#10873: Bad error message for incorrect pattern synonym signature -------------------------------------+------------------------------------- Reporter: mpickering | Owner: Type: bug | Status: new Priority: normal | Milestone: 8.0.1 Component: Compiler (Type | Version: 7.10.2 checker) | Resolution: | Keywords: newcomer Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Description changed by mpickering: Old description:
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.
New description: 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#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler