#9582: Associated Type Synonyms do not unfold in InstanceSigs -------------------------------------+------------------------------------- Reporter: | Owner: andreas.abel | Status: patch Type: bug | Milestone: Priority: normal | Version: 7.8.3 Component: Compiler | Keywords: InstanceSigs (Type checker) | TypeFamilies Resolution: | Architecture: Unknown/Multiple Operating System: | Difficulty: Unknown Unknown/Multiple | Blocked By: Type of failure: | Related Tickets: None/Unknown | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by andreas.abel): Thanks for the help! Now I use PatSigOrigin to swap expected/actual type in the error message generated by tcSubType. {{{#!hs -- Check that type provided in the type signature -- is both a sub- and a super-type of the type -- originating from the method signature in the class. -- As a consequence, the types are equal, and we can discard -- the coercions. (Keep fingers crossed.) let ctOrigin = AmbigOrigin userTypeCtxt void $ tcSubType ctOrigin userTypeCtxt sig_ty local_meth_ty -- In case the provided type is more general than the expected -- type, we take care that the error reports actual and -- expected type correctly. -- Using PatSigOrigin does the job, but it is a bit hacky. void $ tcSubType PatSigOrigin userTypeCtxt local_meth_ty sig_ty }}} This does the job, however, is a bit hacky. I tried a more principled solution with TypeEqOrigin, {{{#!hs let ctOrigin' = TypeEqOrigin { uo_actual = sig_ty , uo_expected = local_meth_ty } void $ tcSubType ctOrigin' userTypeCtxt local_meth_ty sig_ty }}} but except for PatSigOrigin, instances of CtOrigin are ignore in tcSubType, or rather, converted to a standard TypeEqOrigin (with expected/actual ordered not in the way I want to). See TcUnify.lhs: {{{#!hs tcSubType :: CtOrigin -> UserTypeCtxt -> TcSigmaType -> TcSigmaType -> TcM HsWrapper -- Check that ty_actual is more polymorphic than ty_expected -- Both arguments might be polytypes, so we must instantiate and skolemise -- Returns a wrapper of shape ty_actual ~ ty_expected tcSubType origin ctxt ty_actual ty_expected ... unify ty_actual ty_expected ... where -- In the case of patterns we call tcSubType with (expected, actual) -- rather than (actual, expected). To get error messages the -- right way round we have to fiddle with the origin unify ty1 ty2 = uType u_origin ty1 ty2 where u_origin = case origin of PatSigOrigin -> TypeEqOrigin { uo_actual = ty2, uo_expected = ty1 } _other -> TypeEqOrigin { uo_actual = ty1, uo_expected = ty2 } }}} This could be changed of course, bit there is a principal risk of breaking something. In particular, would it not make sense to preserve TypeEqOrigin? {{{#!hs TypeEqOrigin{} -> u_origin }}} If you give your OK, I implement this to make my fix of #9582 cleaner. Working fix on https://github.com/andreasabel/ghc/commit/29a2f93a26f714b79a59617d20b73b2e16... some cleaning needed. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9582#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler