#5412: dataTypeConstrs gives unhelpful error message -------------------------------------+------------------------------------ Reporter: NeilMitchell | Owner: klangner Type: bug | Status: patch Priority: normal | Milestone: 7.6.2 Component: libraries/base | Version: 7.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Comment (by klangner): Ok. I fixed 8 previous messages. But I'm not sure what to do with this code: Case 1. {{{ instance Data Char where toConstr x = mkCharConstr charType x gunfold _ z c = case constrRep c of (CharConstr x) -> z x _ -> error "Data.Data.gunfold(Char)" dataTypeOf _ = charType }}} This instance is defined for Char type. So it is not possible to pass wrong type here. But I guess there still could be a problem with constructor representation. There are similar instances for other data types as well. Lots of them. Case 2. In function: '''repConstr''' exception is thrown when parameters have different data types. So there is a need for another message. I would say something like: "Data.Data.repConstr requires the same data type for both parameters." or similar. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/5412#comment:14 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler