On Thu, Oct 17, 2019 at 5:43 PM Oleg Grenrus <oleg.grenrus@iki.fi> wrote:

I propose adding `Foldable1` type-class into `base1`.

Add Foldable1
=============

Motivation
----------

It's regularly asked whether `Foldable1` could be added to `base`
(e.g. on reddit[^ref1], there're also GHC issue[^ref2] and old
phabricator diff[^ref3])
Also there's work towards non-empty maps and sets[^ref4],
which would benefit from `Foldable1`.

As commented on reddit, `Foldable1` could be added without any pain
to the `base` as it's pure addition - no modifications needed in
existing modules.

[^ref1]:
https://www.reddit.com/r/haskell/comments/6d0vgt/could_we_have_foldable1_and_traversable1_in_base/
[^ref2]: https://gitlab.haskell.org/ghc/ghc/issues/13573
[^ref3]: https://phabricator.haskell.org/D4812
[^ref4]: https://github.com/haskell/containers/pull/616

Change
------

The change exist as merge request[^ref4] on gitlab.haskell.org.

Importantly, this change **doesn't change** anything in other modules
of `base`, except of adding a `Foldable` instance to `Data.Complex`.
In particular, `foldl1` and `foldr1` in `Data.Foldable` remain partial, etc.

My version of `Foldable1` class is big, so I'll comment the motivation
for each member

```haskell
class Foldable t => Foldable1 t where
    {-# MINIMAL foldMap1 | toNonEmpty | foldr1map #-}

    -- the defining member, like foldMap but only asking for Semigroup
    foldMap1 :: Semigroup m => (a -> m) -> t a -> m

    fold1 :: Semigroup m => t m -> 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 :: forall a. Ord a => t a -> a
    minimum1 :: forall a. 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.
    foldr1map :: (a -> b) -> (b -> b -> b) -> t a -> b
    foldl1'map :: (a -> b) -> (b -> b -> b) -> t a -> b
    foldl1map :: (a -> b) -> (b -> b -> b) -> t a -> b
    foldr1'map :: (a -> b) -> (b -> b -> b) -> t a -> b
```

The merge request also adds instances for everything non-empty in `base`.

I propose the `Data.Foldable1` as the module name.
`semigroupoids`[^ref6] uses `Data.Semigroup.Foldable`,
but it's confusing; and using different name could help migration.

The module contains five top-level functions, which should
be self-explanatory:

```haskell
intercalate1 :: (Foldable1 t, Semigroup m) => m -> t m -> m

foldrM1 :: (Foldable1 t, Monad m) => (a -> a -> m a) -> t a -> m a
foldlM1 :: (Foldable1 t, Monad m) => (a -> a -> m a) -> t a -> m a

maximum1By :: Foldable1 t => (a -> a -> Ordering) -> t a -> a
minimum1By :: Foldable1 t => (a -> a -> Ordering) -> t a -> a
```

This is less than in `Data.Semigroup.Foldable`[^ref9],
as without `Apply` they don't make sense.
For example:

```haskell
-- needs Apply, not in Data.Foldable1
traverse1_ :: (Foldable1 t, Apply f) => (a -> f b) -> t a -> f ()
```

And if we relax `Apply` to `Applicative`, we get `traverse_`.

[^ref5]: https://gitlab.haskell.org/ghc/ghc/merge_requests/1973
[^ref9]:
https://hackage.haskell.org/package/semigroupoids-5.3.3/docs/Data-Semigroup-Foldable-Class.html

[^ref5]: https://gitlab.haskell.org/ghc/ghc/merge_requests/1973

Compatibility & migration
-------------------------

I drafted a compatibility package `foldable1` (github[^ref6],
haddocks[^ref7]),
which I hope could be maintained under github.com/haskell organization.
I can act as a maintainer, with a hope that there won't be a lot
of changes happening in `Data.Foldable1`.

To my surprise, there's already a package with this name on
Hackage[^ref8] by
M Farkas-Dyck (cc'd). I hope they would donate the name to Haskell.org /
CLC;
the package won't have any other good use when `Data.Foldable1` is in
`base`,
then act as a compat package.

`Data.Foldable1` contains also instances for `Lift`, `Backwards` and
`Reverse`
data types from `transformers`. Perfectly, the `transformers` bundled
with GHC with this change would implement the instances as well.
This change should propage to `transformers-compat` too.

Similarly, `containers` would have an instance for `Tree` (and non-empty
`Set` and `Map` when they are added).

`semigroupoids` would need a bit of work, to depend on `foldable1`,
yet the public changes can be kept quite minimal.
I don't think that anything in reverse dependencies of `lens` will be
broken by
this change, if "new" `Foldable1` is re-exported from `semigroupoids`'
`Data.Semigroup.Foldable`[^ref9]

Other "compat" packages -- like `tagged`, `bifunctors` -- have to be
dealt with
case by case. For example whether they should depend on `foldable1` or
other way around.

[^ref6]: https://github.com/phadej/foldable1
[^ref7]: https://oleg.fi/haddocks/foldable1/
[^ref8]: https://hackage.haskell.org/package/foldable1
[^ref9]:
https://hackage.haskell.org/package/semigroupoids-5.3.3/docs/Data-Semigroup-Foldable-Class.html

Unresolved questions
--------------------

- Should we add `Bifoldable1` too. That should be trivial.
- GHC-8.10 freeze is quite soon, is targeting GHC-8.12/base-4.15 more
realistic?

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