Non-sensical error message when compiling with PolyKinds and a type family

[goldfire]

When I compile the following code:

{-# LANGUAGE TypeFamilies,
             PolyKinds,
             ScopedTypeVariables
 #-}

convert :: a -> b
convert = undefined

type family Foo a 
type instance Foo Int = Bool

bar :: forall a b c dummya. (b -> c) -> ((Foo a) -> c)
bar f = (convert f :: (Foo a) -> c)

I get the following error message:

Sandbox.hs:13:34:
    Kind mis-match
    Expected kind `OpenKind', but `c' has kind `b'
    In an expression type signature: (Foo a) -> c
    In the expression: (convert f :: (Foo a) -> c)
    In an equation for `bar': bar f = (convert f :: (Foo a) -> c)

This error message does not make sense, for 'b' isn't a kind. It is interesting to note that removing dummya from the list of declared type variables changes the error to a GHC panic.

[igloo]

Thanks for the report. dreixel, if this is easy to fix we may as well get it into 7.4.1, although it's not actually a regression as PolyKinds?? is new.

[simonpj]

Moving to 7.6.1; PolyKinds is not advertised as working in 7.4, and merging fixes will be too destablising.

[simonpj]

Fixed by the massive polykinds patch

commit 3bf54e78cfd4b94756e3f21c00ae187f80c3341d
Author: Simon Peyton Jones <simonpj@microsoft.com>
Date:   Fri Mar 2 16:32:58 2012 +0000

    Hurrah!  This major commit adds support for scoped kind variables,
    which (finally) fills out the functionality of polymorphic kinds.
    It also fixes numerous bugs.