id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
4347	Bug in unification of polymorphic and not-yet-polymorphic type	dolio		"The new type checker in GHC 7 seems to reject some cases of impredicative instantiation that prior versions allowed. I was initially alerted to this by Simon Marlow, who sent a patch for vector-algorithms removing a use of {{{($)}}} where it would have to be instantiated impredicatively.

Initially, I thought this was due to a planned removal of impredicativity, but this cannot be the case, because:

{{{
const :: a -> (forall b. b) -> a
}}}

is accepted by the type checker. However, the simple:

{{{
id :: (forall a. a) -> (forall b. b)
}}}

is not, giving an error message:

{{{
    Couldn't match type `b' with `forall a. a'
      `b' is a rigid type variable bound by
          an expression type signature at <interactive>:1:32
    In the expression: id :: (forall a. a) -> (forall b. b)
}}}

This would seem to indicate that the type is being rewritten to:

{{{
forall b. (forall a. a) -> b
}}}

and then the {{{forall a. a}}} matched with the bare {{{b}}}. It is, of course, fine to rewrite the type this way, since the two are isomorphic, but it is unfortunate that it causes the checker to reject what would otherwise be a valid instantiation."	bug	new	normal	7.6.2	Compiler (Type checker)	7.1			ganesh chowells79@… dimitris@… p.giarrusso@…	Linux	x86_64 (amd64)	GHC rejects valid program