#7891: Can't write type signature for top-level polymorphic pattern binding --------------------------------------+------------------------------------- Reporter: MartijnVanSteenbergen | Owner: Type: bug | Status: new Priority: normal | Component: Compiler (Type checker) Version: 7.6.3 | Keywords: Os: Unknown/Multiple | Architecture: Unknown/Multiple Failure: GHC rejects valid program | Blockedby: Blocking: | Related: --------------------------------------+------------------------------------- Hi, In GHC 7.4.1 and 7.6.3 I can't write a type signature for a top-level polymorphic pattern binding. Consider this program: {{{ {-# LANGUAGE RankNTypes #-} newtype T = T (forall t. t -> t) tf :: T tf = T id -- Can't write this type signature: -- f :: t -> t T f = tf -- But with an indirection we can: g :: t -> t g = f -- We can still use f as it were fully polymorphic (which is good): a :: () a = f () b :: Char b = f 'b' }}} I expect to be able to specify a type for f. The same applies for a data family constructor, which is my original use case. I don't think it matters much, but here is an similar test case that uses a data family: {{{ {-# LANGUAGE RankNTypes #-} {-# LANGUAGE TypeFamilies #-} class C t where data F t :: * mkF :: t -> F t instance C () where data F () = FUnit (forall t. t -> t) mkF () = FUnit id -- Can't write a type for f here either: FUnit f = mkF () }}} -- Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/7891 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler