lisp.html

<script>
  const code = '(print (+ 1 (* 2 3) 3))'

  const parsed = parse(code)
  console.debug('Parsed AST', parsed)
  evaluate(parsed[0])

  function parse(phrase) {
    let seek = 0
    const parsed = []
    let nextToken = ""

    while (seek < phrase.length) {
      const nextChar = phrase[seek]

      if (nextChar == '(') {
        const matchingParen = phrase.length - phrase.split("").reverse().join("").indexOf(')') - 1
        parsed.push(parse(phrase.slice(seek + 1, matchingParen)))
        seek = matchingParen + 1
      } else if (/\s/.test(nextChar)) {
        if (nextToken) parsed.push(nextToken)
        nextToken = ""
      } else {
        nextToken += nextChar
      }

      seek++
    }

    if (nextToken) parsed.push(nextToken)

    return parsed
  }


  function evaluate(ast) {
    const functions = {
      '+': function () { return Array.from(arguments).reduce((a, b) => a + b) },
      '-': function () { return Array.from(arguments).reduce((a, b) => a - b) },
      '*': function () { return Array.from(arguments).reduce((a, b) => a * b) },
      '/': function () { return Array.from(arguments).reduce((a, b) => a / b) },
      'print': console.log,
    }

    const op = functions[ast[0]]
    const args = ast.slice(1).map(x => (x instanceof Array) ? evaluate(x) : parseFloat(x))
    result = op(...args)
    return result
  }

</script>