GHC instantiates levity-polymorphic type variables with foralls

Consider the following use of error.

ghci> :t error
error
  :: forall (v :: GHC.Types.Levity) (a :: TYPE v).
     (?callStack::GHC.Stack.Types.CallStack) =>
     [Char] -> a

ghci> :t error "bad" :: (forall a. a -> a) -> a
error "bad" :: (forall a. a -> a) -> a
  :: (?callStack::GHC.Stack.Types.CallStack) =>
     (forall a1. a1 -> a1) -> a

GHC is instantiating the type variable a in the output with (forall a. a -> a) -> a, which requires impredicative polymorphism. So this example should actually be rejected by the type checker.

Note that GHC also does this with open-kinded type variables, so the behavior with error and undefined has been around for a while. There are tests that depend on it, and a Note that describes the decision to accept these programs. Still, Simon PJ says perhaps we should really just reject these programs.

So here's a ticket to discuss the matter.

For more information, see the discussion starting at https://phabricator.haskell.org/D1739#51408.

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	goldfire, simonpj
Operating system
Architecture