Is there any idiomatic way of dropping `Either<A, B>` and replacing it with `Either<A, Unit>`? I'm ...

# arrow

[David]

Is there any idiomatic way of dropping Either<A, B> and replacing it with Either<A, Unit>? I'm thinking along the way how merge() looks instead of fold(::identity, ::identity) . My case is as follows:

val maybeListOfError = config.listOfItems.map { 
   it.execute() // Returns Either<Error, A>
}
.flattenOrAccumulate() // Either<List<Error>, List<A>>
.onLeft { // Do some recovery }  
.map { }

// I have to to return the list of errors later, if present, but first do some other stuff.
...

maybeListOfError.mapLeft { MyCustomError(it) }.bind()
...

I want to return Either<List<Error>, Unit>, but I don't feel map { } conveys that clearly. I guess I could use leftOrNull() but then it doesn't turn out so nice in a either effect context:

ensure(maybeListOfError == null) { MyCustomError(result!!) }

forces me to use !! and the other alternative:

if(maybeListOfError != null)  
    raise(MyCustomError(maybeListOfError))
}

gives IDE hint of replacing it with ensure again.

Maybe I'm missing something?

[phldavies]

val a = either { forEachAccumulating(config.listOfItems) { it.execute().bind() } }

    @OptIn(ExperimentalRaiseAccumulateApi::class)
    val b = accumulate(::either) { config.listOfItems.forEach { it.execute().bind() } }

either of these should work

[phldavies]

both will return Either<NonEmptyList<Error>, Unit>

[David]

Ah, the top level accumulate solves it. Thanks!

[phldavies]

I'm not sure how stable the new accumulate DSL will be - but I'm assuming it won't change much - option a is probably safer for now