#12814: Should GND infer an instance context when deriving method-free classes? -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: Type: bug | Status: new Priority: normal | Milestone: 8.2.1 Component: Compiler (Type | Version: 8.0.1 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #11369, #12810 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): The rationale is this. * For 'deriving' clauses we need to figure out what the context of the derived instance declaration should be. * What we ''actually'' do is this: we gather together all the constraints that ''will'' be required, and simplify them to make a minimal instance context. * We could instead (a) generate the method bindings etc, (b) typecheck them, (c) figuring out the constraints that are needed to do so, and minimise them (d) turn all that into an instance decl (e) typecheck the instance decl with all its methods. But typechecking instance decls is already fairly complicated and (a-c) sounds tricky to me. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12814#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler