Created
November 13, 2021 22:50
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Tiny LISP parser and interpreter
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// Parser | |
const string = /"(?:\\(?:["\\\/bfnrt]|u[a-fA-F0-9]{4})|[^"\\\0-\x1F\x7F]+)*"/; | |
const number = /-?(?:0|[1-9][0-9]*)(?:\.[0-9]+)?(?:[eE][+-]?[0-9]+)?/; | |
const identifier = /[^()\s,:"']+/; | |
const combine = (...args) => new RegExp(args.map(arg => `(?:${arg.source})`).join("|"), "g"); | |
const tokenize = input => input.matchAll(combine(/[()]/, identifier, string, number)); | |
const construct = iterator => { | |
const root = []; | |
for (const [token] of iterator) { | |
if (token === ")") return root; | |
if (token === "(") root.push(construct(iterator)); | |
else | |
try { | |
root.push(JSON.parse(token)); | |
} catch { | |
root.push(token); | |
} | |
} | |
return root; | |
}; | |
const parse = input => construct(tokenize(input)); | |
// Interpreter | |
const env = { | |
label: (name, value) => (env[name] = resolve(value, env)), | |
head: ([first]) => first, | |
tail: ([_, ...rest]) => rest, | |
list: (head, tail) => [head, ...tail], | |
equal: (a, b, ctx) => resolve(a, ctx) === resolve(b, ctx), | |
if: (cond, cons, alt, ctx) => (resolve(cond, ctx) ? resolve(cons, ctx) : resolve(alt, ctx)), | |
atomic: expression => !Array.isArray(expression), | |
id: expression => expression, | |
}; | |
function apply(op, args, ctx = this ?? environment()) { | |
return typeof ctx[op] === "function" | |
? ctx[op](...args, ctx) | |
: resolve(ctx[op][2], { | |
...ctx, | |
...ctx[op][1].map((value, index) => [args[index], value]), | |
}); | |
} | |
function resolve(expr, ctx = this ?? environment()) { | |
if (ctx.atomic(expr)) return ctx[expr] ?? expr; | |
const op = env.head(expr); | |
const args = | |
Array.isArray(ctx[op]) || | |
(typeof ctx[op] === "function" && !["label", "if", "id"].includes(op)) | |
? env.tail(expr).map(item => resolve(item, ctx)) | |
: env.tail(expr); | |
return apply(op, args, ctx); | |
} | |
const environment = ext => Object.assign(Object.create(env), ext, { resolve, apply }); | |
// Test | |
const source = ` | |
(if (equal a 3) | |
(id "equal") | |
(id "not equal")) | |
`; | |
console.log(source); | |
const tree = parse(source); | |
console.log(tree); | |
const lisp = environment({ | |
a: 4, | |
}); | |
console.log(lisp); | |
// output = ["not equal"] | |
const output = tree.map(expr => lisp.resolve(expr)); | |
console.log(output); |
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