#15050: ScopedTypeVariables could allow more programs -------------------------------------+------------------------------------- Reporter: nomeata | Owner: (none) Type: feature | Status: new request | Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Consider {{{ data P a = P data T1 a where MkT1 :: forall a. P a -> T1 a MkT2 :: forall a. P a -> T1 (a,a) MkT3 :: forall a b. b ~ Int => P a -> P b -> T1 a MkT4 :: forall a b. P a -> P b -> T1 a MkT5 :: forall a b c. b ~ c => P a -> P b -> P c -> T1 a }}} I can write this function {{{ foo :: T1 (Int, Int) -> () foo (MkT1 (P::P (Int,Int))) = () foo (MkT2 (P::P x)) = (() :: x ~ Int => ()) foo (MkT3 P (P::P Int)) = () foo (MkT4 P (P::P b)) = () foo (MkT5 P (P::P b) (P::P b)) = () }}} but this these two equations fail {{{ foo (MkT1 (P::P (Int,x))) = (() :: x ~ Int => ()) foo (MkT1 (P::P x)) = (() :: x ~ (Int,Int) => ()) }}} I am especially surprised by the second one, given that the very similar equation with `MkT2` works. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15050 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler