flatMap

I can’t see a flatMap function or its equivalent which is typical for monads. It should work like Haskell’s >>=.

My suggested implementation:

infix inline fun <V, E, U> Result<V, E>.flatMap(transform: (V) -> Result<U, E>) : Result<U, E> {
    return when (this) {
        is Ok -> transform(value)
        is Err -> this
    }
}

Also flatMapError function could be implemented in simillar way. If you like the idea, I will implement them along with unit tests and create a pull request.

Thanks!

[Ravirael]

You mention the extension function for Kotlin’s Mapclass. However there is also a version for Iterable with signature fun <T, R> Iterable<T>.flatMap(transform: (T) -> Iterable<R>): List<R> . As you can see this is really similar to your andThen by parameters and return type. Both functions are defined for some Monad<T> take function (T) -> Monad<R> and return Monad<R>, that’s why this name seems reasonable for me.

Also in this alternative library that operation is called flatMap.

Of course using Elm’s convention also makes sense.

[Ravirael]

Well, there also is a flatMap method in Java’s Optional and similary flatMap in Scala’s Option. They are equivalent to your andThen as much as they can be!

I agree that the name itself is confusing, however it somehow shows a relationship with map. Still I believe this is a common name for this type of operation in Java’s world.