#14332: Deriving clauses can have forall types -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.2.1 checker) | Resolution: | Keywords: deriving Operating System: Unknown/Multiple | Architecture: Type of failure: GHC accepts | Unknown/Multiple invalid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): OK, so to review: * We need to write up [https://ghc.haskell.org/trac/ghc/ticket/14331#comment:8 the rationalisation] in the user manual. * If we are going to make it a proper feature (which it already is), we should not require silly parens. E.g. this example from the Description {{{ data D a = D deriving ((forall a. C a)) }}} works just as we intend, but should not require the second pair of parens -- a parser bug. * The second example from the Description is also OK {{{ data Proxy (a :: k) = Proxy deriving ((forall k2. (Generic1 :: (k2 -> Type) -> Constraint))) }}} As I put it in the rationalisation, `k` comes from the data type decl, and `k2` from the instance; but it's fine to instantiate `k` to `k2` in that instance decl. And that is just what happens. Worth an example in the user manual perhaps, after the one about `Functor Proxy`. Ryan, might you be able to do this? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14332#comment:14 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler