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a = 1 + 2
10 + a

object SampleParser: RegexParsers<Expr>() {
    val number = literal("[0-9]+".toRegex())
    val identifier = literal("[a-z]+".toRegex())

    val variable = identifier map { Var(it) }
    val value = number map { Value(it.toInt()) }

    val atom = oneOf(variable, value)
    val expr = atom and some(literal("+") and atom) map {
        it.second.fold(it.first) { left, (_, right) -> Add(left, right) }
    }
    val assign = identifier and literal("=") and expr map { Assign(it.first.first, it.second) }
    val stmt = oneOf(assign, expr)

    override val goal = stmt
}

> a = 2
result = 2

> a + 2
Failed to parse expression!
ERROR: Expected: '=' (1:3)

a + 2
  ^

sealed class Expr {
    abstract fun evaluate(ctx: CalcContext): Int
}

class Value(private val value: Int): Expr() {
    override fun evaluate(ctx: CalcContext)= value
}

class Var(private val varName: String): Expr() {
    override fun evaluate(ctx: CalcContext)= ctx.getValue(varName)
}

class Add(private val left: Expr, private val right: Expr): Expr() {
    override fun evaluate(ctx: CalcContext) = left.evaluate(ctx) + right.evaluate(ctx)
}

class Assign(private val varName: String, private val expr: Expr): Expr() {
    override fun evaluate(ctx: CalcContext): Int {
        val value = expr.evaluate(ctx);
        ctx.setValue(varName, value)
        return value
    }
}