#4815: Instance constraints should be used when deriving on associated data types -------------------------------------+------------------------------------- Reporter: batterseapower | Owner: Type: feature request | Status: new Priority: low | Milestone: Component: Compiler | Version: 6.12.3 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 RyanGlScott): I would also be inclined to say "use the standalone route", for the reason that I don't see any reason why the `deriving` clause would infer `Eq (Associated a)` given the current state of things. I was merely imagining a hypothetical scenario in which data family instances //would// reasonable infer such a constraint when deriving instances. If data family instances were more tightly intermingled with their parent instances in terms of instance contexts, I could envision this being the case. P.S. Do you know why GHC decides `DList [] :: D [a]` instead of `DList [] :: Eq a => D [a]`? That is, why GHC chose to make associated data family instance contexts separate from their parent instances' contexts? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/4815#comment:12 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler