_tail = p.alloc(Node {
-            span: node_span,
-            kind: NodeKind::List(node, NodeId::NIL),
-        });
-        if self.head == NodeId::NIL {
-            self.head = new_tail;
-            self.tail = new_tail;
-        } else {
-            let old_tail = p.ast.get_mut(self.tail);
-            let NodeKind::List(expr_before, _) = old_tail.kind else {
-                return;
-            };
-            *old_tail = Node {
-                span: Span::new(old_tail.span.start, node_span.end),
-                kind: NodeKind::List(expr_before, new_tail),
-            };
-            self.tail = new_tail;
-        }
-    }
-}
-
-pub fn parse_list(p: &mut Parser<'_>) -> NodeId {
-    // This could've been a lot simpler if Rust supported tail recursion.
-
-    let start = p.position;
-
-    p.advance(); // skip past opening parenthesis
-    skip_whitespace_and_comments(p);
-
-    let mut list = List::new();
-
-    while p.current() != ')' {
-        if p.current() == '\0' {
-            return p.alloc(Node {
-                span: Span::new(start, p.position),
-                kind: NodeKind::Error("missing ')' to close '('"),
-            });
-        }
-
-        let expr = parse_expr(p);
-        skip_whitespace_and_comments(p);
-
-        list.append(p, expr);
-    }
-    p.advance(); // skip past closing parenthesis
-
-    // If we didn't have any elements, we must not modify the initial Nil with ID 0.
-    if list.head == NodeId::NIL {
-        list.head = p.alloc(Node {
-            span: Span::new(0, 0),
-            kind: NodeKind::Nil,
-        });
-    }
-
-    let end = p.position;
-    p.ast.get_mut(list.head).span = Span::new(start, end);
-
-    list.head
-}
-
-pub fn parse_expr(p: &mut Parser<'_>) -> NodeId {
-    let start = p.position;
-    let kind = match p.current() {
-        '\0' => NodeKind::Eof,
-        c if is_decimal_digit(c) => parse_number(p),
-        // NOTE: Because of the `match` order, this prevents identifiers from starting with a digit.
-        c if is_ident(c) => parse_ident(p),
-        '(' => return parse_list(p),
-        _ => {
-            p.advance();
-            NodeKind::Error("unexpected character")
-        }
-    };
-    let end = p.position;
-
-    p.alloc(Node {
-        span: Span::new(start, end),
-        kind,
-    })
-}
-
-pub fn parse_toplevel(p: &mut Parser<'_>) -> NodeId {
-    let start = p.position;
-
-    let mut nodes = List::new();
-
-    skip_whitespace_and_comments(p);
-    while p.current() != '\0' {
-        let expr = parse_expr(p);
-        skip_whitespace_and_comments(p);
-
-        nodes.append(p, expr);
-    }
-
-    let end = p.position;
-
-    p.alloc(Node {
-        span: Span::new(start, end),
-        kind: NodeKind::Toplevel(nodes.head),
-    })
-}
-
-#[cfg(test)]
-mod tests {
-    use core::error::Error;
-
-    use alloc::{boxed::Box, string::String};
-
-    use super::*;
-
-    #[track_caller]
-    fn parse(
-        f: fn(&mut Parser<'_>) -> NodeId,
-        source: &str,
-        expected: &str,
-    ) -> Result<(), Box<dyn Error>> {
-        let ast = Ast::new(16);
-        let code = SourceCode::unlimited_len(source);
-        let mut p = Parser::new(ast, code);
-        let node = f(&mut p);
-        let ast = p.ast;
-
-        let mut s = String::new();
-        ast.write(code, node, &mut s, AstWriteMode::Spans)?;
-
-        assert_eq!(s, expected);
-
-        Ok(())
-    }
-
-    #[test]
-    fn parse_number() -> Result<(), Box<dyn Error>> {
-        parse(parse_expr, "123", "123@0..3")?;
-        parse(parse_expr, "123.456", "123.456@0..7")?;
-        Ok(())
-    }
-
-    #[test]
-    fn parse_ident() -> Result<(), Box<dyn Error>> {
-        parse(parse_expr, "abc", "abc@0..3")?;
-        parse(parse_expr, "abcABC_01234", "abcABC_01234@0..12")?;
-        parse(parse_expr, "+-*/\\^!=<>", "+-*/\\^!=<>@0..10")?;
-        Ok(())
-    }
-
-    #[test]
-    fn parse_list() -> Result<(), Box<dyn Error>> {
-        parse(parse_expr, "()", "()@0..2")?;
-        parse(parse_expr, "(a a)", "(a@1..2 a@3..4)@0..5")?;
-        parse(parse_expr, "(a a a)", "(a@1..2 a@3..4 a@5..6)@0..7")?;
-        parse(parse_expr, "(() ())", "(()@1..3 ()@4..6)@0..7")?;
-        parse(
-            parse_expr,
-            "(nestedy (nest OwO))",
-            "(nestedy@1..8 (nest@10..14 OwO@15..18)@9..19)@0..20",
-        )?;
-        Ok(())
-    }
-
-    #[test]
-    fn oom() -> Result<(), Box<dyn Error>> {
-        parse(parse_expr, "(a a a a a a a a)", "(a@1..2 a@3..4 a@5..6 a@7..8 a@9..10 a@11..12 a@13..14 . #error(program is too big)@0..0)@0..17")?;
-        parse(parse_expr, "(a a a a a a a a a)", "(a@1..2 a@3..4 a@5..6 a@7..8 a@9..10 a@11..12 a@13..14 . #error(program is too big)@0..0)@0..19")?;
-        parse(parse_expr, "(a a a a a a a a a a)", "(a@1..2 a@3..4 a@5..6 a@7..8 a@9..10 a@11..12 a@13..14 . #error(program is too big)@0..0)@0..21")?;
-        parse(parse_expr, "(a a a a a a a a a a a)", "(a@1..2 a@3..4 a@5..6 a@7..8 a@9..10 a@11..12 a@13..14 . #error(program is too big)@0..0)@0..23")?;
-        Ok(())
-    }
-
-    #[test]
-    fn toplevel() -> Result<(), Box<dyn Error>> {
-        parse(
-            parse_toplevel,
-            r#"
-                (hello world)
-                (abc)
-            "#,
-            "(hello@18..23 world@24..29)@17..30\n(abc@48..51)@47..52@0..65",
-        )?;
-        Ok(())
-    }
-}
diff --git a/crates/haku/src/source.rs b/crates/haku/src/source.rs
new file mode 100644
index 0000000..3eac60c
--- /dev/null
+++ b/crates/haku/src/source.rs
@@ -0,0 +1,55 @@
+use core::{fmt, ops::Deref};
+
+#[derive(Clone, Copy, PartialEq, Eq)]
+pub struct Span {
+    pub start: u32,
+    pub end: u32,
+}
+
+impl Span {
+    pub fn new(start: u32, end: u32) -> Self {
+        Self { start, end }
+    }
+
+    pub fn slice<'a>(&self, source: &'a SourceCode) -> &'a str {
+        &source.code[self.start as usize..self.end as usize]
+    }
+}
+
+impl fmt::Debug for Span {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}..{}", self.start, self.end)
+    }
+}
+
+/// Source code string with a verified size limit.
+/// An exact size limit is not enforced by this type - it only ensures the string isn't longer than
+/// intended, to not stall the parser for an unexpected amount of time.
+#[derive(Debug, PartialEq, Eq)]
+#[repr(transparent)]
+pub struct SourceCode {
+    code: str,
+}
+
+impl SourceCode {
+    pub fn limited_len(code: &str, max_len: u32) -> Option<&Self> {
+        if code.len() <= max_len as usize {
+            Some(Self::unlimited_len(code))
+        } else {
+            None
+        }
+    }
+
+    pub fn unlimited_len(code: &str) -> &Self {
+        // SAFETY: SourceCode is a transparent wrapper around str, so converting between them is safe.
+        unsafe { core::mem::transmute(code) }
+    }
+}
+
+impl Deref for SourceCode {
+    type Target = str;
+
+    fn deref(&self) -> &Self::Target {
+        &self.code
+    }
+}
diff --git a/crates/haku/src/system.rs b/crates/haku/src/system.rs
index ef5686e..7b04bcd 100644
--- a/crates/haku/src/system.rs
+++ b/crates/haku/src/system.rs
@@ -16,10 +16,17 @@ pub type SystemFn = fn(&mut Vm, FnArgs) -> Result<Value, Exception>;
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub struct ChunkId(u32);
 
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum SystemFnArity {
+    Unary,
+    Binary,
+    Nary,
+}
+
 #[derive(Debug, Clone)]
 pub struct System {
     /// Resolves a system function name to an index into `fn`s.
-    pub resolve_fn: fn(&str) -> Option<u8>,
+    pub resolve_fn: fn(SystemFnArity, &str) -> Option<u8>,
     pub fns: [Option<SystemFn>; 256],
     pub chunks: Vec<Chunk>,
 }
@@ -30,7 +37,7 @@ pub struct SystemImage {
 }
 
 macro_rules! def_fns {
-    ($($index:tt $name:tt => $fnref:expr),* $(,)?) => {
+    ($($index:tt $arity:tt $name:tt => $fnref:expr),* $(,)?) => {
         pub(crate) fn init_fns(system: &mut System) {
             $(
                 debug_assert!(system.fns[$index].is_none());
@@ -38,9 +45,9 @@ macro_rules! def_fns {
             )*
         }
 
-        pub(crate) fn resolve(name: &str) -> Option<u8> {
-            match name {
-                $($name => Some($index),)*
+        pub(crate) fn resolve(arity: SystemFnArity, name: &str) -> Option<u8> {
+            match (arity, name){
+                $((SystemFnArity::$arity, $name) => Some($index),)*
                 _ => None,
             }
         }
@@ -106,43 +113,44 @@ pub mod fns {
         vm::{Exception, FnArgs, Vm},
     };
 
-    use super::System;
+    use super::{System, SystemFnArity};
 
     impl System {
         def_fns! {
-            0x00 "+" => add,
-            0x01 "-" => sub,
-            0x02 "*" => mul,
-            0x03 "/" => div,
+            0x00 Binary "+" => add,
+            0x01 Binary "-" => sub,
+            0x02 Binary "*" => mul,
+            0x03 Binary "/" => div,
+            0x04 Unary "-" => neg,
 
-            0x40 "not" => not,
-            0x41 "=" => eq,
-            0x42 "<>" => neq,
-            0x43 "<" => lt,
-            0x44 "<=" => leq,
-            0x45 ">" => gt,
-            0x46 ">=" => geq,
+            0x40 Unary "!" => not,
+            0x41 Binary "==" => eq,
+            0x42 Binary "!=" => neq,
+            0x43 Binary "<" => lt,
+            0x44 Binary "<=" => leq,
+            0x45 Binary ">" => gt,
+            0x46 Binary ">=" => geq,
 
-            0x80 "vec" => vec,
-            0x81 ".x" => vec_x,
-            0x82 ".y" => vec_y,
-            0x83 ".z" => vec_z,
-            0x84 ".w" => vec_w,
+            0x80 Nary "vec" => vec,
+            0x81 Nary "vecX" => vec_x,
+            0x82 Nary "vecY" => vec_y,
+            0x83 Nary "vecZ" => vec_z,
+            0x84 Nary "vecW" => vec_w,
 
-            0x85 "rgba" => rgba,
-            0x86 ".r" => rgba_r,
-            0x87 ".g" => rgba_g,
-            0x88 ".b" => rgba_b,
-            0x89 ".a" => rgba_a,
+            0x85 Nary "rgba" => rgba,
+            0x86 Nary "rgbaR" => rgba_r,
+            0x87 Nary "rgbaG" => rgba_g,
+            0x88 Nary "rgbaB" => rgba_b,
+            0x89 Nary "rgbaA" => rgba_a,
 
-            0x90 "list" => list,
+            0x90 Nary "list" => list,
 
-            0xc0 "to-shape" => to_shape_f,
-            0xc1 "line" => line,
-            0xc2 "rect" => rect,
-            0xc3 "circle" => circle,
-            0xe0 "stroke" => stroke,
-            0xe1 "fill" => fill,
+            0xc0 Nary "toShape" => to_shape_f,
+            0xc1 Nary "line" => line,
+            0xc2 Nary "rect" => rect,
+            0xc3 Nary "circle" => circle,
+            0xe0 Nary "stroke" => stroke,
+            0xe1 Nary "fill" => fill,
         }
     }
 
@@ -196,6 +204,11 @@ pub mod fns {
         Ok(Value::Number(result))
     }
 
+    pub fn neg(vm: &mut Vm, args: FnArgs) -> Result<Value, Exception> {
+        let x = args.get_number(vm, 0, "`-` can only work with numbers")?;
+        Ok(Value::Number(-x))
+    }
+
     pub fn not(vm: &mut Vm, args: FnArgs) -> Result<Value, Exception> {
         if args.num() != 1 {
             return Err(vm.create_exception("(not) expects a single argument to negate"));
diff --git a/crates/haku/src/token.rs b/crates/haku/src/token.rs
new file mode 100644
index 0000000..d8d621d
--- /dev/null
+++ b/crates/haku/src/token.rs
@@ -0,0 +1,143 @@
+use core::{error::Error, fmt::Display};
+
+use alloc::vec::Vec;
+
+use crate::source::Span;
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum TokenKind {
+    Eof,
+
+    Ident,
+    Tag,
+    Number,
+    Color,
+
+    // Operators
+    Plus,
+    Minus,
+    Star,
+    Slash,
+    EqualEqual,
+    NotEqual,
+    Less,
+    LessEqual,
+    Greater,
+    GreaterEqual,
+    Not,
+
+    // Punctuation
+    Newline,
+    LParen,
+    RParen,
+    LBrack,
+    RBrack,
+    Comma,
+    Equal,
+    Backslash,
+    RArrow,
+
+    // Keywords
+    Underscore,
+    And,
+    Or,
+    If,
+    Else,
+    Let,
+
+    // NOTE: This must be kept last for TokenSet to work correctly.
+    Error,
+}
+
+#[derive(Debug, Clone)]
+pub struct Lexis {
+    pub kinds: Vec<TokenKind>,
+    pub spans: Vec<Span>,
+}
+
+impl Lexis {
+    pub fn new(capacity: usize) -> Self {
+        assert!(capacity < u32::MAX as usize);
+
+        Self {
+            kinds: Vec::with_capacity(capacity),
+            spans: Vec::with_capacity(capacity),
+        }
+    }
+
+    pub fn len(&self) -> u32 {
+        self.kinds.len() as u32
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
+    pub fn push(&mut self, kind: TokenKind, span: Span) -> Result<(), TokenAllocError> {
+        if self.kinds.len() >= self.kinds.capacity() {
+            return Err(TokenAllocError);
+        }
+
+        self.kinds.push(kind);
+        self.spans.push(span);
+
+        Ok(())
+    }
+
+    pub fn kind(&self, position: u32) -> TokenKind {
+        self.kinds[position as usize]
+    }
+
+    pub fn span(&self, position: u32) -> Span {
+        self.spans[position as usize]
+    }
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct TokenAllocError;
+
+impl Display for TokenAllocError {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        f.write_str("too many tokens")
+    }
+}
+
+impl Error for TokenAllocError {}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct TokenKindSet {
+    bits: [u32; Self::WORDS],
+}
+
+impl TokenKindSet {
+    const WORDS: usize = ((TokenKind::Error as u32 + u32::BITS - 1) / (u32::BITS)) as usize;
+
+    const fn word(kind: TokenKind) -> usize {
+        (kind as u32 / u32::BITS) as usize
+    }
+
+    const fn bit(kind: TokenKind) -> u32 {
+        1 << (kind as u32 % u32::BITS)
+    }
+
+    pub const fn new(elems: &[TokenKind]) -> Self {
+        let mut set = Self {
+            bits: [0; Self::WORDS],
+        };
+        let mut i = 0;
+        while i < elems.len() {
+            set = set.include(elems[i]);
+            i += 1;
+        }
+        set
+    }
+
+    pub const fn include(mut self, kind: TokenKind) -> Self {
+        self.bits[Self::word(kind)] |= Self::bit(kind);
+        self
+    }
+
+    pub fn contains(&self, kind: TokenKind) -> bool {
+        self.bits[Self::word(kind)] & Self::bit(kind) != 0
+    }
+}
diff --git a/crates/haku/src/value.rs b/crates/haku/src/value.rs
index 97b8113..b169e81 100644
--- a/crates/haku/src/value.rs
+++ b/crates/haku/src/value.rs
@@ -1,6 +1,6 @@
 use alloc::vec::Vec;
 
-use crate::system::ChunkId;
+use crate::{compiler::ClosureSpec, system::ChunkId};
 
 // TODO: Probably needs some pretty hardcore space optimization.
 // Maybe when we have static typing.
@@ -156,9 +156,25 @@ pub struct Closure {
     pub start: BytecodeLoc,
     pub name: FunctionName,
     pub param_count: u8,
+    pub local_count: u8,
     pub captures: Vec<Value>,
 }
 
+impl Closure {
+    pub fn chunk(chunk_id: ChunkId, spec: ClosureSpec) -> Self {
+        Self {
+            start: BytecodeLoc {
+                chunk_id,
+                offset: 0,
+            },
+            name: FunctionName::Anonymous,
+            param_count: 0,
+            local_count: spec.local_count,
+            captures: Vec::new(),
+        }
+    }
+}
+
 #[derive(Debug, Clone, PartialEq)]
 pub struct List {
     pub elements: Vec<Value>,
diff --git a/crates/haku/src/vm.rs b/crates/haku/src/vm.rs
index d9cb816..549988e 100644
--- a/crates/haku/src/vm.rs
+++ b/crates/haku/src/vm.rs
@@ -123,8 +123,9 @@ impl Vm {
 
     fn push(&mut self, value: Value) -> Result<(), Exception> {
         if self.stack.len() >= self.stack.capacity() {
-            // TODO: can this error message be made clearer?
-            return Err(self.create_exception("too many local variables"));
+            return Err(self.create_exception(
+                "too many temporary values (local variables and expression operands)",
+            ));
         }
         self.stack.push(value);
         Ok(())
@@ -136,6 +137,14 @@ impl Vm {
         })
     }
 
+    fn get_mut(&mut self, index: usize) -> Result<&mut Value, Exception> {
+        if self.stack.get(index).is_some() {
+            Ok(&mut self.stack[index])
+        } else {
+            Err(self.create_exception("corrupted bytecode (set local variable out of bounds)"))
+        }
+    }
+
     fn pop(&mut self) -> Result<Value, Exception> {
         self.stack
             .pop()
@@ -168,6 +177,11 @@ impl Vm {
         let mut bottom = self.stack.len();
         let mut fuel = self.fuel;
 
+        let init_bottom = bottom;
+        for _ in 0..closure.local_count {
+            self.push(Value::Nil)?;
+        }
+
         #[allow(unused)]
         let closure = (); // Do not use `closure` after this! Use `get_ref` on `closure_id` instead.
 
@@ -200,6 +214,12 @@ impl Vm {
                     self.push(value)?;
                 }
 
+                Opcode::SetLocal => {
+                    let index = chunk.read_u8(&mut pc)? as usize;
+                    let new_value = self.pop()?;
+                    *self.get_mut(index)? = new_value;
+                }
+
                 Opcode::Capture => {
                     let index = chunk.read_u8(&mut pc)? as usize;
                     let closure = self.get_ref(closure_id).as_closure().unwrap();
@@ -226,26 +246,14 @@ impl Vm {
                     }
                 }
 
-                Opcode::DropLet => {
-                    let count = chunk.read_u8(&mut pc)? as usize;
-                    if count != 0 {
-                        let new_len = self.stack.len().checked_sub(count).ok_or_else(|| {
-                            self.create_exception(
-                            "corrupted bytecode (Drop tried to drop too many values off the stack)",
-                        )
-                        })?;
-                        let value = self.pop()?;
-                        self.stack.resize_with(new_len, || unreachable!());
-                        self.push(value)?;
-                    }
-                }
-
                 Opcode::Function => {
                     let param_count = chunk.read_u8(&mut pc)?;
                     let then = chunk.read_u16(&mut pc)? as usize;
                     let body = pc;
                     pc = then;
 
+                    let local_count = chunk.read_u8(&mut pc)?;
+
                     let capture_count = chunk.read_u8(&mut pc)? as usize;
                     let mut captures = Vec::with_capacity(capture_count);
                     for _ in 0..capture_count {
@@ -272,6 +280,7 @@ impl Vm {
                         },
                         name: FunctionName::Anonymous,
                         param_count,
+                        local_count,
                         captures,
                     }))?;
                     self.push(Value::Ref(id))?;
@@ -327,6 +336,11 @@ impl Vm {
                             )
                         })?;
 
+                    // NOTE: Locals are only pushed _after_ we do any stack calculations.
+                    for _ in 0..closure.local_count {
+                        self.push(Value::Nil)?;
+                    }
+
                     self.push_call(frame)?;
                 }
 
@@ -381,10 +395,13 @@ impl Vm {
             }
         }
 
-        Ok(self
+        let result = self
             .stack
             .pop()
-            .expect("there should be a result at the top of the stack"))
+            .expect("there should be a result at the top of the stack");
+        self.stack.resize_with(init_bottom, || unreachable!());
+
+        Ok(result)
     }
 
     fn store_context(&mut self, context: Context) {
diff --git a/crates/haku/tests/language.rs b/crates/haku/tests/language.rs
index 787c46f..71e8423 100644
--- a/crates/haku/tests/language.rs
+++ b/crates/haku/tests/language.rs
@@ -1,10 +1,14 @@
 use std::error::Error;
 
 use haku::{
+    ast::{dump::dump, Ast},
     bytecode::{Chunk, Defs},
     compiler::{compile_expr, Compiler, Source},
-    sexp::{self, Ast, Parser, SourceCode},
+    lexer::{lex, Lexer},
+    parser::{self, Parser, ParserLimits},
+    source::SourceCode,
     system::System,
+    token::Lexis,
     value::{BytecodeLoc, Closure, FunctionName, Ref, RefId, Value},
     vm::{Vm, VmLimits},
 };
@@ -12,11 +16,16 @@ use haku::{
 fn eval(code: &str) -> Result<Value, Box<dyn Error>> {
     let mut system = System::new(1);
 
-    let ast = Ast::new(1024);
     let code = SourceCode::unlimited_len(code);
-    let mut parser = Parser::new(ast, code);
-    let root = sexp::parse_toplevel(&mut parser);
-    let ast = parser.ast;
+
+    let mut lexer = Lexer::new(Lexis::new(1024), code);
+    lex(&mut lexer)?;
+
+    let mut ast = Ast::new(1024);
+    let mut parser = Parser::new(&lexer.lexis, &ParserLimits { max_events: 1024 });
+    parser::toplevel(&mut parser);
+    let (root, mut parser_diagnostics) = parser.into_ast(&mut ast)?;
+    println!("{}", dump(&ast, root, Some(code)));
     let src = Source {
         code,
         ast: &ast,
@@ -27,21 +36,29 @@ fn eval(code: &str) -> Result<Value, Box<dyn Error>> {
     let mut chunk = Chunk::new(65536).unwrap();
     let mut compiler = Compiler::new(&mut defs, &mut chunk);
     compile_expr(&mut compiler, &src, root)?;
+    let closure_spec = compiler.closure_spec();
     let defs = compiler.defs;
 
-    for diagnostic in &compiler.diagnostics {
+    let mut diagnostics = lexer.diagnostics;
+    diagnostics.append(&mut parser_diagnostics);
+    diagnostics.append(&mut compiler.diagnostics);
+
+    for diagnostic in &diagnostics {
         println!(
-            "{}..{}: {}",
-            diagnostic.span.start, diagnostic.span.end, diagnostic.message
+            "{}..{} {:?}: {}",
+            diagnostic.span().start,
+            diagnostic.span().end,
+            diagnostic.span().slice(code),
+            diagnostic.message()
         );
     }
 
-    if !compiler.diagnostics.is_empty() {
-        panic!("compiler diagnostics were emitted")
+    if !diagnostics.is_empty() {
+        panic!("diagnostics were emitted")
     }
 
     let limits = VmLimits {
-        stack_capacity: 256,
+        stack_capacity: 1024,
         call_stack_capacity: 256,
         ref_capacity: 256,
         fuel: 32768,
@@ -50,16 +67,9 @@ fn eval(code: &str) -> Result<Value, Box<dyn Error>> {
     let mut vm = Vm::new(defs, &limits);
     let chunk_id = system.add_chunk(chunk)?;
     println!("bytecode: {:?}", system.chunk(chunk_id));
+    println!("closure spec: {closure_spec:?}");
 
-    let closure = vm.create_ref(Ref::Closure(Closure {
-        start: BytecodeLoc {
-            chunk_id,
-            offset: 0,
-        },
-        name: FunctionName::Anonymous,
-        param_count: 0,
-        captures: Vec::new(),
-    }))?;
+    let closure = vm.create_ref(Ref::Closure(Closure::chunk(chunk_id, closure_spec)))?;
     let result = vm.run(&system, closure)?;
 
     println!("used fuel: {}", limits.fuel - vm.remaining_fuel());
@@ -87,49 +97,52 @@ fn literal_number() {
 
 #[test]
 fn literal_bool() {
-    assert_eq!(eval("false").unwrap(), Value::False);
-    assert_eq!(eval("true").unwrap(), Value::True);
+    assert_eq!(eval("False").unwrap(), Value::False);
+    assert_eq!(eval("True").unwrap(), Value::True);
 }
 
 #[test]
 fn function_nil() {
-    assert_eq!(eval("(fn () ())").unwrap(), Value::Ref(RefId::from_u32(1)));
+    assert_eq!(
+        eval(r#" \_ -> () "#).unwrap(),
+        Value::Ref(RefId::from_u32(1))
+    );
 }
 
 #[test]
 fn function_nil_call() {
-    assert_eq!(eval("((fn () ()))").unwrap(), Value::Nil);
+    assert_eq!(eval(r#"(\_ -> ()) ()"#).unwrap(), Value::Nil);
 }
 
 #[test]
 fn function_arithmetic() {
-    expect_number("((fn (x) (+ x 2)) 2)", 4.0, 0.0001);
+    expect_number(r#"(\x -> x + 2) 2"#, 4.0, 0.0001);
 }
 
 #[test]
 fn function_let() {
-    expect_number("((fn (add-two) (add-two 2)) (fn (x) (+ x 2)))", 4.0, 0.0001);
+    expect_number(r#"(\addTwo -> addTwo 2) \x -> x + 2"#, 4.0, 0.0001);
 }
 
 #[test]
 fn function_closure() {
-    expect_number("(((fn (x) (fn (y) (+ x y))) 2) 2)", 4.0, 0.0001);
+    expect_number(r#"((\x -> \y -> x + y) 2) 2"#, 4.0, 0.0001);
 }
 
 #[test]
 fn if_literal() {
-    expect_number("(if 1 1 2)", 1.0, 0.0001);
-    expect_number("(if () 1 2)", 2.0, 0.0001);
-    expect_number("(if false 1 2)", 2.0, 0.0001);
-    expect_number("(if true 1 2)", 1.0, 0.0001);
+    expect_number("if (1) 1 else 2", 1.0, 0.0001);
+    expect_number("if (()) 1 else 2", 2.0, 0.0001);
+    expect_number("if (False) 1 else 2", 2.0, 0.0001);
+    expect_number("if (True) 1 else 2", 1.0, 0.0001);
 }
 
 #[test]
 fn def_simple() {
     let code = r#"
-        (def x 1)
-        (def y 2)
-        (+ x y)
+        x = 1
+        y = 2
+        x + y
     "#;
     expect_number(code, 3.0, 0.0001);
 }
@@ -137,13 +150,13 @@ fn def_simple() {
 #[test]
 fn def_fib_recursive() {
     let code = r#"
-        (def fib
-            (fn (n)
-                (if (< n 2)
-                    n
-                    (+ (fib (- n 1)) (fib (- n 2))))))
-
-        (fib 10)
+        fib = \n ->
+            if (n < 2)
+                n
+            else
+                fib (n - 1) + fib (n - 2)
+    
+        fib 10
     "#;
     expect_number(code, 55.0, 0.0001);
 }
@@ -151,27 +164,30 @@ fn def_fib_recursive() {
 #[test]
 fn def_mutually_recursive() {
     let code = r#"
-        (def f
-            (fn (x)
-                (if (< x 10)
-                    (g (+ x 1))
-                    x)))
+        f = \x ->
+            if (x < 10)
+                g (x + 1)
+            else
+                x
 
-        (def g
-            (fn (x)
-                (if (< x 10)
-                    (f (* x 2))
-                    x)))
+        g = \x ->
+            if (x < 10)
+                f (x * 2)
+            else
+                x
 
-        (f 0)
+        f 0
     "#;
     expect_number(code, 14.0, 0.0001);
 }
 
 #[test]
 fn def_botsbuildbots() {
-    let result = eval("(def botsbuildbots (fn () (botsbuildbots))) (botsbuildbots)");
-    if let Err(error) = result {
+    let code = r#"
+        botsbuildbots = \_ -> botsbuildbots ()
+        botsbuildbots ()
+    "#;
+    if let Err(error) = eval(code) {
         assert_eq!(
             error.to_string(),
             "Exception {\n    message: \"too much recursion\",\n}"
@@ -184,8 +200,8 @@ fn def_botsbuildbots() {
 #[test]
 fn let_single() {
     let code = r#"
-        (let ((x 1))
-            (+ x 1))
+        let x = 1
+        x + 1
     "#;
     expect_number(code, 2.0, 0.0001);
 }
@@ -193,9 +209,9 @@ fn let_single() {
 #[test]
 fn let_many() {
     let code = r#"
-        (let ((x 1)
-              (y 2))
-            (+ x y))
+        let x = 1
+        let y = 2
+        x + y
     "#;
     expect_number(code, 3.0, 0.0001);
 }
@@ -203,9 +219,9 @@ fn let_many() {
 #[test]
 fn let_sequence() {
     let code = r#"
-        (let ((x 1)
-              (y (+ x 1)))
-            (+ x y))
+        let x = 1
+        let y = x + 1
+        x + y
     "#;
     expect_number(code, 3.0, 0.0001);
 }
@@ -213,59 +229,40 @@ fn let_sequence() {
 #[test]
 fn let_subexpr() {
     let code = r#"
-        (+
-            (let ((x 1)
-                  (y 2))
-                (* x y)))
+        (let x = 1
+         let y = 2
+         x * y) + 2
     "#;
-    expect_number(code, 2.0, 0.0001);
+    expect_number(code, 4.0, 0.0001);
 }
 
 #[test]
-fn let_empty() {
+fn let_subexpr_two() {
     let code = r#"
-        (let () 1)
-    "#;
-    expect_number(code, 1.0, 0.0001);
-}
-
-#[test]
-fn let_subexpr_empty() {
-    let code = r#"
-        (+ (let () 1) (let () 1))
-    "#;
-    expect_number(code, 2.0, 0.0001);
-}
-
-#[test]
-fn let_subexpr_many() {
-    let code = r#"
-        (+
-            (let ((x 1)
-                  (y 2))
-                (* x y))
-            (let () 1)
-            (let ((x 1)) x))
+        (let x = 1
+         2) +
+        (let x = 1
+         x)
     "#;
     expect_number(code, 3.0, 0.0001);
 }
 
 #[test]
-fn system_arithmetic() {
-    expect_number("(+ 1 2 3 4)", 10.0, 0.0001);
-    expect_number("(+ (* 2 1) 1 (/ 6 2) (- 10 3))", 13.0, 0.0001);
+fn let_subexpr_many() {
+    let code = r#"
+        (let x = 1
+         let y = 2
+         x * y) +
+        (let x = 1
+         2) +
+        (let x = 1
+         x)
+    "#;
+    expect_number(code, 5.0, 0.0001);
 }
 
 #[test]
-fn practical_fib_recursive() {
-    let code = r#"
-        ((fn (fib)
-            (fib fib 10))
-
-         (fn (fib n)
-             (if (< n 2)
-                 n
-                 (+ (fib fib (- n 1)) (fib fib (- n 2))))))
-    "#;
-    expect_number(code, 55.0, 0.0001);
+fn system_arithmetic() {
+    expect_number("1 + 2 + 3 + 4", 10.0, 0.0001);
+    expect_number("(2 * 1) + 1 + (6 / 2) + (10 - 3)", 13.0, 0.0001);
 }
diff --git a/crates/rkgk/src/haku.rs b/crates/rkgk/src/haku.rs
index 81b88a9..58a0e90 100644
--- a/crates/rkgk/src/haku.rs
+++ b/crates/rkgk/src/haku.rs
@@ -5,11 +5,15 @@
 
 use eyre::{bail, Context, OptionExt};
 use haku::{
+    ast::Ast,
     bytecode::{Chunk, Defs, DefsImage},
     compiler::{Compiler, Source},
+    lexer::{lex, Lexer},
+    parser::{self, Parser, ParserLimits},
     render::{tiny_skia::Pixmap, Renderer, RendererLimits},
-    sexp::{Ast, Parser, SourceCode},
+    source::SourceCode,
     system::{ChunkId, System, SystemImage},
+    token::Lexis,
     value::{BytecodeLoc, Closure, FunctionName, Ref, Value},
     vm::{Vm, VmImage, VmLimits},
 };
@@ -22,9 +26,11 @@ use crate::schema::Vec2;
 // because we do some dynamic typing magic over on the JavaScript side to automatically call all
 // the appropriate functions for setting these limits on the client side.
 pub struct Limits {
-    pub max_source_code_len: usize,
+    pub max_source_code_len: u32,
     pub max_chunks: usize,
     pub max_defs: usize,
+    pub max_tokens: usize,
+    pub max_parser_events: usize,
     pub ast_capacity: usize,
     pub chunk_capacity: usize,
     pub stack_capacity: usize,
@@ -88,12 +94,21 @@ impl Haku {
     pub fn set_brush(&mut self, code: &str) -> eyre::Result<()> {
         self.reset();
 
-        let ast = Ast::new(self.limits.ast_capacity);
         let code = SourceCode::limited_len(code, self.limits.max_source_code_len)
             .ok_or_eyre("source code is too long")?;
-        let mut parser = Parser::new(ast, code);
-        let root = haku::sexp::parse_toplevel(&mut parser);
-        let ast = parser.ast;
+
+        let mut lexer = Lexer::new(Lexis::new(self.limits.max_tokens), code);
+        lex(&mut lexer)?;
+
+        let mut parser = Parser::new(
+            &lexer.lexis,
+            &ParserLimits {
+                max_events: self.limits.max_parser_events,
+            },
+        );
+        parser::toplevel(&mut parser);
+        let mut ast = Ast::new(self.limits.ast_capacity);
+        let (root, parser_diagnostics) = parser.into_ast(&mut ast)?;
 
         let src = Source {
             code,
@@ -107,7 +122,10 @@ impl Haku {
         haku::compiler::compile_expr(&mut compiler, &src, root)
             .context("failed to compile the chunk")?;
 
-        if !compiler.diagnostics.is_empty() {
+        if !lexer.diagnostics.is_empty()
+            || !parser_diagnostics.is_empty()
+            || !compiler.diagnostics.is_empty()
+        {
             bail!("diagnostics were emitted");
         }
 
diff --git a/rkgk.toml b/rkgk.toml
index 8a0be66..ef0107d 100644
--- a/rkgk.toml
+++ b/rkgk.toml
@@ -61,6 +61,12 @@ max_chunks = 2
 # Maximum amount of defs across all source code chunks.
 max_defs = 256
 
+# Maximum amount of tokens a single chunk can have.
+max_tokens = 4096
+
+# Maximum amount of events that the parser may emit in a single chunk.
+max_parser_events = 4096
+
 # Maximum amount of AST nodes in a single parse.
 ast_capacity = 4096