#14132: Report an error for a missing class instance before an error for type family instances of an associated type. -------------------------------------+------------------------------------- Reporter: duog | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.2.1 checker) | Resolution: | Keywords: 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): Yes, I think it'd be possible to improve this. At the moment we actually ''suppress'' the class error because of the type-equality error. But with a little re-ordering in `TcErrors.reportWanteds` we could fix that. Then we'd at least report both. Suppressing the equality error because it involves an associated type would be significantly harder. But at least you'd get both. Specifically the code in `TcErrors` is this {{{ report1 = [ ("custom_error", is_user_type_error,True, mkUserTypeErrorReporter) , given_eq_spec , ("insoluble2", utterly_wrong, True, mkGroupReporter mkEqErr) , ("skolem eq1", very_wrong, True, mkSkolReporter) , ("skolem eq2", skolem_eq, True, mkSkolReporter) , ("non-tv eq", non_tv_eq, True, mkSkolReporter) , ("Out of scope", is_out_of_scope, True, mkHoleReporter) , ("Holes", is_hole, False, mkHoleReporter) -- The only remaining equalities are alpha ~ ty, -- where alpha is untouchable; and representational equalities -- Prefer homogeneous equalities over hetero, because the -- former might be holding up the latter. -- See Note [Equalities with incompatible kinds] in TcCanonical , ("Homo eqs", is_homo_equality, True, mkGroupReporter mkEqErr) , ("Other eqs", is_equality, False, mkGroupReporter mkEqErr) ] -- report2: we suppress these if there are insolubles elsewhere in the tree report2 = [ ("Implicit params", is_ip, False, mkGroupReporter mkIPErr) , ("Irreds", is_irred, False, mkGroupReporter mkIrredErr) , ("Dicts", is_dict, False, mkGroupReporter mkDictErr) ] }}} I think that moving the "non-tv-eq" line from `report1` into `report2` would do the job. (And maybe change that True to False.) The "Homo eqs" and "Other eqs" could move too. I'm swamped right now, and this would make lots of error messages in `validate` change slightly, each of which needs a check and accept. But maybe someone else can have a go? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14132#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler