Proposal: Add 'Foldable1' to base

[Tony Morris]

Delete head1 and last1 and you'll get my +1.

(they are better written as optics, when we also get Apply into base)

On 12/10/20 4:37 PM, Reed Mullanix wrote:
> With the recent discussion around the addition of 'intersection' to containers,
> I think it might be a good time to re-open the discussion surrounding adding
> 'Foldable1' to base.
> 
> For context, 'Foldable1' would be a subclass of 'Foldable' that abstracts
> folds over non-empty containers. Alternatively, it can be seen as a method
> of combining together the elements of a container using a semigroup.
> The contents of this class have been discussed previously (See [1,2]),
> and the version presented in this proposal is taken from [1].
> 
>   class Foldable t => Foldable1 t where
>      {-# MINIMAL foldMap1 | foldr1map #-}
> 
>      fold1 :: Semigroup m => t m -> m
> 
>      -- the defining member, like foldMap but only asking for Semigroup
>      foldMap1 :: Semigroup m => (a -> m) -> t a -> m
> 
>      -- strict foldMap1, cf foldMap'
>      foldMap1' :: Semigroup m => (a -> m) -> t a -> m
> 
>      -- analogue of toList
>      toNonEmpty :: t a -> NonEmpty a
> 
>      -- left&right, strict&non-strict folds
>      foldr1  :: (a -> a -> a) -> t a -> a
>      foldr1' :: (a -> a -> a) -> t a -> a
>      foldl1  :: (a -> a -> a) -> t a -> a
>      foldl1' :: (a -> a -> a) -> t a -> a
> 
>      -- these can have efficient implementation for NonEmptySet
>      maximum1 :: Ord a => t a -> a
>      minimum1 :: Ord a => t a -> a
> 
>      -- head1 have efficient implementation for NonEmpty and Tree
>      -- last1 for symmetry
>      head1 :: t a -> a
>      last1 :: t a -> a
> 
>      -- fold variants with premap.
>      -- Without this map, we cannot implement foldl using foldr etc.
>      foldrMap1  :: (a -> b) -> (b -> b -> b) -> t a -> b
>      foldlMap1' :: (a -> b) -> (b -> b -> b) -> t a -> b
>      foldlMap1  :: (a -> b) -> (b -> b -> b) -> t a -> b
>      foldrMap1' :: (a -> b) -> (b -> b -> b) -> t a -> b
> 
> 
> This has a couple of benefits. On the practical side, we can provide
> total alternatives
> to existing partial functions (IE: 'foldr1' and friends). It also
> enables us to fold
> over containers using a semigroup instance, which comes up suprisingly often.
> 
> Naming:
> --------------------------------------------------------------------------------
> Historically, the biggest source of controversy with this proposal has
> been over the
> name. The class currently exists in semigroupoids [3] under the name
> 'Foldable1', though
> there was some discussion around renaming it to 'SemiFoldable' [4].
> However, if we keep
> the name unchanged, it makes the migration path nice and
> straightforward, and the possible
> name conflict with Data.Functor.Classes seems unlikely.
> 
> Migration:
> --------------------------------------------------------------------------------
> If we decide to go with 'Foldable1' as the name, we should be able to
> perform this change with
> 0 breakage.
> 
> References:
> [1] https://mail.haskell.org/pipermail/libraries/2019-November/030059.html
> [2] https://gitlab.haskell.org/ghc/ghc/-/issues/13573
> [3] https://hackage.haskell.org/package/semigroupoids-5.3.4/docs/Data-Semigroup-Foldable.html#t:Foldable1
> [4] https://github.com/ekmett/semigroupoids/issues/26
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