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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 :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 closed normal 7.10.1 Compiler (Type checker) 7.1 wontfix ImpredicativeTypes ganesh chowells79@… dimitris@… p.giarrusso@… Linux x86_64 (amd64) GHC rejects valid program