Hi, I see `Either.fx<L,R>{}` works with only...

# arrow

[Gopal S Akshintala]

Hi, I see Either.fx<L,R>{} works with only one type of Either<L,R> . Is there any way to work with different types of Either to avoid nested fold?

val a: Either<Int, String> = "a".right()
    val b: Either<String, Int> = 1.right()
    
    Either.fx<String, Int> {
        val bind = a.bind() // ^^^ Compiler Error
        b.bind()
    }

[simon.vergauwen]

Hi, that doesn’t work since L is where flatMap works over.

[simon.vergauwen]

Or rather it short-circuits on L being present or in other words it can only return L as a final result.

[simon.vergauwen]

fun <A, B, C> EitherOf<A, B>.flatMap(f:(B) -> Either<A, C>): Either<A, C>

[simon.vergauwen]

So this code is a.flatMap { b } which doesn’t compile

[Gopal S Akshintala]

Yes @simon.vergauwen I understand why the error is, but I am looking for any concise way to work around the problem, so I don’t have to write nested folds

[Gopal S Akshintala]

val fold = a.fold(
            { it.toString() },
            {
                b.fold(
                        { it },
                        { it.toString() }
                )
            }
    )

[simon.vergauwen]

Ah, you an use mapLeft https://arrow-kt.io/docs/apidocs/arrow-core-data/arrow.core/-either/map-left.html

[simon.vergauwen]

Other functions such as flatMapLeft can still be implemented / contributed to Arrow Core 😉

[simon.vergauwen]

Then you can do

[simon.vergauwen]

Either.fx<String, Int> {
        val bind = a.mapLeft { it.toString() }.bind()
        b.bind()
    }

[Gopal S Akshintala]

Thanks @simon.vergauwen