#13573: Add Foldable1 to base -------------------------------------+------------------------------------- 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: | -------------------------------------+------------------------------------- Description changed by Iceland_jack: @@ -24,1 +24,3 @@ - minimum1 :: (Ord a, Foldable1 f) => f a -> a + import qualified Data.Semigroup as S + + minimum1, maximum1 :: (Ord a, Foldable1 f) => f a -> a @@ -26,2 +28,0 @@ - - maximum1 :: (Ord a, Foldable1 f) => f a -> a @@ -30,1 +30,1 @@ - head1 :: Foldable1 f => f a -> a + head1, last1 :: Foldable1 f => f a -> a @@ -32,0 +32,1 @@ + last1 = S.getLast . foldMap1 S.Last @@ -33,4 +34,1 @@ - last1 :: Foldable1 f => f a -> a - last1 = S.getLast . foldMap1 S.Last - - foldr1 :: Foldable1 f => (a -> a -> a) -> (f a -> a) + foldr1, foldl1 :: Foldable1 f => (a -> a -> a) -> (f a -> a) @@ -38,2 +36,0 @@ - - foldl1 :: Foldable1 f => (a -> a -> a) -> (f a -> a) 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/0f0e14f51cf1ecd7ebf2d8c52204bd91ae... 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:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler