ImpredicativeTypes and ConstraintKinds don't interact as expected
Given this code:
{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, ImpredicativeTypes, ConstraintKinds #-}
class P a b
instance P a b
type Test a = forall b. P a bI would expect to be able to actually use the Test Constraint in a function, like so:
foo :: Test a => a -> a
foo = idBut GHC complains that:
Illegal polymorphic or qualified type: Test a
In the type signature for ‘foo’: foo :: Test a => a -> aI get the exact same error message if I replace ImpredicativeTypes with RankNTypes. If this is intended behavior under ImpredicativeTypes, how difficult would it be to add another extension that allows this type of polymorphism?
Trac metadata
| Trac field | Value |
|---|---|
| Version | 7.8.2 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture |