#10873: Bad error message for incorrect pattern synonym signature -------------------------------------+------------------------------------- Reporter: mpickering | Owner: Type: bug | Status: new Priority: normal | Milestone: 8.0.1 Component: Compiler | Version: 7.10.2 (Type checker) | Keywords: newcomer | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Revisions: | -------------------------------------+------------------------------------- Consider the following program {{{ {-# LANGUAGE PatternSynonyms #-} pattern Pat :: Show a => () => a -> Maybe a pattern Pat a = Just a }}} GHC complains that {{{ test.hs:4:9: No instance for (Show a) arising from a pattern }}} I think this is quite difficult to understand. The problem is that matching on `Just a` doesn't provide the show constraint (it provides no constraints). A better error message here would explain this fact and maybe a short explanation of the difference between prov/req in a pattern synonym signature. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10873 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler