Relax constraints for Free monad from applicative

[turion]

Some instances for Functor and Applicative for free monad transformers needlessly require Monad m for the inner monad. This PR relaxes these constraints to Functor and Applicative, respectively, and makes some trivial code changes.

[turion]

This is a smaller version of https://github.com/ekmett/free/pull/186.

[turion]

It's a bit ridiculous to write & debug for 7.* GHCs which I can't even install locally, but well.

[RyanGlScott]

I would be in favor of dropping support for pre-8.0 GHCs, given that they are increasingly becoming a barrier to contributors. I have already been dropping pre-8.0 support on other @ekmett libraries whenever this sort of situation arises, and I think it is time to do the same for free.

Would you be willing to wait until that happens before proceeding with this PR? (Alternatively, you could even submit a PR that drops pre-8.0 support if you'd like, but if you'd prefer, I could also do this myself.)

[turion]

I would be in favor of dropping support for pre-8.0 GHCs, given that they are increasingly becoming a barrier to contributors. I have already been dropping pre-8.0 support on other @ekmett libraries whenever this sort of situation arises, and I think it is time to do the same for free.

Yes :tada: highly in favour of this.

Would you be willing to wait until that happens before proceeding with this PR?

Yes sure!

(Alternatively, you could even submit a PR that drops pre-8.0 support if you'd like, but if you'd prefer, I could also do this myself.)

I would in principle, but I don't know how to find all the places where there are needless Functor m constraints. I don't know the codebase as well, so I'd rather leave that part to you.

[RyanGlScott]

I don't know the codebase as well, so I'd rather leave that part to you.

That is entirely reasonable—thank you for confirming. As a first pass, I think I will build free with -Wredundant-constraints to see which constraints can simply be removed without needing further code changes. After that, I'll hand it over to you.

(Also, my apologies for not noticing #154/#186 previously. I think these came about before I become a co-maintainer of the library, so they slipped under my radar.)

[RyanGlScott]

I've submitted #227 to drop support for pre-8.0 GHCs and remove "trivially" redundant constraints (i.e., constraints that are redundant without requiring any further code changes).

[RyanGlScott]

To help improve my own understanding, I've collected all of the places in #154, #186, and this PR where the implementation of class methods can be changed to relax superclass constraints:

• Relaxing this Monad m constraint in Control.Monad.Trans.Free:

  https://github.com/ekmett/free/blob/a7a30928778cd184f9723c335531f89db69f3c78/src/Control/Monad/Trans/Free.hs#L203-L204

  To a Functor m constraint:

  instance (Functor f, Functor m) => Functor (FreeT f m) where
  fmap f (FreeT m) = FreeT (fmap f' m) where

  This is the only implementation change in #154, and the first of three implementation changes in this PR. (As a result, this PR supersedes #154.)

• The second implementation change in this PR is to relax this Monad m constraint in Control.Monad.Trans.Free.Ap:

  https://github.com/ekmett/free/blob/a7a30928778cd184f9723c335531f89db69f3c78/src/Control/Monad/Trans/Free/Ap.hs#L199-L200

  To a Functor m constraint:

  instance (Functor f, Functor m) => Functor (FreeT f m) where
  fmap f (FreeT m) = FreeT (fmap f' m) where

• The third implementation change in this PR is to relax this Monad constraint in Control.Monad.Trans.Free.Ap:

  https://github.com/ekmett/free/blob/a7a30928778cd184f9723c335531f89db69f3c78/src/Control/Monad/Trans/Free/Ap.hs#L204-L205

  To an Applicative m constraint:

  instance (Applicative f, Applicative m) => Applicative (FreeT f m) where
  pure a = FreeT (pure (Pure a))

186 contains all of the changes above plus the following:

• Relaxing this Monad m constraint in Control.Monad.Trans.Free:

  https://github.com/ekmett/free/blob/a7a30928778cd184f9723c335531f89db69f3c78/src/Control/Monad/Trans/Free.hs#L208-L212

  To an Applicative m constraint:

  instance (Functor f, Functor m, Applicative m) => Applicative (FreeT f m) where
  pure a = FreeT (pure (Pure a))
  {-# INLINE pure #-}
  FreeT mf <*> FreeT mx = FreeT $ liftA2 q mf mx
    where
      Pure a `q` Pure b  = Pure $ a b
      Pure a `q` Free b  = Free $ fmap a <$> b
      Free a `q` b       = Free $ (<*> FreeT (pure b)) <$> a
  {-# INLINE (<*>) #-}

• Relaxing this Monad m constraint in Control.Monad.Trans.Free.Church:

  https://github.com/ekmett/free/blob/a7a30928778cd184f9723c335531f89db69f3c78/src/Control/Monad/Trans/Free/Church.hs#L118-L128

  To an Applicative m constraint:

  instance (Foldable f, Foldable m, Applicative m) => Foldable (FT f m) where
  foldr f r xs = F.foldr (<<<) id inner r
    where
      inner = runFT xs (pure . f) (\xg xf -> F.foldr (liftA2 (<<<) . xg) (pure id) xf)
  {-# INLINE foldr #-}
  
  foldl' f z xs = F.foldl' (!>>>) id inner z
    where
      (!>>>) h g = \r -> g $! h r
      inner = runFT xs (pure . flip f) (\xg xf -> F.foldr (liftA2 (>>>) . xg) (pure id) xf)
  {-# INLINE foldl' #-}

• Relaxing this Monad m constraint in Control.Monad.Trans.Iter:

  https://github.com/ekmett/free/blob/a7a30928778cd184f9723c335531f89db69f3c78/src/Control/Monad/Trans/Iter.hs#L162-L163

  To a Functor m constraint:

  instance Functor m => Functor (IterT m) where
  fmap f = IterT . fmap (bimap f (fmap f)) . runIterT

• Relaxing the Monad m constraint in Control.Monad.Trans.Free's hoistFreeT and transFreeT functions to Functor m. Similarly for the functions of the same name in Control.Monad.Trans.Free.Ap.
• Relaxing the Monad m constraint in Control.Monad.Trans.Free's cutoff function to Applicative m. Similarly for the function of the same name in Control.Monad.Trans.Free.Ap.
• Relaxing the Monad m constraint in Control.Monad.Trans.Free.Church's joinFT function to Applicative m.

There are likely other constraints downstream that can be relaxed as a result of these changes, but the changes above are the primary ones.

[turion]

I agree with your assessment. I've rebased and pushed.

[turion]

CI weather isn't good it seems... can we rerun?

[RyanGlScott]

Excellent, thanks for being patient! Although CI is being temperamental, the code itself looks great. I'll go ahead and land this.