Hi Alexander, > 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.
interesting idea. It's ":t" 's verbose representation mode. The ghci represents true type (not lie) and beginners may intuitively understand the relation between Foldable type class and instances. Beginners will be overcome FTP more easily. > 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. It's one of the options. But I feel that Levity (or RuntimeRep) is more deep than the type class. They may feel difficult to understand the difference of two patterns in ($). (If it will be long, it's better to separate thread =) ) Regards, Takenobu 2016-02-16 16:28 GMT+09:00 Alexander Kjeldaas <alexander.kjeld...@gmail.com> : > > > On Fri, Feb 5, 2016 at 2:16 PM, Takenobu Tani <takenobu...@gmail.com> > wrote: > >> Hi, >> >> I'll worry about the learning curve of beginners. >> Maybe, beginners will try following session in their 1st week. >> >> ghci> :t foldr >> ghci> :t ($) >> >> 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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