Bug in unification of polymorphic and not-yet-polymorphic type
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.
Trac metadata
Trac field | Value |
---|---|
Version | 7.1 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler (Type checker) |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |