#16116: Explicit foralls in associated type family equations are oblivious to class-bound variables -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.7 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): That example is rejected with a `Not in scope: type variable ‘b’` error, before and after the patch, since `b` is simply not bound anywhere. A more interesting example is the one from `T5515`: {{{#!hs instance ctx a => Bome ctx where type BArg ctx = a }}} This time, `a` technically //is// in scope over the type family equation. However, it's not bound on the LHS of the equation, so it's rejected. After this patch, the new error message becomes: {{{#!diff diff -uw "indexed-types/should_fail/T5515.run/T5515.stderr.normalised" "indexed-types/should_fail/T5515.run/T5515.comp.stderr.normalised" --- indexed-types/should_fail/T5515.run/T5515.stderr.normalised 2019-01-09 11:08:03.005471816 -0500 +++ indexed-types/should_fail/T5515.run/T5515.comp.stderr.normalised 2019-01-09 11:08:03.005471816 -0500 @@ -1,8 +1,24 @@ -T5515.hs:9:3: - The RHS of an associated type declaration mentions out-of-scope variable ‘a’ - All such variables must be bound on the LHS +T5515.hs:6:16: + Expecting one more argument to ‘ctx’ + Expected a type, but ‘ctx’ has kind ‘* -> Constraint’ + In the first argument of ‘Arg’, namely ‘ctx’ + In the first argument of ‘ctx’, namely ‘(Arg ctx)’ + In the class declaration for ‘Bome’ -T5515.hs:15:3: - The RHS of an associated type declaration mentions out-of-scope variable ‘a’ - All such variables must be bound on the LHS +T5515.hs:14:1: + Type variable ‘a’ is mentioned in the RHS, + but not bound on the LHS of the family instance + In the type instance declaration for ‘Arg’ + In the instance declaration for ‘Some f’ + +T5515.hs:14:10: + Could not deduce (C f a0) + from the context: C f a + bound by an instance declaration: + forall f a. C f a => Some f + at T5515.hs:14:10-24 + The type variable ‘a0’ is ambiguous + In the ambiguity check for an instance declaration + To defer the ambiguity check to use sites, enable AllowAmbiguousTypes + In the instance declaration for ‘Some f’ }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16116#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler