kotlin.Result - flatMap extension

ResultExtensionTest.kt

import net.jqwik.api.ForAll
import net.jqwik.api.Property

// requires compiler option '-Xallow-result-return-type'
fun <T, U> Result<T>.flatMap(function: (T) -> Result<U>): Result<U> {
    return if (isSuccess) {
        function(getOrNull()!!)
    } else {
        Result.failure(this.exceptionOrNull()!!)
    }
}

class ResultExtensionsTest {

    @Property
    fun `flatmap check -- monad left identity`(@ForAll value: String): Boolean {
        val initial = point(value)
        return initial.flatMap(::f) == f(value)
    }

    @Property
    fun `flatmap check -- monad right identity`(@ForAll value: String): Boolean {
        val initial = f(value)
        return initial.flatMap { point(it) } == initial
    }

    @Property
    fun `flatmap check -- monad associativity`(@ForAll value: String): Boolean {
        val initial = point(value)
        return initial.flatMap(::f).flatMap(::g) == initial.flatMap { f(it).flatMap(::g) }
    }

    companion object {
        fun f(s: String) = Result.success("$s something")
        fun g(s: String) = Result.success("$s nonething")

        fun <T> point(x: T): Result<T> = Result.success(x)
    }
}