Splitter is a contravariant functor

[eayus]

The Splitter type seems to form a contravariant functor: given a splitter than works on a and a preprocessing step b -> a, we can get a splitter that works on b.

The package cotravariant adds a type class for this, though if you wanted to avoid dependencies a simple function would also work.

Is this something you would consider adding to this library? I'd be happy to implement it.

[byorgey]

Hi @eayus , thanks for this suggestion! I definitely do not want to add a dependency on the contravariant package, since the split package currently only has a dependency on base, and adding a bunch of dependencies could have a big impact (currently 7441 packages on Hackage import split either directly or transitively).

However, I think you are right that Splitter is a contravariant functor and I would be happy to merge a PR that adds a function to witness this. Perhaps mapSplitter :: (b -> a) -> Splitter a -> Splitter b? I'm open to other naming suggestions as well. Also, I suppose while we're at it we should also add mapDelimiter :: (b -> a) -> Delimiter a -> Delimiter b, and mapSplitter would be implemented in terms of it.

[eayus]

I agree and think that those are sensible names for the functions. I'll try to get around to submitting a PR in the next few days.

[Bodigrim]

The package cotravariant adds a type class for this, though if you wanted to avoid dependencies a simple function would also work.

Contravariant has been in base for quite a long time: https://hackage.haskell.org/package/base-4.19.0.0/docs/Data-Functor-Contravariant.html#t:Contravariant

[byorgey]

Oh, thanks @Bodigrim , I had forgotten that! @eayus would you like to also add a Contravariant instance (just by setting contramap = mapSplitter)?