#4815: Instance constraints should be used when deriving on associated data types -------------------------------------+------------------------------------- Reporter: batterseapower | Owner: Type: feature request | Status: new Priority: low | Milestone: Component: Compiler | Version: 6.12.3 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 simonpj): I think it's more than just #11008. The question of this ticket is just "what context should GHC infer for a 'deriving' clause." But the ticket concerns the specific case of a `data instance` inside a `class` instance decl, thus: {{{ instance <context> => C (T a b) where data D (T a b) = D1 a | D2 b }}} Now arguably, whether or not the `deriving Eq` part ''needs'' `<context>` it would do no harm to add it, so we get the derived instance {{{ instance <context> => Eq (DT a b) where (==) = <blah> }}} (where `DT` is the representation data type for `D (T a b)`). The point is that `<context>` might help satisfy the `Eq` constraints arising from `<blah>`. Max argues that `<context>` must already be satisfied (because the class instance holds). But that's not really true {{{ f x y = D1 x == D2 y }}} Here we need equality on `DT` but we don't need `C (T a b)`. So `<context>` might be over-constraining. So I'm unconvinced that this is worth the bother. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/4815#comment:17 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler