#14046: “Illegal type synonym family application in instance” is too strict in the presence of functional dependencies -------------------------------------+------------------------------------- Reporter: lexi.lambda | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: #3485 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by AntC):
GHC rejects the following instance, complaining about the use of a type synonym family in the instance head ...
? This isn't a bug. GHC has never allowed type family applications in the instance head. (With FlexibleInstances, it does allow type synonyms.) #3485 is an enhancement request (and closed, for good reasons IMO). So the FunctionalDependency is a red herring. I agree with your suggestion on lens#754 the Template Haskell for lenses should be smarter. If there's an opportunity for an enhancement, I think there's a more general suggestion. Borrow the irrefutable patterns idea like: {{{ instance HasValue (Rec a) ~(Fam a) where ... }}} And allow `~( ... )` to be any type or family application. Treat it as syntactic sugar for: {{{ instance (b0 ~ Fam a) => HasValue (Rec a) b0 where ... -- b0 is fresh }}} Also there's an opportunity for the compiler to see that instance (as compared with others): a. doesn't really overlap b. is consistent with FunDeps c. doesn't break the Coverage Condition -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14046#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler