#12883: Unexpected constraint when using ExistentialQuantification -------------------------------------+------------------------------------- Reporter: danilo2 | Owner: Type: bug | Status: new Priority: high | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: GHC rejects Unknown/Multiple | valid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Hello! Consider this simple code: {{{ data Elem t = Typeable t => Elem Int tst1 :: forall a. Elem a -> TypeRep tst1 (Elem _) = typeRep (Proxy :: Proxy a) }}} It works and it should work. `Elem` is created only to be sure that if we've got a value of type `Elem t` we don't have to check that `t` is `Typeable`. <offtopic> By the way, exystential newtypes would be so awesome in GHC! </offtopic> And now a strange thing. If we create another function: {{{ tst2 :: forall a. Elem a -> TypeRep tst2 _ = typeRep (Proxy :: Proxy a) }}} GHC throws error that there is no `Typeable a` constraint. The only difference between `tst1` and `tst2` is different pattern binding. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12883 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler