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%% title = "haku - writing a little programming language for fun"
scripts = ["treehouse/vendor/codejar.js", "treehouse/components/literate-programming.js"]
% id = "01J3K8A0D1774SFDPKDK5G9GPV"
- I've had this idea on my mind as of late, of a little pure functional programming language that would run in your browser.
% id = "01J3K8A0D1WTM2KHERFZG2FWBJ"
+ the primary use case would be writing fun audiovisual sketches you can inspect and edit live, because after all everything is declarative.
this was motivated by my discovery of [glisp][], which was recently on the front page of [Lobsters][glisp lobsters].
[glisp]: https://glisp.app
[glisp lobsters]: https://lobste.rs/s/amanh7/glisp_graphical_lisp
% id = "01J3K8A0D16PAM5AV11E8JF3AF"
- [I even commented about it!](https://lobste.rs/s/amanh7/glisp_graphical_lisp#c_oqa6ap)
% id = "01J3K8A0D1N4EGRKPFTP0FNZSW"
- so let's get going!
% id = "01J3K8A0D1ZXQ9NJ8CVGBQ7FZB"
- ### parsing
% id = "01J3K8A0D11KMK6MWCRT5KQV09"
- I don't know about you, but I like writing parsers.
however, since I'm trying to keep this language absolutely _tiny_, I think S-expressions might be the best fit for this purpose.
% id = "01J3K8A0D1PT058QRSXS142Y5T"
- honestly I don't even like S-expressions that much.
I find them extremely hard to read, but I dunno - maybe my mind will change after having written a language using them.
we can always swap the syntax out for something else later.
% id = "01J3K8A0D1198QXV2GFWF7JCV0"
- let me show you an example of how I'd like haku to look.
I find that is the best way of breaking down syntax into smaller parts.
```haku
; Recursive fibonacci
(def fib
(fn (n)
(if (< n 2)
n
(+ (fib (- n 1)) (fib (- n 2))))))
(print (fib 10))
```
% id = "01J3K8A0D1KNHJ10WCVX8C88WP"
- we have a handful of lexical elements: parentheses, identifiers, and numbers.
there are also comments and whitespace, of course.
those will get skipped over by the lexer, because we're not really building a production-grade language to need them.
% id = "01J3K8A0D14Z8W5K6KDEJQ6DZJ"
- syntactically, we only really have two types of productions.
there are literals, and there are lists.
% id = "01J3K8A0D1N8SP9J8EMBNEVG9C"
- when I say _literals_, I'm referring to both identifiers and integers.
we will of course differentiate between them in the syntax, because they mean different things.
% id = "01J3K8A0D14A94S2RNFV97DX18"
- we will start by writing the lexical analysis part of our parser, to join single characters up to slightly more managable pieces.
{:program=haku}
```javascript
export const lexer = {};
```
% id = "01J3K8A0D1YZMHNSRZMSBNQVD4"
- the entire idea of a lexer is that you read the input string left to right, top to bottom, and piece together larger _tokens_ out of that.
% id = "01J3K8A0D1C9YBXWK257GFMR68"
- for instance, for the input string
```haku
(example s-expression)
```
we will produce the tokens
| type | start | end | text |
| --- | --: | --: | --- |
| ( | 0 | 1 | `(` |
| identifier | 1 | 8 | `example` |
| identifier | 9 | 21 | `s-expression` |
| ) | 21 | 22 | `)` |
| end of file | 22 | 22 | |
% id = "01J3K8A0D1GGQ292D4MQBCGHWC"
- to lex the input into tokens, we'll need to know the input string (of course), and where we currently are in the string.
{:program=haku}
```javascript
lexer.init = (input) => {
return {
input,
position: 0,
};
};
```
% id = "01J3K8A0D139JN9J5TTA2WAP4R"
- we'll also define a few helper functions to make reading text a little easier, without having to perform any bounds checks whenever we read tokens.
{:program=haku}
```javascript
export const eof = "end of file";
lexer.current = (state) => {
return state.position < state.input.length
? state.input.charAt(state.position)
: eof;
};
lexer.advance = (state) => ++state.position;
```
% id = "01J3K8A0D1GPMDD8S063K6ETM3"
- our lexer will run in a loop, producing tokens until it hits the end of input or an error.
{:program=haku}
```javascript
export function lex(input) {
let tokens = [];
let state = lexer.init(input);
while (true) {
let start = state.position;
let kind = lexer.nextToken(state);
let end = state.position;
tokens.push({ kind, start, end });
if (kind == eof || kind == "error") break;
}
return tokens;
}
```
% id = "01J3K8A0D10GZMN36TDZWYH632"
- remember that error handling is important!
we mustn't forget that the user can produce invalid input - such as this string:
```haku
{example}
```
haku does not have curly braces in its syntax, so that's clearly an error!
reporting this to the user will be a much better experience than, perhaps... getting stuck in an infinite loop. :oh:
% id = "01J3K8A0D117B6AQ8YKMCX4KAK"
- now for the most important part - that `lexer.nextToken` we used will be responsible for reading back a token from the input, and returning what kind of token it has read.
for now, let's make it detect parentheses.
we of course also need to handle end of input - whenever our lexer runs out of characters to consume, as well as when it encounters any characters we don't expect.
{:program=haku}
```javascript
lexer.nextToken = (state) => {
let c = lexer.current(state);
if (c == "(" || c == ")") {
lexer.advance(state);
return c;
}
if (c == eof) return eof;
lexer.advance(state);
return "error";
};
```
% id = "01J3K8A0D1C5C5P32WQFW1PD0R"
- with all that frameworking in place, let's test if our lexer works!
{:program=haku}
```javascript
export function printTokens(input) {
let tokens = lex(input);
for (let { kind, start, end } of tokens) {
if (kind == "error") {
let errorString = input.substring(start, end);
console.log(`unexpected characters at ${start}..${end}: '${errorString}'`);
} else {
console.log(`${kind} @ ${start}..${end}`);
}
}
}
printTokens(`()((()))`);
```
{:program=haku}
```output
( @ 0..1
) @ 1..2
( @ 2..3
( @ 3..4
( @ 4..5
) @ 5..6
) @ 6..7
) @ 7..8
end of file @ 8..8
```
...seems pretty perfect!
% id = "01J3K8A0D1AV280QZ0Y10CPN62"
- except, of course, we're not handling whitespace or comments.
{:program=haku}
```javascript
printTokens(`( )`);
```
{:program=haku}
```output
( @ 0..1
unexpected characters at 1..2: ' '
```
% id = "01J3K8A0D1RHK349974Y23DG56"
- so let's write another function that will lex those.
{:program=haku}
```javascript
lexer.skipWhitespaceAndComments = (state) => {
while (true) {
let c = lexer.current(state);
if (c == " " || c == "\t" || c == "\n" || c == "\r") {
lexer.advance(state);
continue;
}
if (c == ";") {
while (
lexer.current(state) != "\n" &&
lexer.current(state) != eof
) {
lexer.advance(state);
}
lexer.advance(state); // skip over newline, too
continue;
}
break;
}
};
```
% id = "01J3K8A0D10F11DPN5TN0Y7AAX"
- except instead of looking at whitespace and comments in the main token reading function, we'll do that _outside_ of it, to avoid getting whitespace caught up in the actual tokens' `start`..`end` spans.
{:program=haku}
```javascript
export function lex(input) {
let tokens = [];
let state = lexer.init(input);
while (true) {
lexer.skipWhitespaceAndComments(state); // <--
let start = state.position;
let kind = lexer.nextToken(state);
let end = state.position;
tokens.push({ kind, start, end });
if (kind == eof || kind == "error") break;
}
return tokens;
}
```
% id = "01J3K8A0D1AQWFJHSC9XCCKNKF"
- now if we look at the output...
{:program=haku}
```javascript
printTokens(`( )`);
```
{:program=haku}
```output
( @ 0..1
) @ 2..3
end of file @ 3..3
```
the whitespace is ignored just fine!
% id = "01J3K8A0D1S7MCHYYVYMPWEHEF"
- and comments of course follow:
{:program=haku}
```javascript
printTokens(`
( ; comment comment!
)
`);
```
{:program=haku}
```output
( @ 5..6
) @ 30..31
end of file @ 32..32
```
% id = "01J3K8A0D16NF69K3MNNYH1VJ1"
- it'd be really nice if we could use identifiers though...
{:program=haku}
```javascript
printTokens(`(hello world)`);
```
{:program=haku}
```output
( @ 0..1
unexpected characters at 1..2: 'h'
```
so I guess that's the next thing on our TODO list!
% id = "01J3K8A0D1SF46M2E7DEP6V44N"
- we'll introduce a function that will tell us if a given character is a valid character in an identifier.
since S-expressions are so minimal, it is typical to allow all sorts of characters in identifiers -
in our case, we'll allow alphanumerics, as well as a bunch of symbols that seem useful.
and funky!
{:program=haku}
```javascript
export const isIdentifier = (c) =>
/^[a-zA-Z0-9+~!@$%^&*=<>+?/.,:\\|-]$/.test(c);
```
% id = "01J3K8A0D10TTSM7TV0C05PVNJ"
- this could probably be a whole lot faster if I had used a simple `c >= 'a' && c <= 'z'` chain, but I'm lazy, so a regex it is.
% id = "01J3K8A0D16VA5D4JGT26YZ4KP"
- when I said funky, I wasn't joking - have you ever seen `,` in an identifier?
% id = "01J3K8A0D11GYDHXVZJXVWAGHN"
- I'm allowing it since it isn't really gonna hurt anything.
I _did_ disallow `#` though, because that's commonly used for various extensions.
who knows what I might be able to cram under that symbol!
% id = "01J3K8A0D17S0FTBXHP36VVP8C"
- with a character set established, we can now stuff identifiers into our lexer.
I'll start by introducing a function that'll chew as many characters that meet a given condition as it can:
{:program=haku}
```javascript
lexer.advanceWhile = (state, fn) => {
while (fn(lexer.current(state))) {
lexer.advance(state);
}
};
```
% id = "01J3K8A0D1YV77A2TR64R74HRD"
- now we can add identifiers to `nextToken`:
{:program=haku}
```javascript
lexer.nextToken = (state) => {
let c = lexer.current(state);
if (isIdentifier(c)) {
lexer.advanceWhile(state, isIdentifier);
return "identifier";
}
if (c == "(" || c == ")") {
lexer.advance(state);
return c;
}
if (c == eof) return eof;
lexer.advance(state);
return "error";
};
```
% id = "01J3K8A0D1DKA8YCBCJVZXXGR4"
- let's try lexing that `(hello world)` string now.
{:program=haku}
```javascript
printTokens(`(hello world)`);
```
{:program=haku}
```output
( @ 0..1
identifier @ 1..6
identifier @ 7..12
) @ 12..13
end of file @ 13..13
```
nice!
% id = "01J3K8A0D15G77YG2A0CN8P0M6"
- in the original example, there were also a couple of numbers:
```haku
(+ (fib (- n 1)) (fib (- n 2)))
```
so let's also add support for some basic integers; we'll add decimals later if we ever need them.
% id = "01J3K8A0D18MA59WFYW7PCPQ30"
- defining integers is going to be a similar errand to identifiers, so I'll spare you the details and just dump all the code at you:
{:program=haku}
```javascript
export const isDigit = (c) => c >= "0" && c <= "9";
lexer.nextToken = (state) => {
let c = lexer.current(state);
if (isDigit(c)) {
lexer.advanceWhile(state, isDigit);
return "integer";
}
if (isIdentifier(c)) {
lexer.advanceWhile(state, isIdentifier);
return "identifier";
}
if (c == "(" || c == ")") {
lexer.advance(state);
return c;
}
if (c == eof) return eof;
lexer.advance(state);
return "error";
};
```
% id = "01J3K8A0D1SZ4YSR1KD2HYAWPV"
- note how we check `isDigit` _before_ `isIdentifier` -
this is really important, because otherwise identifiers would take precedence over integers!
% id = "01J3K8A0D1B5J858DJ6BKNJRKT"
- now let's see the results of all that hard work.
{:program=haku}
```javascript
printTokens(`(fib (- n 1))`);
```
{:program=haku}
```output
( @ 0..1
identifier @ 1..4
( @ 5..6
identifier @ 6..7
identifier @ 8..9
integer @ 10..11
) @ 11..12
) @ 12..13
end of file @ 13..13
```
looks good!
% id = "01J3K8A0D148R9B0HVMH79A3CK"
- #### an amen break
% id = "01J3K8A0D1WX6EH5H61BVR1X31"
- to let your head rest a bit after reading all of this, here are some fun numbers:
% id = "01J3K8A0D11D479PJKY22AQFTC"
- there are a total of
{:program=haku}
```javascript
console.log(Object.keys(lexer).length);
```
{:program=haku}
```output
6
```
functions in the `lexer` namespace.
not a whole lot, huh?
% id = "01J3K8A0D19XK0MRH4Z461G2J0"
- I was personally quite surprised how tiny an S-expression lexer can be.
they were right about S-expressions being a good alternative for when you don't want to write syntax!
the entire thing fits in *86 lines of code.*
% id = "01J3K8A0D1CG89X84KM2DN14ZT"
+ :bulb: for the curious: *here's why I implement lexers like this!*
% id = "01J3K8A0D1FYBKJ6X2W17QAK3Z"
- many tutorials will have you implementing lexers such that data is _parsed_ into the language's data types.
for instance, integer tokens would be parsed into JavaScript `number`s.
I don't like this approach for a couple reasons.
% id = "01J3K8A0D1P258JKRVG11M7B64"
- pre-parsing data like this pollutes your lexer code with wrangling tokens into useful data types.
I prefer it if the lexer is only responsible for _reading back strings_.
implemented my way, it can concern itself only with chewing through the source string; no need to extract substrings out of the input or anything.
% id = "01J3K8A0D14VZTKBPJTG3BGD0M"
- there's also a performance boost from implementing it this way: _lazy_ parsing, as I like to call it, allows us to defer most of the parsing work until it's actually needed.
if the token never ends up being needed (e.g. due to a syntax error,) we don't end up doing extra work eagerly!
% id = "01J3K8A0D1GYZ9Y9MK6K24JME7"
- if that doesn't convince you, consider that now all your tokens are the exact same data structure, and you can pack them neatly into a flat array.
if you're using a programming language with flat arrays, that is.
such as Rust or C.
I'm implementing this in JavaScript of course, but it's still neat not having to deal with mass `if`osis when extracting data from tokens - you're always guaranteed a token will have a `kind`, `start`, and `end`.
% id = "01J3K8A0D1NTPSD77WM84KVMRX"
- now. back to your regularly scheduled programming!
% id = "01J3K8A0D1X6A68K6TGX00FCTE"
- it's time for us to implement a parser for our S-expressions.
{:program=haku}
```javascript
export const parser = {};
```
% id = "01J3K8A0D1ZMJJHDMW24D1GESE"
- the goal is to go from this flat list of tokens:
| type | start | end | text |
| --- | --: | --: | --- |
| ( | 0 | 1 | `(` |
| identifier | 1 | 8 | `example` |
| identifier | 9 | 21 | `s-expression` |
| ) | 21 | 22 | `)` |
| end of file | 22 | 22 | |
to a nice recursive tree that represents our S-expressions:
```haku.ast
list
identifier example
identifier s-expression
```
% id = "01J3K8A0D1SSWPAKSNG8TA4N1H"
- there are many parsing strategies we could go with, but in my experience you can't go simpler than good ol' [recursive descent][].
[recursive descent]: https://en.wikipedia.org/wiki/Recursive_descent_parser
% id = "01J3K8A0D1NHD7QGQ1NZTDQRWX"
- the idea of recursive descent is that you have a stream of tokens that you read from left to right, and you have a set of functions that parse your non-terminals.
essentially, each function corresponds to a single type of node in your syntax tree.
% id = "01J3K8A0D1F01CKXP10M7WD6VV"
- does the "stream of tokens that you read from left to right" ring a bell?
if it does, that's because lexing operates on a _very_ similar process - it's just non-recursive!
% id = "01J3K8A0D111A22X9WW8NP3T3X"
- knowing that similarity, we'll start off with a similar set of helper functions to our lexer.
{:program=haku}
```javascript
parser.init = (tokens) => {
return {
tokens,
position: 0,
};
};
parser.current = (state) => state.tokens[state.position];
parser.advance = (state) => {
if (state.position < state.tokens.length - 1) {
++state.position;
}
};
```
note however that instead of letting `current` read out of bounds, we instead clamp `advance` to the very last token - which is guaranteed to be `end of file`.
% id = "01J3K8A0D1XF9PEBQ6D4F1P3BA"
- the S-expression grammar can compose in the following ways:
% id = "01J3K8A0D1CWFBC9JTM6PFZRR8"
- an S-expression is a literal integer, identifier, or a list.
% id = "01J3K8A0D1BM9QGDHWCX7PANPR"
- literal integers `65` and identifiers `owo` stand alone on their own.
they do not nest anything else inside of them.
% id = "01J3K8A0D19BXABXNV75N93A18"
- lists `(a b c)` are sequences of S-expressions enclosed in parentheses.
inside, they can contain literal integers and identifiers, or even other lists recursively.
% id = "01J3K8A0D1G43KZDVH7EW0ZAKQ"
- this yields the following [EBNF][] grammar:
```ebnf
Expr = "integer" | "identifier" | List;
List = "(" , { Expr } , ")";
```
[EBNF]: https://en.wikipedia.org/wiki/Extended_Backus%E2%80%93Naur_form
% id = "01J3K8A0D1FPZE52S1RVWCR66Y"
- we'll start by implementing the `Expr = "integer" | "identifier"` rule.
parsing integers and identifiers is as simple as reading their single token, and returning a node for it:
{:program=haku}
```javascript
parser.parseExpr = (state) => {
let token = parser.current(state);
switch (token.kind) {
case "integer":
case "identifier":
parser.advance(state);
return { ...token };
default:
parser.advance(state);
return {
kind: "error",
error: "unexpected token",
start: token.start,
end: token.end,
};
}
};
```
% id = "01J3K8A0D1ENMQV0ZSP8C5ZX5A"
- of course again, we mustn't forget about errors!
it's totally possible for our lexer to produce a token we don't understand - such as an `error`, or an `end of file`.
or really any token we choose to introduce in the future, but choose to not be valid as an `Expr` starter.
% id = "01J3K8A0D1QRSPTYPH2JQ77HW9"
+ we'll wrap initialization and `parseExpr` in another function, which will accept a list of tokens and return a syntax tree, hiding the complexity of managing the parser state underneath.
{:program=haku}
```javascript
parser.parseRoot = (state) => parser.parseExpr(state);
export function parse(input) {
let state = parser.init(input);
let expr = parser.parseRoot(state);
if (parser.current(state).kind != eof) {
let strayToken = parser.current(state);
return {
kind: "error",
error: `found stray '${strayToken.kind}' token after expression`,
start: strayToken.start,
end: strayToken.end,
};
}
return expr;
}
```
this function also checks that there aren't any tokens after we're done parsing the root `Expr` production.
it wouldn't be very nice UX if we let the user input tokens that didn't do anything!
% id = "01J3K8A0D1KE4JRKEXWPAQJFDV"
- I'm adding that `parseRoot` alias in so that it's easy to swap the root production to something else than `Expr`.
% id = "01J3K8A0D1GP31XPC0VVZTJPMV"
- now we can try to parse a tree out of a little expression...
{:program=haku}
```javascript
export function printTree(input) {
let tokens = lex(input);
let tree = parse(tokens);
console.log(JSON.stringify(tree, null, " "));
}
```
...and print it into the console:
{:program=haku}
```javascript
printTree("-w-")
```
{:program=haku}
```output
{
"kind": "identifier",
"start": 0,
"end": 3
}
```
nice!
% id = "01J3K8A0D14YEA038BD8KAAECC"
- now it's time to parse some lists.
for that, we'll introduce another function, which will be called by `parseExpr` with an existing `(` token.
its task will be to read as many expressions as it can, until it hits a closing parenthesis `)`, and then construct a node out of that.
{:program=haku}
```javascript
parser.parseList = (state, leftParen) => {
parser.advance(state);
let children = [];
while (parser.current(state).kind != ")") {
if (parser.current(state).kind == eof) {
return {
kind: "error",
error: "missing closing parenthesis ')'",
start: leftParen.start,
end: leftParen.end,
};
}
children.push(parser.parseExpr(state));
}
let rightParen = parser.current(state);
parser.advance(state);
return {
kind: "list",
children,
start: leftParen.start,
end: rightParen.end,
};
};
```
% id = "01J3K8A0D1YZ93B7X3A14X1W0N"
- and the last thing left to do is to hook it up to our `parseExpr`, in response to a `(` token:
{:program=haku}
```javascript
parser.parseExpr = (state) => {
let token = parser.current(state);
switch (token.kind) {
case "integer":
case "identifier":
parser.advance(state);
return { ...token };
case "(":
return parser.parseList(state, token); // <--
default:
parser.advance(state);
return {
kind: "error",
error: "unexpected token",
start: token.start,
end: token.end,
};
}
};
```
% id = "01J3K8A0D1RHWQAA9FMDC654S9"
- now let's try parsing an S-expression!
{:program=haku}
```javascript
printTree("(hello! ^^ (nested nest))");
```
{:program=haku}
```output
{
"kind": "list",
"children": [
{
"kind": "identifier",
"start": 1,
"end": 7
},
{
"kind": "identifier",
"start": 8,
"end": 10
},
{
"kind": "list",
"children": [
{
"kind": "identifier",
"start": 12,
"end": 18
},
{
"kind": "identifier",
"start": 19,
"end": 23
}
],
"start": 11,
"end": 24
}
],
"start": 0,
"end": 25
}
```
% id = "01J3K8A0D1AJP9WHVKBBKKC3B7"
- I don't know about you, but I personally find the JSON output quite distracting and long.
I can't imagine how long it'll be on even larger expressions!
to counteract that, let's write an S-expression pretty printer:
{:program=haku}
```javascript
export function exprToString(expr, input) {
let inputSubstring = input.substring(expr.start, expr.end);
switch (expr.kind) {
case "integer":
case "identifier":
return inputSubstring;
case "list":
return `(${expr.children.map((expr) => exprToString(expr, input)).join(" ")})`;
case "error":
return `<error ${expr.start}..${expr.end} '${inputSubstring}': ${expr.error}>`;
}
}
```
% id = "01J3K8A0D1CB6B8BEY65ADJZSV"
- obviously this loses some information compared to the JSON - we no longer report start and end indices, but that is easy enough to add if you need it.
I don't need it, so I'll conveniently skip it for now.
% id = "01J3K8A0D1G1BPN5W4GT26EJX4"
- let's see if our pretty printer works!
{:program=haku}
```javascript
export function printTree(input) {
let tokens = lex(input);
let tree = parse(tokens);
console.log(exprToString(tree, input));
}
printTree("(hello! -w- (nestedy nest))");
```
{:program=haku}
```output
(hello! -w- (nestedy nest))
```
that's... the same string.
% id = "01J3K8A0D1XP4FQB2HZR9GV5CJ"
- let's try something more complicated, with comments and such.
{:program=haku}
```javascript
export function printTree(input) {
let tokens = lex(input);
let tree = parse(tokens);
console.log(exprToString(tree, input));
}
printTree(`
(def add-two
; Add two to a number.
(fn (n) (+ n 2)))
`);
```
{:program=haku}
```output
(def add-two (fn (n) (+ n 2)))
```
looks like it works!
% id = "01J3K8A0D10DRSP49WF8YH5WSH"
- of course this is hardly the _prettiest_ printer in the world.
% id = "01J3K8A0D1VCJ7TV6CN7M07N5J"
- for one, it does not even preserve your comments.
% id = "01J3K8A0D1K3M9223YM96PS68B"
- it does not add indentation either, it just blindly dumps a minimal S-expression into the console.
% id = "01J3K8A0D1P2EF0C657J1REV9Z"
- but it proves that our parser _works_ - we're able to parse an arbitrary S-expression into a syntax tree, and then traverse that syntax tree again, performing various recursive algorithms on it.
isn't that cool?
% id = "01J3K8A0D1PB6MSPBS1K6K6KR3"
- and that's all there'll be to parsing, at least for now!
% id = "01J3K8A0D11M0NJCBKKPAMVJ2J"
- maybe in the future I'll come up with something more complex, with a more human-friendly syntax.
who knows!
right now it's experimentation time, so these things don't really matter.
% id = "01J3K8A0D1HB566XYSET099Q26"
- #### amen break, part two
% id = "01J3K8A0D1KX5EWV5NW29PF525"
- the S-expression parser consists of a whopping
{:program=haku}
```javascript
console.log(Object.keys(parser).length);
```
{:program=haku}
```output
6
```
functions.
just like the lexer!
% id = "01J3K8A0D1RZE0F75S2C7PPTAZ"
- the parser is *99 lines of code*. quite tiny, if you ask me!
% id = "01J3K8A0D1K91SY17T780S7MPK"
- together with the lexer, the entire S-expression parser is *185 lines of JavaScript.*
that's a pretty small amount, especially given that it's extremely simple code!
% id = "01J3K8A0D1PJNDGKJH8DXN4G3G"
- I wouldn't call this parser production-ready, though.
a production-ready parser would have some way of _preserving comments_ inside the syntax tree, such that you can pretty-print it losslessly.
if you're bored, you can try to add that in!
% id = "01J3K8A0D1PJQJFAG2YADEKVNB"
+ here's a fun piece of trivia: I'm wrote a [Nim S-expression parser for Rosetta Code][nim s-expr] way back in [July 2019][nim s-expr diff].
[nim s-expr]: https://rosettacode.org/wiki/S-expressions#Nim
[nim s-expr diff]: https://rosettacode.org/wiki/S-expressions?diff=prev&oldid=202824
% id = "01J3K8A0D1BWG3TFFXDD6BCPP2"
- you can see it's quite different from how I wrote this parser - in particular, because I didn't need to focus so much on the parser being hot-patchable and reusable, it came out quite a lot more compact, despite having fully static types!
% id = "01J3K8A0D1F4R8KPHETV9N08YP"
- it's definitely not how I would write a parser nowadays.
it's pretty similar, but the syntax tree structures are quite different - it doesn't use the [lazy parsing][branch:01J3K8A0D1FYBKJ6X2W17QAK3Z] trick I talked about before.
% id = "01J3K8A0D178J6W49AFCE9HEQ6"
- I mean, it's only a trick I learned last year!
% id = "01J3K8A0D12VCHW6AJX0ZGPQBY"
- code style-wise it's also not my prettiest Nim code ever - it kind of abuses `template`s for referring to the current character with a single word, but that doesn't convey the fact that it's an effectful operation very well.
- ### interpretation
- with a parser now ready, it would be nice if we could execute some actual code!
- we'll again start off by setting a goal.
I want to be able to evaluate arbitrary arithmetic expressions, like this one:
```haku
(+ (* 2 1) 1 (/ 6 2) (- 10 3))
```
- the simplest way to get some code up and running would be to write a _tree-walk interpreter_.
{:program=haku}
```javascript
export const treewalk = {};
```
this kind of interpreter is actually really simple!
it just involves walking through your syntax tree, executing each node one by one.
- we'll again start off by defining a function that initializes our interpreter's state.
right now there isn't really anything to initialize, but recall that we don't have our tokens parsed into any meaningful data yet, so we'll have to have access the source string to do that.
{:program=haku}
```javascript
treewalk.init = (input) => {
return { input };
};
```
- the core of our interpretation will be a function that descends down the node tree and _evaluates_ each node, giving us a result.
{:program=haku}
```javascript
treewalk.eval = (state, node) => {
switch (node.kind) {
default:
throw new Error(`unhandled node kind: ${node.kind}`);
}
};
```
for now we'll leave it empty.
- in the meantime, let's prepare a couple convenient little wrappers to run our code:
{:program=haku}
```javascript
export function run(input, node) {
let state = treewalk.init(input);
return treewalk.eval(state, node);
}
export function printEvalResult(input) {
try {
let tokens = lex(input);
let ast = parse(tokens);
let result = run(input, ast);
console.log(result);
} catch (error) {
console.log(error.toString());
}
}
```
- now we can try running some code!
let's see what happens.
{:program=haku}
```javascript
printEvalResult("65");
```
{:program=haku}
```output
Error: unhandled node kind: integer
```
...of course.
- so let's patch those integers in!
this is where we'll need that source string of ours - we don't actually have a JavaScript `number` representation of the integers, so we'll need to parse them into place.
{:program=haku}
```javascript
treewalk.eval = (state, node) => {
switch (node.kind) {
case "integer":
let sourceString = state.input.substring(node.start, node.end);
return parseInt(sourceString);
default:
throw new Error(`unhandled node kind: ${node.kind}`);
}
};
```
- now when we run the program above...
{:program=haku}
```javascript
printEvalResult("65");
```
{:program=haku}
```output
65
```
we get sixty five!
- but that's of course a bit boring - it would be nice if we could like, y'know, _perform some arithmetic_.
- traditionally, in Lisp-like languages, a list expression always represents a function application, with the head of the list being the function to call, and the tail of the function being the arguments to apply to the function.
let's implement that logic then!
{:program=haku}
```javascript
export const builtins = {};
treewalk.eval = (state, node) => {
switch (node.kind) {
case "integer":
let sourceString = state.input.substring(node.start, node.end);
return parseInt(sourceString);
case "list": // <--
let functionToCall = node.children[0];
let builtin = builtins[state.input.substring(functionToCall.start, functionToCall.end)];
return builtin(state, node);
default:
throw new Error(`unhandled node kind: ${node.kind}`);
}
};
```
- we'm putting all of our built-in magic functions into a separate object `builtins`, so that they're easy to patch partially later.
you've seen my tricks already with hot-patching functions in objects, so this shouldn't be too surprising.
+ you'll note I'm kind of cheating here - because we have no mechanism to represent variables just yet, I'm using the node's text as the key to our `builtins` table.
- heck, I'm not even validating that this is an identifier - so you can technically do something like this, too:
```haku
((what the fuck) lol)
```
which will call the builtin named `(what the fuck)`.
- we could try this out now, except we don't actually have any builtins! so I'll add a few in, so that we can _finally_ perform our glorious arithmetic:
{:program=haku}
```javascript
function arithmeticBuiltin(op) {
return (state, node) => {
let result = treewalk.eval(state, node.children[1]);
for (let i = 2; i < node.children.length; ++i) {
result = op(result, treewalk.eval(state, node.children[i]));
}
return result;
};
}
builtins["+"] = arithmeticBuiltin((a, b) => a + b);
builtins["-"] = arithmeticBuiltin((a, b) => a - b);
builtins["*"] = arithmeticBuiltin((a, b) => a * b);
builtins["/"] = arithmeticBuiltin((a, b) => a / b);
```
- one thing of note is how `arithmeticBuiltin` accepts two or more arguments.
you're free to pass in more than that, which is common among Lisps.
- now let's try running our full arithmetic expression! drum roll please...
{:program=haku}
```javascript
printEvalResult("(+ (* 2 1) 1 (/ 6 2) (- 10 3))");
```
{:program=haku}
```output
13
```
- #### a brief intermission
- I will now pause here to say, I'm kind of tired of writing this `printEvalResult` ceremony over and over again.
so I took a bit of time to enhance the treehouse's capabilities, and it's now capable of running languages other than JavaScript!
- all we have to do is swap out the evaluation [kernel][]{title="like in Jupyter! Jupyter kernels are basically just support for different programming languages" style="cursor: help; text-decoration: 1px dotted underline;"}...
[kernel]: https://docs.jupyter.org/en/latest/projects/kernels.html
{:program=haku}
```javascript
import { getKernel } from "treehouse/components/literate-programming/eval.js";
let kernel = getKernel();
export const defaultKernelInit = kernel.init;
kernel.init = () => {
return defaultKernelInit();
};
export const defaultKernelEvalModule = kernel.evalModule;
kernel.evalModule = async (state, source, language, params) => {
if (language == "haku") {
printEvalResult(source);
return true;
} else {
return await defaultKernelEvalModule(state, source, language, params);
}
};
```
- and now we can write haku in code blocks!
{:program=haku}
```haku
(+ (* 2 1) 1 (/ 6 2) (- 10 3))
```
{:program=haku}
```output
13
```
% stage = "Draft"
id = "01J3K8A0D1D0NTT3JYYFMRYVSC"
- ### tests
% id = "01J3K8A0D1DQZCZSX4H82QQBHR"
- parser
{:program=test-parser}
```javascript
import { lex, parse, exprToString } from "haku/sexp.js";
let input = "(example s-expression)";
let tokens = lex(input);
tokens.forEach(token => console.log(`${token.kind} ${token.start}..${token.end} '${input.substring(token.start, token.end)}'`));
let ast = parse(tokens);
console.log(exprToString(ast, input));
```
{:program=test-parser}
```output
( 0..1 '('
identifier 1..8 'example'
identifier 9..21 's-expression'
) 21..22 ')'
end of file 22..22 ''
(example s-expression)
```
- treewalk
{:program=test-treewalk}
```javascript
import { lex, parse, exprToString } from "haku/sexp.js";
import { run } from "haku/treewalk.js";
let input = "(+ (* 2 1) 1 (/ 6 2) (- 10 3))";
let tokens = lex(input);
let ast = parse(tokens);
console.log(run(input, ast));
```
{:program=test-treewalk}
```output
```
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