id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
7328	"Seemingly inappropriate ""expecting one more argument"" error in polykinded class"	illissius		"Triggered by the following code:

{{{
{-# LANGUAGE PolyKinds, GADTs #-}

data Proxy a

class Foo a where
    foo :: a ~ f i => Proxy (Foo f)
}}}

GHC writes:

{{{
test.hs:6:34:
    Expecting one more argument to `f'
    In the type `a ~ f i => Proxy (Foo f)'
    In the class declaration for `Foo'
}}}

It seems to me that given Foo :: forall k. k -> Constraint, Proxy :: forall k. k -> *, a :: k, f :: k1 -> k, that Foo in Proxy (Foo f) should be instantiated at k1 -> k, and Proxy at Constraint, and that this should work."	bug	closed	normal		Compiler	7.6.1	fixed			Unknown/Multiple	Unknown/Multiple	GHC rejects valid program	Unknown	polykinds/T7328