lisp parser

list_parser.kt

fun parseLisp(input: String): Any? {
    val tokens = tokenize(input).iterator()
    return if (tokens.hasNext()) parse(tokens) else null
}

fun parse(tokens: Iterator<String>): Any? {
    val token = tokens.next()
    return when (token) {
        "(" -> parseList(tokens)
        ")" -> null // This shouldn't happen if the input is well-formed
        else -> token
    }
}

fun parseList(tokens: Iterator<String>): List<Any?> {
    val list = mutableListOf<Any?>()
    while (tokens.hasNext()) {
        val token = tokens.next()
        if (token == ")") {
            return list
        }
        list.add(if (token == "(") parseList(tokens) else token)
    }
    throw IllegalArgumentException("Unmatched parentheses")
}

fun tokenize(exp: String): List<String> =
    exp.replace("(", "( ") // Prep for split
        .replace(")", " )")
        .split(" ")
        .filter { it.isNotEmpty() }

fun main() {
    val lispExp = "(first (list 1 (+ 2 3) 9))"
    println("Input: $lispExp")
    val ast = parseLisp(lispExp)
    println("Output: $ast")
}

Input: (first (list 1 (+ 2 3) 9))
Output: [first, [list, 1, [+, 2, 3], 9]]