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

[Andres Löh]

I also find this scary. Even this works:

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

But:

Prelude> let test :: (Eq a) => a => b => a; test = undefined

<interactive>:35:13:
    Illegal polymorphic or qualified type: (b) => a
    Perhaps you intended to use -XRankNTypes or -XRank2Types
    In the type signature for `test': test :: Eq a => a => b => a
Prelude> :set -XRankNTypes
Prelude> let test :: (Eq a) => a => b => a; test = undefined

<interactive>:8:13:
    Illegal constraint: b (Use -XConstraintKinds to permit this)
    In the type signature for `test': test :: Eq a => a => b => a

So this is what I'd also expect for "a" to happen. I think this is a bug.

Cheers,
  Andres

-- 
Andres Löh, Haskell Consultant
Well-Typed LLP, http://www.well-typed.com

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