#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 RyanGlScott): Replying to [comment:5 goldfire]:
Can't we just type-check the generated code and then posit whatever constraints are necessary for that code to be accepted? That's what we do for `stock` instances, no?
Currently, no. The code for this is [http://git.haskell.org/ghc.git/blob/2e8463b232054b788b73e6551947a9434aa76009... inferConstraints] in `TcDerivInfer`. For stock deriving, it walks over each field of each constructor for a datatype, puts the class constraint on the field's type, and then simplifies. So `data Foo a b = Foo a Int deriving Show` becomes `(Show a, Show Int)` becomes `Show a`. As for why this is done this way, I'm not sure, but I do know that GHC figures out the instance context //before// ever generating the class method implementations (possibly for typechecking purposes). -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12814#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler