Re: New type of ($) operator in GHC 8.0 is problematic

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 :

On Fri, Feb 5, 2016 at 2:16 PM, Takenobu Tani 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