[Haskell-cafe] Confused about type seen in the wild

[Albert Y. C. Lai]

On 14-04-08 11:11 PM, Alexander Solla wrote:
> mathJaxField  ::  Monad  m
>
>               =>  RenderMessage  (HandlerSite  m)  FormMessage
>
>               =>  Field  m  MathJax
>
>
> Notice that there are two type arrows (=>).  Interestingly,
>
>
> mathJaxField  ::  (Monad  m
>
>
>                  ,  RenderMessage  (HandlerSite  m)  FormMessage
>
>
>                  )  =>  Field  m  MathJax
>
>
>
> compiles too.  So it looks like currying/uncurrying, at the type level.  But when did this start?  I'm using GHC 7.6.3

Yes, GHC accepts "X => Y => Z" and makes it "(X, Y) => Z". In fact, go 
wilder:

whee :: Show a => Eq a => a => Bool
whee x = x == x && null (show x)

It is valid and it means (Show a, Eq a) => a -> Bool. Try it!

My understanding is that when the type checker has to support all those 
generalizations like ConstraintKind, DataKinds, etc etc, it becomes so 
general that some traditional distinctions vanish.