Proposal: Add 'Foldable1' to base

[George Wilson]

I like David's more conservative class size.

I would prefer to see the semigroupoids classes renamed [1] before
they get added to base, since it's much harder to rename them once
they're there. However, it's not clear to me that there's the required
will to move forward with such a gargantuan renaming effort
(semigroupoids is depended on directly and transitively by a lot of
hackage). There was a recent attempt that didn't gain traction [2]. I
would also prefer that the renaming and the move to base are two
discrete steps -- not combined, since that would make the migration
awkward and unpleasant (migrating to a differently-named class from a
different package).

I want to see the spirit of this happen, but overall I'm -1 on this
proposal at this time.

[1] https://github.com/ekmett/semigroupoids/issues/26
[2] https://github.com/ekmett/semigroupoids/pull/90

Cheers,
George


On Fri, 11 Dec 2020 at 22:37, Tony Morris <tonymorris at gmail.com> wrote:
>
> 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
> > _______________________________________________
> > Libraries mailing list
> > Libraries at haskell.org
> > http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
> >
> _______________________________________________
> Libraries mailing list
> Libraries at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries