Explicitly define `length`, `elem`, etc. in `Foldable` instance for `Data.Functor.Compose`

[cdsmith]

Currently, the Foldable instance for Data.Functor.Compose looks like this:

instance (Foldable f, Foldable g) => Foldable (Compose f g) where
    foldMap f (Compose t) = foldMap (foldMap f) t

All other methods besides foldMap get default implementations, which delegate to foldMap. I propose that the following should be added:

    length (Compose t) = getSum (foldMap' (Sum . length) t)
    elem x (Compose t) = getAny (foldMap' (Any . elem x) t)
    sum (Compose t) = getSum (foldMap' (Sum . sum) t)
    product (Compose t) = getProduct (foldMap' (Product . product) t)
    ... and so on

The inner container in a Compose may have a more efficient implementation for other methods of Foldable. In case it does, that should be used instead of falling back to foldMap as happens now.

This applies to all members of Foldable, which presumably are in the class precisely because they can have more efficient implementations. It seems particularly troublesome for length and elem, which are likely to have much more efficient implementations for commonly used containers.

[treeowl]

I think it should be foldMap'.

[cdsmith]

Yes, you are right. I edited my proposal to use foldMap' for length.

[cdsmith]

On further reflection, I realized that essentially the same situation applies to elem and other members, so I have updated this issue to also include them.

[Bodigrim]

@cdsmith could you please put up a draft MR?

[cdsmith]

@Bodigrim Is libraries/base at https://gitlab.haskell.org/ghc/ghc the right place to do so?

[Bodigrim]

Yes

[cdsmith]

I probably cannot put up a draft MR in a timely fashion. I spent some time this evening trying to figure out how to get that checked out and building. Among the things that stumped me were how to get HLS working (it complains about missing a file related to hadrian), how to build just base (instead of rebuilding all of GHC and everything), where to put tests (I can barely find any tests in libraries now)... hopefully there's someone else interested in this that is already familiar with developing in base, and for whom the hours it would take me to figure this stuff out would not be required.

[Bodigrim]

@cdsmith you do not even need to build GHC yourself, CI will do it for you; and at this stage tests are not obligatory.

[Bodigrim]

@cdsmith unless there is a progress within two weeks, I'll close this as abandoned. If anyone else has bandwidth to create an MR, feel free to do so.

[cdsmith]

I will try to work on this over the weekend. It would be a shame to lose what seems like a clear improvement, and I should get better at working in the GHC tree anyway.

[Bodigrim]

@cdsmith ping. Feel free to advertise this job on irc / twitter / discourse / reddit, but if there is no MR very soon, I'll close.

core-libraries-committee/PROPOSALS.md

Lines 54 to 57 in dcaf3f2

	The issue creator is responsible for seeing their proposal through.
	The Core Libraries Committee's focus is around management,
	not implementation: you should do the work for your own proposals
	(or others' if you like them!) to show us that you think it's worth doing.

[cdsmith]

Is there a more traditional issue tracker for base somewhere where I can file this, then? I'm not trying to force anyone else to implement anything, but if this were any other library, I'd think it's worth having an open issue. I cannot seem to find such a thing for base.