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,,,,,