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

[Jason Dagit]

On Thu, Apr 10, 2014 at 10:21 AM, Kim-Ee Yeoh <ky3 at atamo.com> wrote:

> No problems here:
>
> # ghci
> GHCi, version 7.6.3: http://www.haskell.org/ghc/  :? for help
> Loading package ghc-prim ... linking ... done.
> Loading package integer-gmp ... linking ... done.
> Loading package base ... linking ... done.
> Prelude> let whee :: Show a => Eq a => a => Bool; whee x = x == x && null
> (show x)
> Prelude> :t whee
> whee :: (Eq a, Show a) => a -> Bool
>

It seems to be an issue with giving everything on a single line:

$ ghci
GHCi, version 7.6.3: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Prelude λ> let whee x = x == x && null (show x) :: Show a => Eq a => a =>
Bool

<interactive>:2:14:
    Couldn't match expected type `a -> Bool' with actual type `Bool'
    In the expression:
        x == x && null (show x) :: Show a => Eq a => a => Bool
    In an equation for `whee':
        whee x = x == x && null (show x) :: Show a => Eq a => a => Bool

I see now to make it work as a one liner I could do it this way:
Prelude λ> :t (\x -> x == x && null (show x)) :: Show a => Eq a => a => Bool
(\x -> x == x && null (show x)) :: Show a => Eq a => a => Bool
  :: (Eq a, Show a) => a -> Bool

Thanks!
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