#8567: Poor error message when GeneralizedNewtypeDeriving fails -------------------------------------+------------------------------------ Reporter: goldfire | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.7 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 simonpj): There are two parts to this: * It would be dead easy to generate a better error message for an unsolved constraint `Coerce Int Bool`. Rather than generating a generic type-class error message, it'd be easy to produce a message about the inability to coerce `Int` to `Bool`. Error messages are generated by `TcErrors`. We already generate special error messages for implicit parameters, which are already type-class constraints; `?x:Int` is short for `IP "x" Int`. * The "arising from a use of corece" part is also generated in `TcErrors`. Again, it'd be easy to change the form of the message for a call to `coerce`. * A bit harder to omit the `In the expression`, `In the definition of meth` parts. They might even be helpful, if we explain in the manual how `newtype deriving` actually works (via `corece`). I'd be happy to advise Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8567#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler