Oh okay, you want `leftFlatMap` but that currently...

# arrow

[simon.vergauwen]

Oh okay, you want leftFlatMap but that currently doesn’t exist in Arrow.

[Robert Menke]

If it’s still ok to add flatMapLeft I did create a branch with the functionality + test. I get an error pushing my branch up to origin because of permissions. No big deal if you’d prefer not to add extra code to maintain @simon.vergauwen @pakoito. Up to you guys.

[pakoito]

I';d rather not, we aligned the names not so long ago

[pakoito]

and we moved from mapLeft to handleError for example

[simon.vergauwen]

But this is different from handleErrorWith

[simon.vergauwen]

It doesn’t fix L but rather leaves it generic.

[simon.vergauwen]

It’s an operator of the bifunctor hierarchy, right?

[Robert Menke]

Cool, handleErrorWith does work perfectly for my use case so I can stick with that. Thanks for pointing me in the right direction!

[pakoito]

flatMapLeft is a specialized handleErrorWith

[pakoito]

or just a plain alias

[simon.vergauwen]

It’s the other way around

[pakoito]

because you can return handleErrorWith { Either.Left("") }

[pakoito]

I thought we kept the handleError naming so it was consistent across datatypes

[simon.vergauwen]

fun <A, B, C> Either<A, B>.leftFlatmap(f: (A) -> Either<C, B>): Either<C, B> = when(this) {
  is Either.Left -> f(this.a)
  is Either.Right -> this
}

fun <A, B> Either<A, B>.handleErrorWith(f: (A) -> Either<A, B>): Either<A, B> = leftFlatMap(f)

[simon.vergauwen]

I’m not favoring one over the other. I’d include both since I expect leftFlatMap to come back when we complete the bifunctor hiearchy.

[simon.vergauwen]

It’s a very handy operator for IO<E, A> over handleErrorWith

[simon.vergauwen]

Because it can to change E to Nothing, well I guess you could name it handleErrorWith in a bifunctor hierarchy but then it would be an alias. Here it’s not.

[simon.vergauwen]

@Robert Menke you’re very welcome! Glad to help any time. Sorry for the confusion.

[Robert Menke]

So 1 other consideration is that handleErrorWith is not an inline function so it actually doesn’t work for my use case because I need to call suspend functions inside. I can get by with fold for now, just voicing my experience.

[simon.vergauwen]

That is where IO comes in, Either cannot deal with the powers of suspend. It cannot control the async jumps, exception being thrown etc.

[Robert Menke]

Gotcha 👍 so just out of curiosity (and ignorance) why inline certain functions like flatMap or fold but not something like handleErrorWith?

[simon.vergauwen]

With IO you'd do io.handleErrorWith { e -> IO.effect { suspendFunction() } }

[simon.vergauwen]

If those functions are still inline then that will probably change soon.

[Robert Menke]

Ok that makes sense! Thanks!