Either.kt

sealed class Either<L, R> {
    class Left<L, R>(val l: L) : Either<L, R>() {
        override fun toString(): String = "Left $l"
    }
    
    class Right<L, R>(val r: R) : Either<L, R>() {
        override fun toString(): String = "Right $r"
    }
    
    infix fun <Rp> bind(f: (R) -> (Either<L, Rp>)): Either<L, Rp> {
        return when (this) {
            is Either.Left<L, R> -> Left<L, Rp>(this.l)
            is Either.Right<L, R> -> f(this.r)
        }
    }
    
    infix fun <Rp> seq(e: Either<L, Rp>): Either<L, Rp> = e
    
    companion object {
        fun <L, R> ret(a: R) = Either.Right<L, R>(a)
        fun <L, R> fail(msg: String): Either.Right<L, R> = throw Exception(msg)
    }
}

fun main(args: Array<String>) {
    
    println(Either.Right<String, Int>(5) bind { it -> Either.ret<String, Boolean>(true)} )
    
    println(Either.Left<String, Int>("error").bind({ it -> Either.ret<String, Int>(it + 1) }))
    
    val right = Either.Right<String, Int>(5)
    val condition = { it: Int -> if (it > 3) {
        Either.Left<String, Int>("number is too big")
    } else {
        Either.Right<String, Int>(it * 2)
    }}
    
    println(right bind condition)
    println(Either.Right<String, Int>(2).bind(condition).bind(condition).seq(Either.Right<String, Float>(2.0f)))
    
    try {
        println(Either.Right<String, Int>(5) bind { Either.fail<String, Int>("asdf") })
    } catch (e: Exception) {
        println("exception: ${e.message}")
    }
}