haku: add len and index functions for reading lists

crates/haku/src/system.rs

@@ -247,9 +247,8 @@ pub mod fns {
             0x88 Nary "rgbaB" => rgba_b,
             0x89 Nary "rgbaA" => rgba_a,
 
-            // NOTE: Not used right now, has been replaced with Opcode::List.
-            // Keeping it around to reserve a slot for data structure operations.
-            0x90 Nary "list (unused)" => list,
+            0x90 Nary "len" => len,
+            0x91 Nary "index" => index,
 
             0xc0 Nary "toShape" => to_shape_f,
             0xc1 Nary "line" => line,
@@ -552,11 +551,33 @@ pub mod fns {
         Ok(Value::Number(rgba.r))
     }
 
-    pub fn list(vm: &mut Vm, args: FnArgs) -> Result<Value, Exception> {
-        let elements: Vec<_> = (0..args.num()).map(|i| args.get(vm, i)).collect();
-        vm.track_array(&elements)?;
-        let id = vm.create_ref(Ref::List(List { elements }))?;
-        Ok(Value::Ref(id))
+    pub fn len(vm: &mut Vm, args: FnArgs) -> Result<Value, Exception> {
+        if args.num() != 1 {
+            return Err(vm.create_exception("`len` expects a single argument (len list)"));
+        }
+
+        let list = args.get_list(vm, 0, "argument to (len list) must be a list")?;
+        Ok(Value::Number(list.elements.len() as f32))
+    }
+
+    pub fn index(vm: &mut Vm, args: FnArgs) -> Result<Value, Exception> {
+        if args.num() != 2 {
+            return Err(vm.create_exception("`index` expects two arguments (index list i)"));
+        }
+
+        let list = args.get_list(vm, 0, "1st argument to (index list i) must be a list")?;
+        let i = args.get_number(vm, 1, "2nd argument to (index list i) must be a number")?;
+        if i >= 0.0 {
+            let i = i as usize;
+            if i < list.elements.len() {
+                return Ok(list.elements[i]);
+            }
+        }
+        Err(vm.create_exception(format!(
+            "list index out of bounds (length of list is {}, index is {})",
+            list.elements.len(),
+            i
+        )))
     }
 
     fn to_shape(value: Value, vm: &Vm) -> Option<Shape> {

crates/haku/src/vm.rs

@@ -622,6 +622,23 @@ impl FnArgs {
         };
         Ok(closure)
     }
+
+    #[inline(never)]
+    pub fn get_list<'vm>(
+        &self,
+        vm: &'vm Vm,
+        index: usize,
+        message: &'static str,
+    ) -> Result<&'vm List, Exception> {
+        let value = self.get(vm, index);
+        let (_, any_ref) = vm
+            .get_ref_value(value)
+            .ok_or_else(|| vm.create_exception(message))?;
+        let Ref::List(list) = any_ref else {
+            return Err(vm.create_exception(message));
+        };
+        Ok(list)
+    }
 }
 
 #[derive(Debug, Clone, PartialEq, Eq)]

crates/haku/tests/language.rs

@@ -292,3 +292,20 @@ fn with_dotter_identity() {
     "#;
     assert_eq!(eval(code).unwrap(), Value::Ref(RefId::from_u32(2)))
 }
+
+#[test]
+fn system_len() {
+    expect_number("len []", 0.0, 0.0001);
+    expect_number("len [1, 2]", 2.0, 0.0001);
+}
+
+#[test]
+fn system_index() {
+    expect_number("index [1] 0", 1.0, 0.0001);
+    expect_number("index [1, 2] 0", 1.0, 0.0001);
+    expect_number("index [1, 2] 1", 2.0, 0.0001);
+    expect_number("index [1] 0.5", 1.0, 0.0001);
+    expect_number("index [1, 2] 0.5", 1.0, 0.0001);
+    assert!(eval("index [1] (-1)").is_err());
+    assert!(eval("index [1] 1").is_err());
+}