`fromLeft` / `fromRight` shouldn't be partial

[kritzcreek]

I'd like to have

fromLeft :: forall a b. a -> Either a b -> a
fromRight ::  forall a b. b -> Either a b -> b

over the current partial

fromLeft :: Partial => forall a b. Either a b -> a
fromLeft :: Partial => forall a b. Either a b -> b

as typing either (const a) identity becomes old rather quickly and I need these functions much more frequently than their partial counterparts. I'm not sure how confusing it would be to define these functions differently in two modules in this package, but even if don't rename the old ones I think this would be a win.

[JordanMartinez]

Agreed, and shouldn't the current fromLeft be unsafeFromLeft?

If the current type signature is swapped out for your type signature, how will this affect the results one sees when one searches for fromLeft via Pursuit? Will both versions of the same function appear?

[hdgarrood]

Pursuit only includes the latest version of each library in the search index, so only the new version would show up.

[hdgarrood]

I’d advocate dropping the partial variant actually: you can easily recover it with fromLeft (crashWith “whatever”)

[hdgarrood]

I think it is a little unfortunate that doing this would introduce an inconsistency with maybe though, as fromJust is partial. It also departs from Haskell as well right?

[hdgarrood]

On reflection I’m a little less enthusiastic about this, given how low down in the ecosystem it is.

[JordanMartinez]

I think it is a little unfortunate that doing this would introduce an inconsistency with maybe though, as fromJust is partial.

That's what I thought after reading your comment.

What if the function was called something other than fromLeft? For example
eitherL default (\leftValue -> body) either
and the same for eitherR?

[hdgarrood]

fromLeftWithDefault maybe?

[JordanMartinez]

I feel like that name is too long. What about eitherLeft and eitherRight?

[hdgarrood]

Abbreviations are overrated imo, especially in cases like this where the function we are considering doesn’t have an existing agreed-upon name. We also have editor tooling to reduce the burden of typing it out. I’m not keen on eitherLeft.

[JordanMartinez]

Hard to argue with those points, haha. I've been on the side where a terse function name left me confused about it for quite a while...

[kritzcreek]

doesn’t have an existing agreed-upon name

base has them fwiw: http://hackage.haskell.org/package/base-4.12.0.0/docs/Data-Either.html#v:fromLeft

[hdgarrood]

Oh! I wasn’t aware of that, that does change things actually. I’ll withdraw my objection then.

[JordanMartinez]

To help with migration, should there be a fromLeft' and vice versa for those who still want the Partial version of this function?

[hdgarrood]

I’d suggest not, unless we intend to keep them in the library. Updating fromLeft to fromLeft’ is, in practice, going to be pretty much equally arduous as updating it to fromLeft (crashWith “unexpected Right”), because the time consuming part is finding the files you need to change and checking that stuff still compiles after making the changes.

[garyb]

We could restore the Unsafe module like we had pre-Partial and move the Partial versions in there.

I don't see the PR getting merged until we have another reason to break the whole ecosystem (like a breaking compiler release), just so you know @JordanMartinez.