#12975: Suggested type signature for a pattern synonym causes program to fail to type check -------------------------------------+------------------------------------- Reporter: ocharles | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.0.1 checker) | Keywords: Resolution: | PatternSynonyms Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by mpickering): * keywords: patternsynonyms => PatternSynonyms Comment: The type signature you are looking for is... {{{#!hs pattern Cast :: Typeable b => b -> T pattern Cast a <- MkT (cast -> Just a) }}} `cast` has type `(Typeable a, Typeable b) => a -> Maybe b`, as `MkT` provides `Typeable a` for us, we require `Typeable b` in order to use the function. The inferred type is still wrong in HEAD. Thanks! -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12975#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler