[GHC] #13573: Add Foldable1 to base

[GHC]

#13573: Add Foldable1 to base
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #10365            |  Differential Rev(s):
       Wiki Page:                    |
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Description changed by Iceland_jack:

@@ -9,1 +9,1 @@
-   foldMap1 :: Semigroup m => (a -> m) -> f a -> m
+   foldMap1 :: Semigroup m => (a -> m) -> (f a -> m)

New description:

 This is a proposal to add
 [https://hackage.haskell.org/package/semigroupoids-5.1/docs/Data-
 Semigroup-Foldable.html Foldable1] (non-empty `Foldable`) to base

 {{{#!hs
 -- Data.Semigroup.Foldable
 class Foldable f => Foldable1 f where
   fold1    :: Semigroup m => f m -> m
   foldMap1 :: Semigroup m => (a -> m) -> (f a -> m)

   -- Possible methods
   head1      :: f a -> a
   last1      :: f a -> a
   toNonEmpty :: f a -> NonEmpty a
 }}}

 along with instances and function that are only valid for non-empty
 structures (details: [https://github.com/ekmett/semigroupoids/issues/49
 semigroupoids issue #49],
 [https://github.com/arkeet/difference/blob/0f0e14f51cf1ecd7ebf2d8c52204bd91ae3b2969/src/Data/Semigroup/Difference.hs
 github])

 {{{#!hs
 import qualified Data.Semigroup as S

 minimum1, maximum1 :: (Ord a, Foldable1 f) => f a -> a
 minimum1 = S.getMin . foldMap1 S.Min
 maximum1 = S.getMax . foldMap1 S.Max

 head1, last1 :: Foldable1 f => f a -> a
 head1 = S.getFirst . foldMap1 S.First
 last1 = S.getLast  . foldMap1 S.Last

 foldr1, foldl1 :: Foldable1 f => (a -> a -> a) -> (f a -> a)
 foldr1 f = unimprove . foldMap1 (\a -> Diff (f a) a)
 foldl1 f = unimprove . getDual . foldMap1 (\a -> Dual $ Diff (flip f a) a)
 }}}

 Adding `foldM`, `foldM_`, `foldrM` and `foldlM` for non-empty structures
 is also a possibility.

 ----

 Currently these are partial functions in `Foldable`. This proposal does
 '''not''' propose replacing partial `Foldable` functions.

 ----

 I wanted to test the waters before submitting it to the libraries mailing
 list. This may be controversial but it gives us a path to avoid partial
 functions in `Foldable`.

--

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13573#comment:11>
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