WhimsyTest.kt

import norswap.autumn.Grammar
import norswap.autumn.Parser
import norswap.autumn.parsers.*

interface Expression
data class Number(val value: Int): Expression
data class Add(val left: Expression, val right: Expression): Expression
data class Sub(val left: Expression, val right: Expression): Expression

/*
Equivalent TatSu Grammar

@@left_recursion :: True

start = expression $;
number = /\d+/;

addition = expression '+' number;
subtraction = expression '-' number;

expression =
    | addition
    | subtraction
    | number;
 */
class CalcGrammar: Grammar() {
    fun number() = build_str (
        syntax = { repeat1 { char_range('0', '9') } },
        value = { Number(it.toInt()) }
    )

    fun addition(expr: Parser) = build (
        syntax = { seq { expr() && string("+") && number() } },
        effect = { Add(it(0), it(1)) }
    )

    fun subtraction(expr: Parser) = build (
        syntax = { seq { expr() && string("-") && number() } },
        effect = { Sub(it(0), it(1)) }
    ) 

    val expression = leftrec { self ->
        choice {
            addition(self) || subtraction(self) || number()
        }
    }

    override fun root() = expression()
}

fun diagnose (grammar: Grammar, input: String) {
    if (grammar.parse(input)) println("success: " + grammar.stack)
    else println("failure: " + grammar.failure?.invoke())
    grammar.reset()
}


fun main (args: Array<String>) {
    val grammar = CalcGrammar()
    diagnose(grammar, "1+1-1")
    diagnose(grammar, "1-1+1")
}

Output:

success: [Sub(left=Add(left=Number(value=1), right=Number(value=1)), right=Number(value=1))]
success: [Add(left=Sub(left=Number(value=1), right=Number(value=1)), right=Number(value=1))]