On Fri, Feb 5, 2016 at 2:16 PM, Takenobu Tani <takenobu.hs@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