Re: [GHC] #9612: Use functional dependencies to give more specific error messages

#9612: Use functional dependencies to give more specific error messages -------------------------------------+------------------------------------- Reporter: rwbarton | Owner: Type: feature | Status: new request | Milestone: Priority: normal | Version: 7.8.3 Component: Compiler | Keywords: (Type checker) | Architecture: Unknown/Multiple Resolution: | Difficulty: Unknown Operating System: | Blocked By: Unknown/Multiple | Related Tickets: Type of failure: | None/Unknown | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by Simon Peyton Jones ): In [changeset:"20632d37b5bcb68bb0ca34238f1ed49c7be3a8f7/ghc"]: {{{ #!CommitTicketReference repository="ghc" revision="20632d37b5bcb68bb0ca34238f1ed49c7be3a8f7" Do not discard insoluble Derived constraints This is preparing for a fix to Trac #9612. The idea is that insoluble constraints are nice solid errors that we should not discard before we have a chance to report them. So TcRnTypes.dropDerivedWC now keeps insoluble Derived constrains, and instead TcSimplify.solve_wanteds filters them out We get somewhat better error message for kind-equality failures too. A slight downside is that to avoid *duplicate* kind-equality failures when we float a kind-incompatible equality (e.g. alpha:* ~ Int#), I've disabled constraint-floating when there are insolubles. But that in turn makes a handful of error messages a little less informative; good examples are mc21, mc22, mc25. But I am re-jigging the constraint floating machinery in another branch, which will make this go back to the way it was before. }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9612#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler