#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 RyanGlScott): Why should `C` be illegal? The type parameters of `T` are a permutation of a proper subset of the class parameters (`a` and `k`), so it [https://wiki.haskell.org/GHC/Type_families#Associated_family_declarations_2 should be legal]. But moreover, I think the answer to my question in comment:14 is: you can't newtype-derive `C` for `MyInt`, as there's an associated type that //doesn't mention the last type parameter of `C`//. This is crucial, because if this doesn't hold, then we won't have a `T` instance to fall back on, making the whole derivation crumble. We'd just need to check for this condition beforehand. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8165#comment:16 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler