#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

#14132: Report an error for a missing class instance before an error for type family instances of an associated type. -------------------------------------+------------------------------------- Reporter: duog | Owner: duog 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 duog): I had a go at this yesterday, but I ran into a problem. There is a comment on report1: {{{ -- report1: ones that should *not* be suppresed by -- an insoluble somewhere else in the tree -- It's crucial that anything that is considered insoluble -- (see TcRnTypes.trulyInsoluble) is caught here, otherwise -- we might suppress its error message, and proceed on past -- type checking to get a Lint error later }}} This means moving `ReporterSpec`s between report1 and report2 doesn't work as is. It would be possible to make it work by adding some complexity to the suppression logic; make `ReporterSpec`s in report2 unable to suppress `ReporterSpec`s in report1. Alternatively, I think I see how to suppress the equality error in favour of the class instance error: The tuples in report1 and report2 are of type `ReporterSpec`: {{{ type Reporter = ReportErrCtxt -> [Ct] -> TcM () type ReporterSpec = ( String -- Name , Ct -> PredTree -> Bool -- Pick these ones , Bool -- True <=> suppress subsequent reporters , Reporter) -- The reporter itself }}} In `tryReporter`, the current list of unsatisfiable constraints is partitioned with the 2nd member of the tuple. The matching constraints are passed to the `Reporter`, and the non-matching constraints are returned for the next iteration of `tryReporter`. I propose to change the definition to: {{{ type Reporter = ReportErrCtxt -> [Ct] -> TcM [Ct] type ReporterSpec = ( String -- Name , Bool -- True <=> suppress subsequent reporters , Reporter) -- The reporter itself }}} Now the partitioning will be done inside the `Reporter`, and we can write something like: {{{ associated_type_eq :: Reporter associated_type_eq ctxt cts = let associated_type_cts = [ (tf_ct, dict_cts) | tf_ct <- cts , EqPred NomEq ty1 _ <- [classifyPredType (ctPred ct)] , (FamilyTyCon{famTcParent = Just cls}, tf_tys) <- ["some function 1" ty1] , let dict_cts = [ ct' | ct' <- cts , ClassPred cls' cls_tys <- [classifyPredType (ctPred ct)] , cls' == cls && tf_tys "some function 2" cls_tys ] ] yeses = [x | (tf_ct, dict_cts) <- associated_type_cts , x <- tf_ct : dict_cts ] noes = filter (`notElem` yeses) cts in do mkGroupReporter mkDictErr ctxt $ concat [dict_cts | (_, dict_cts) <- associated_type_cts] return noes }}} where "some function 1" gives the type constructor and it's arguments, and "some function 2" checks that this associated type comes from this instance. These functions must exist right? Do you know of any other cases that could be improved by this more powerful factoring? This would affect tracing, since currently in `tryReporter` the partitioning is done, and if the predicate matches anything `tryReporter` traces the `Ct`s that matched. With the partitioning inside the `Reporter` this doesn't work anymore. Perhaps the `Reporter` could also return the yeses. Please let me know whether you think this is a good idea, otherwise I will go ahead and do the simpler option of adding some complexity to the suppression logic. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14132#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#14132: Report an error for a missing class instance before an error for type family instances of an associated type. -------------------------------------+------------------------------------- Reporter: duog | Owner: duog 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: | -------------------------------------+------------------------------------- Changes (by duog): * Attachment "T12170a.comp.stderr" added. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14132 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#14132: Report an error for a missing class instance before an error for type family instances of an associated type. -------------------------------------+------------------------------------- Reporter: duog | Owner: duog 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): Ah. That turned out to be a separate bug: #14149. I'll commit a fix for it shortly. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14132#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler