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 b
I would expect to be able to actually use the Test Constraint in a function, like so:
foo :: Test a => a -> a
foo = id
But GHC complains that:
Illegal polymorphic or qualified type: Test a
In the type signature for ‘foo’: foo :: Test a => a -> a
I 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 | |
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Blocking | |
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