ModellingErrorsWithMap.kt

sealed class MathExpression {
    fun eval(): Result = when (this) {
        is Addition -> left.eval().flatMap { ll -> right.eval().map { rr -> ll + rr } }
        is Subtraction -> left.eval().flatMap { ll -> right.eval().map { rr -> ll - rr } }
        is Division -> right.eval().flatMap { rr -> if (rr == 0)  Failure("Division by zero") else left.eval().map { ll -> ll/rr }}
        is Number -> Success(value)
    }
}