#5462: Deriving clause for arbitrary classes -------------------------------------+------------------------------------- Reporter: simonpj | Owner: dreixel Type: feature | Status: new request | Milestone: ⊥ Priority: normal | Version: 7.2.1 Component: Compiler | Keywords: Generics Resolution: | Architecture: Unknown/Multiple Operating System: | Difficulty: Unknown Unknown/Multiple | Blocked By: Type of failure: | Related Tickets: #7346 None/Unknown | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by rwbarton): Well, I was imagining that you would simply reduce the constraints using whatever instances are in scope, just like when inferring the type of a top-level definition. However, now I see that this falls apart in the presence of non-uniform recursion. Quite annoying... but in this case there seems to potentially be a genuine choice involved, so it might be sensible to just reject "deriving" then and require the user to write their own instance head. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/5462#comment:32 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler