#11008: Difficulties around inferring exotic contexts -------------------------------------+------------------------------------- Reporter: crockeea | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 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 crockeea): With the deriving clause, which instance does GHC try to create? 1. instance (Eq (Foo r)) => Eq (Bar r) 2. instance (C (F r), Eq r) => Eq (Bar r) If option 1, GHC has no business inspecting the constraints on the `Foo r` instance. If option 2, why? Is it due to [https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/deriving.htm... #newtype-deriving slower instances]? In either case, since I *have* a matching `Eq` instance for `Foo`, GHC should assume that I knew what I was doing when I wrote it, and just use it. This doesn't change the behavior when no matching instance is found, like in your example. Whatever the result, the docs definitely need to be clearer. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11008#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler