#8165: Use GeneralizedNewtypeDeriving to automatically create associated type families -------------------------------------+------------------------------------- Reporter: MikeIzbicki | Owner: goldfire Type: feature request | Status: new Priority: normal | Milestone: 8.2.1 Component: Compiler (Type | Version: 7.6.3 checker) | Resolution: | Keywords: TypeFamilies Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #2721 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): The recipe gave in comment:4 doesn't work for the example in comment:14. I could elaborate the recipe a bit thus: {{{ class C x y z where type T y z x op :: x -> [y] -> z newtype N a = MkN <rep-type> deriving( C ) -- Here the deriving clause would generate instance C x y <rep-type> => C x y (N a) where type T y (N a) x = T y <rep-type> x op = coerce (op :: x -> [y] -> <rep-type>) }}} The recipe for generating the `type instance` is to replace the occurrence of `z` (which must occur) with `<rep-type>` on the LHS and RHS. I don't think it need be the last parameter of `T`, as the example shows. I'm far from sure if this is really worth the effort of explaining and implementing it. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8165#comment:18 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler