#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 goldfire): This is by design. GHC refuses to infer so-called exotic contexts, believing that sufficiently exotic contexts should be written by the user. This behavior is independent of whether GND or other `deriving` features are used. Here is an extreme example: {{{ data X a b = MkX (a -> b) deriving Eq }}} This fails, complaining about a missing `Eq (a -> b)` instance. (My choice of `data` vs. `newtype` is utterly irrelevant here.) But I can write this: {{{ deriving instance Eq (a -> b) => Eq (X a b) }}} Should GHC infer the context automatically here? I think not -- it masks a deeper problem. Now, returning to the original post: should GHC infer that context? It looks just like the example I just gave, where the context requires an `Eq` constraint on some other type constructor. It all boils down to yet another knob we can turn in the internals of GHC. It's a very easy knob to turn, with no interactions throughout the code. See the relevant function [https://github.com/ghc/ghc/blob/8f5ad1a009eddd05447ff8057792b4d03983cd35/com... here], with a Note directly above. If you can suggest a better setting for the knob, go right ahead. :) Regardless, we should document this in the manual somewhere, as it is all user-facing. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11008#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler