#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: @@ -5,3 +5,1 @@ - Semigroup-Foldable.html Foldable1], - [https://hackage.haskell.org/package/semigroupoids-5.1/docs/Data- - Semigroup-Traversable.html Traversable1] in ''base'' as well. + Semigroup-Foldable.html Foldable1] in ''base'' as well. New description: `NonEmpty` and `Semigroup` are in ''base'' and we are on our way to add `Semigroup` as a superclass of `Monoid` (#10365), this is a fine time to ask if we should include [https://hackage.haskell.org/package/semigroupoids-5.1/docs/Data- Semigroup-Foldable.html Foldable1] in ''base'' as well. `Foldable` has partial functions already, functions that only make sense for non-empty structures such as `foldr1`, `foldl1`, `minimum` and `maximum`. There are also functions which ''could'' be defined in terms of `Foldable` like `head` / `tail` (for definition of `unimprove` and `Diff` see [https://github.com/ekmett/semigroupoids/issues/49 semigroupoids issue #49] and this [https://github.com/arkeet/difference/blob/0f0e14f51cf1ecd7ebf2d8c52204bd91ae... github] page) {{{#!hs minimum1 :: (Ord a, Foldable1 f) => f a -> a minimum1 = S.getMin . foldMap1 S.Min maximum1 :: (Ord a, Foldable1 f) => f a -> a maximum1 = S.getMax . foldMap1 S.Max head1 :: Foldable1 f => f a -> a head1 = S.getFirst . foldMap1 S.First last1 :: Foldable1 f => f a -> a last1 = S.getLast . foldMap1 S.Last foldr1 :: Foldable1 t => (a -> a -> a) -> (t a -> a) foldr1 f = unimprove . foldMap1 (\a -> Diff (f a) a) foldl1 :: Foldable1 t => (a -> a -> a) -> (t a -> a) foldl1 f = unimprove . getDual . foldMap1 (\a -> Dual $ Diff (flip f a) a) }}} There is also the possibility of adding further functions such as `foldM`, `foldM_`, `foldrM` and `foldlM` for non-empty structures. As usual I don't have more concrete ideas, but let's start by testing the waters before submitting a ghc-proposal. I understand this may be controversial but I never liked so many partial functions in `Foldable`. -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13573#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler