railway-oriented-programming (kotlin, playgorund)

railway.kt

package com.example.demo

// based on ...
// https://www.reddit.com/r/Kotlin/comments/15b16px/little_example_of_railway_oriented_programming_in/
// https://github.com/GabrielLasso/Kotlin-railway-example

fun main() {
    val pipeline: (data: Either<Throwable, Person>) -> Either<Throwable, Response> =
        (::validate `→` ::save `→` ::notify `→` ::buildResponse)

    val validInput = Person("Gabriel", 26)
    val validResponse: Either<Throwable, Response> = validInput
        .let { Either.Right(it) }
        .let(pipeline::invoke)

    val invalidInput = Person("Invalid age", -3)
    val invalidResponse: Either<Throwable, Response> = invalidInput
        .let { Either.Right(it) }
        .let(pipeline::invoke)

    println("input: $validInput ==> response: $validResponse")
    println("input: $invalidInput ==> response: $invalidResponse")
}



sealed class Either<out L, out R> {
    data class Left<L>(val value: L) : Either<L, Nothing>()
    data class Right<R>(val value: R) : Either<Nothing, R>()
}

typealias Arrow<A, B> = (A) -> B

infix fun <A, B, C> Arrow<A, B>.`→`(f: Arrow<B, C>): Arrow<A, C> {
    return {
        f(this(it))
    }
}

data class Person(val name: String, val age: Int)
data class Response(val body: String, val code: Int)

fun validate(data: Either<Throwable, Person>): Either<Throwable, Person> {
    if (data is Either.Left) {
        return data
    }
    data as Either.Right

    if (data.value.age < 0) {
        return Either.Left(Exception("Age must be non negative"))
    }

    return data
}

fun save(data: Either<Throwable, Person>): Either<Throwable, Person> {
    if (data is Either.Left) {
        return data
    }

    println("Saved $data")
    return data
}

fun notify(data: Either<Throwable, Person>): Either<Throwable, Person> {
    if (data is Either.Left) {
        return data
    }

    println("Notified user $data")
    return data
}

fun buildResponse(data: Either<Throwable, Person>): Either<Throwable, Response> {
    if (data is Either.Left) {
        return data
    }

    return Either.Right(Response("Ok: $data", 200))
}