Re: Error when ($) is used, but no error without

[Brian Hulley]

Robin Bate Boerop wrote:

This code compiles properly (with -fglasgow-exts on GHC 6.4.1):

class CC a
type C x = CC a => a x
f, g :: C a -> Int
f _ = 3
g x = f x

But, this code:

class CC a
type C x = CC a => a x
f, g :: C a -> Int
f _ = 3
g x = f $ x  -- the only change

gives this error:

    Inferred type is less polymorphic than expected
      Quantified type variable `a' escapes
      Expected type: a a1 -> b
      Inferred type: C a1 -> Int
    In the first argument of `($)', namely `f'
    In the definition of `g': g x = f $ x

What's going on here?

I think the type declaration is actually equivalent to:

type C x = forall a. CC a => a x

so that you are declaring:

f,g :: (forall a. CC a => a Int) -> Int -- not allowed

instead of:

f,g :: forall a. (CC a => a Int ->Int)

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