#13582: Confusing error message with multiparameter type classes. -------------------------------------+------------------------------------- Reporter: | Owner: (none) facundo.dominguez | Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: type-checking | errors multiparameter type classes Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): Are you saying that GHC 7.8's error message is better because it mentions `b` rather than `b0`? HEAD also use `b0` rather than `b`. Really there are two unsolved constraint: {{{ [WD] $dFirst_a2yX {1}:: First Int b0_a2yW[tau:1] Int (CDictCan) [WD] $dTypeable_a2yY {1}:: Typeable b0_a2yW[tau:1] (CDictCan) }}} Both mention the same unification variable. I don't know why 7.8 reports it differently. But you are right. The Big Thing is that ''unification variables currently do not record their origin''. Would it be better to say this? {{{ b0 is a unification variable arising from instantiating the call to 'second' on line 18 }}} But actually even that is not right. `b0` arises from using the instance declaration to simplify the constraint `Second Int Int`, which itself arises from the call to `second`. What would you LIKE it to say? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13582#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler