On Fri, Feb 5, 2016 at 2:16 PM, Takenobu Tani <takenobu...@gmail.com> wrote:
> Hi, > > I'll worry about the learning curve of beginners. > Maybe, beginners will try following session in their 1st week. > > ghci> :t foldr > ghci> :t ($) > > They'll get following result. > > > Before ghc7.8: > > Prelude> :t foldr > foldr :: (a -> b -> b) -> b -> [a] -> b > > Prelude> :t ($) > ($) :: (a -> b) -> a -> b > > Beginners should only understand about following: > > * type variable (polymorphism) > > > After ghc8.0: > > Prelude> :t foldr > foldr :: Foldable t => (a -> b -> b) -> b -> t a -> b > > If the output was the following it would be more understandable (and more encouraging!) """ Prelude> :t foldr foldr :: Foldable t => (a -> b -> b) -> b -> t a -> b For example: foldr :: (a -> b -> b) -> b -> [a] -> b foldr :: (a -> b -> b) -> b -> Maybe a -> b foldr :: (a -> b -> b) -> b -> Identity a -> b foldr :: (a -> b -> b) -> b -> (c, a) -> b and more """ It is easy to see a pattern here. The order of the instances used could be the load order, so the ones from Prelude would come first. > Prelude> :t ($) > ($) > :: forall (w :: GHC.Types.Levity) a (b :: TYPE w). > (a -> b) -> a -> b > > I'm not sure how this would work here, but when Levity is *, this should collapse into the old syntax, so: """ Prelude> :t ($) ($) :: <"unreadable blurb"> For example: ($) :: (a -> b) -> a -> b ($) :: forall a (b :: #). (a -> b) -> a -> b """ At least one of those lines should be understandable. Alexander
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