Copying ghc-devs. Oleg says: | Sorry for reporting a problem via e-mail. It seems track no | longer accepts anonymous (guest) submissions. When I tried to register | the account I was told that my submission is a spam. The tracker is | really well protected. Can anyone help him? I'll open a ticket. Simon | -----Original Message----- | From: oleg@okmij.org [mailto:oleg@okmij.org] | Sent: 16 June 2014 11:46 | To: Simon Peyton Jones | Subject: Regression in the typechecker in GHC 7.8.2 | | | Hello! | | Sorry for reporting a problem via e-mail. It seems track no | longer accepts anonymous (guest) submissions. When I tried to register | the account I was told that my submission is a spam. The tracker is | really well protected. | | Here is the problem: what used to type check in GHC 7.4.1 (and I think | in 7.6.2, although I no longer have access to that version) fails in GHC | 7.8.2. | | The following program type-checks with GHC 7.4.1 and GHC 7.8.2: | | {-# LANGUAGE RankNTypes #-} | {-# LANGUAGE TypeFamilies #-} | | module T where | | foo :: (forall f g. (Functor f) => f a -> f b) -> [a] -> [b] | -- foo :: (forall f g. (Functor f, g ~ f) => g a -> g b) -> [a] -> [b] | foo tr x = tr x | | t = foo (fmap not) [True] | | The following code (which differs only in the signature of foo) | | {-# LANGUAGE RankNTypes #-} | {-# LANGUAGE TypeFamilies #-} | | module T where | | -- foo :: (forall f g. (Functor f) => f a -> f b) -> [a] -> [b] foo :: | (forall f g. (Functor f, g ~ f) => g a -> g b) -> [a] -> [b] foo tr x = | tr x | | t = foo (fmap not) [True] | | | type-checks with 7.4.1 but not with 7.8.2. The latter reports the error | | Couldn't match type `b' with `Bool' | `b' is untouchable | inside the constraints (Functor f, g ~ f) | bound by a type expected by the context: | (Functor f, g ~ f) => g Bool -> g b | at /tmp/t.hs:12:5-25 | `b' is a rigid type variable bound by | the inferred type of t :: [b] at /tmp/t.hs:12:1 | Expected type: Bool -> b | Actual type: Bool -> Bool | Relevant bindings include t :: [b] (bound at /tmp/t.hs:12:1) | In the first argument of `fmap', namely `not' | In the first argument of `foo', namely `(fmap not)' | | Giving t the type signature [Bool] fixes the problem. Alas, I come | across the similar untouchable error in situations where giving the type | signature is quite difficult (in local bindings, with quite large | types). | | Sorry again for the off-line report, | Oleg