The type variable ‘a0’ is ambiguous

Vote count:

0

I don't understand why the following code won't compile:

{-# LANGUAGE GADTs, ScopedTypeVariables #-}

data A = A

class C a where
  c :: a -> Bool

instance C A where
  c _ = True

data D a where
  D :: C a => D a

toBool :: D a -> Bool
toBool D = c (undefined::a)

Here is the error message:

Could not deduce (C a0) arising from a use of ‘c’
from the context (C a)
  bound by a pattern with constructor
             D :: forall a. C a => D a,
           in an equation for ‘toBool’
  at test.hs:15:8
The type variable ‘a0’ is ambiguous
Note: there is a potential instance available:
  instance C A -- Defined at test.hs:8:10
In the expression: c (undefined :: a)
In an equation for ‘toBool’: toBool D = c (undefined :: a)

Can someone explain what's going on ?                                        

asked 34 secs ago

John Smith

1

The type variable ‘a0’ is ambiguous