Add Validation data type to Extra modules

[vrom911]

It's not in base, but in either package:

• https://hackage.haskell.org/package/either-5.0.1/docs/Data-Either-Validation.html#t:Validation

Though you don't usually want to drag all that dependencies, so maybe it makes sense to reimplement it in some Extra module?

[chshersh]

@vrom911 Completely agree with this 👍 💯 You probably don't want to depend on either package in lightweight libraries or applications. But Validation data type is extremely useful. And since we have Extra modules, it's relatively easy to add known and common things to the relude without worrying about name clashes.

[kahlil29]

Would this include a reimplementation of Validation with new enhancements/features or we would just reuse the current data declaration and instances from Data.Either ?

[chshersh]

@kahlil29 Couple hours ago I've received an unexpected email from @mauriciofierrom with the attempt to resolve this issue. Here is the gist:

• https://gist.github.com/mauriciofierrom/bdb7a5d0ff06c4a23b51ef7bfa7d81d9

I will duplicate my comments to this solution here:

1. You can also add Bifoldable and Bitraversable instances. These typeclasses are already in base and we're going to reexport them as well. See relevant issues:
   • Reexport some of the Data.Bifoldable #98
   • Reexport some of the Data.Bitraversable #99
2. It would be really nice to use -XInstanceSigs extension and all type signatures to all methods in all instances. In relude we want our package to be as beginner-friendly as possible, and having such type signatures significantly increases code readability. See example in this module:
   • https://github.com/kowainik/relude/blob/master/src/Relude/Extra/Foldable1.hs
3. Also, usually, when you're writing general purpose library, it's a good idea to implement all methods from the typeclasses. Default implementations might not be efficient.
4. In addition to previous point: adding {-# INLINE #-} pragma to every implemented method helps performance as well.
5. Also, in relude we're using 4 spaces for indentation :)

[mauriciofierrom]

@chshersh thanks for the comments. If it's ok with you (and @kahlil29) I will work on this issue then.

[kahlil29]

@chshersh Noted :)
@mauriciofierrom Go ahead 👍