ModelingErrorsWithTypes.kt

sealed class MathExpression {
    fun eval(): Result = when (this) {

        is Addition -> {
            val resultL = left.eval()
            when (resultL) {
                is Failure -> resultL
                is Success -> {
                    val resultR = right.eval()
                    when (resultR) {
                        is Failure -> resultR
                        is Success -> Success(resultL.value + resultR.value)
                    }
                }
            }
        }

        is Subtraction -> {
            val resultL = left.eval()
            when (resultL) {
                is Failure -> resultL
                is Success -> {
                    val resultR = right.eval()
                    when (resultR) {
                        is Failure -> resultR
                        is Success -> Success(resultL.value - resultR.value)
                    }
                }
            }
        }

        is Division -> {
            val resultL = left.eval()
            when (resultL) {
                is Failure -> resultL
                is Success -> {
                    val resultR = right.eval()
                    when (resultR) {
                        is Failure -> resultR
                        is Success -> if (resultR.value != 0) Success(resultL.value - resultR.value) else Failure("Division by zero")
                    }
                }
            }
        }
        is Number -> Success(value)
    }
}