The original expression is ill typed because, as you say, the type of the body of the alternative mentions the existentially-bound type varaible. Core Lint should jolly well reject it. (Maybe, though, it doesn't test for this?) In Haskell it would look like this data T where MkT :: forall a . a -> T f (MKT x) = x This is illegal. So if GHC is producing this program, please file a ticket Simon From: ghc-devs-bounces@haskell.org [mailto:ghc-devs-bounces@haskell.org] On Behalf Of Andrew Farmer Sent: 13 June 2013 04:20 To: ghc-devs@haskell.org Subject: exprType and existentials Calling `exprType` on: case enumFromStepN Int $fNumInt (I# 1) (I# 1) x of wild Stream s1 ostep t ds1 -> Left s1 ((Int, (Int, Int)), s1) t gives: Either s1 ((Int, (Int, Int)), s1) Which is problematic, as the type variable 's1' is unbound outside of the alternative RHS! Unfolding the scrutinee and case-reducing gives: Left (Int, Int) ((Int, (Int, Int)), (Int, Int)) ((,) Int Int (I# 1) (case x of n1 I# ipv ? n1)) Calling `exprType` now gives: Either (Int, Int) ((Int, (Int, Int)), (Int, Int)) That is, `s1` was `(Int, Int)` in this case. Is it expected that `exprType` behaves this way? Or should I file a ticket? I realize the type of the scrutinee (in this case Stream Int) gives no indication as to the existential type... and I'm not sure how you would get it without making exprType do some kind of evaluation, but it would be nice if exprType at least failed if it doesn't have enough information... Andrew