Rename and deprecate mapToFst/Snd

[vrom911]

In #194 I've noticed that functions mapToFst/mapToSnd/mapBoth have a bit misleading name. Check out the type:

mapToFst :: (a -> b) -> a -> (b, a)

when I would expect it to have the following type:

mapToFst :: (a -> b) -> f a -> f (b, a)

(which is exactly what was needed in the mentioned PR, and we used map (mapToFst f) in there).

I also understand, that this is breaking change and I am not sure if it worth it. Need to thing about renaming or deprecating process a bit harder.

Anyway, at least adding functions which would actually map the containers is also a possible solution for the issue. I would really appreciate the discussion of this problem.

cc @kowainik/dev

[chshersh]

@vrom911 I think renaming to less confusing names worth it in the long term. Regarding deprecation process: we can use the process described in my blog post (which includes multi-stage process with DEPRECATED pragmas and custom type errors):

• https://kodimensional.dev/type-errors#deprecation-better-migration-guide

Regarding mapBoth: we have a separate issue #174. Probably instead of only generalisation we can also rename the function to bimapBoth and have bimapBothF in the spirit of other functions in the Relude.Extra.Bifunctor module.

We can introduce new functions first and deprecate the old ones:

??? :: (a -> b) -> a -> (b, a)
fmapToFst :: Functor f => (a -> b) -> f a -> f (b, a)  -- uses ???

It's hard to come up with the good name for mapToFst... I can propose the following options:

• withFst
• mapKeepFst/keepFst
• toFst

What do you think?

[willbasky]

Map implicates some f, therefore it is better to abandon it.
withFst i vote from you polling.

[josephcsible]

I'm not a fan of renaming. In my mind, it makes sense that map in mapTo* refers to mapping over the first/second half of the tuple (and similarly for traverse). For example, note the following identities:

• mapToFst f = bimap f id . dup = first f . dup
• mapToSnd f = bimap id f . dup = second f . dup = fmap f . dup
• mapBoth f = bimap f f
• traverseToFst f = bitraverse f pure . dup
• traverseToSnd f = bitraverse pure f . dup = traverse f . dup
• traverseBoth f = bitraverse f f

Also, if you did rename the map* functions, what would it even make sense to rename the traverse* functions too?

[chshersh]

I propose the following change:

1. Rename mapToFst and mapToSnd to toFst and toSnd correspondingly.
2. Change the type of mapToFst (and mapToSnd) to the following:

fmapToFst :: Functor f => (a -> b) -> f a -> f (b, a)
fmapToSnd :: Functor f => (a -> b) -> f a -> f (a, b)

3. Deprecate mapToFst and mapToSnd.

Now we have [fmap|traverse]to[Fst|Snd] and a sound naming scheme.