#14331: Overzealous free-floating kind check causes deriving clause to be rejected -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: merge Priority: normal | Milestone: 8.2.2 Component: Compiler (Type | Version: 8.2.1 checker) | Resolution: | Keywords: deriving Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: GHC rejects | Test Case: valid program | deriving/should_compile/T14331 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): I'm afraid I mostly disagree with comment:28. Your first example is {{{#!hs -- C :: Type -> Type -> Constraint data D k (a :: k) = ... deriving (forall x. C x) }}} I think we get an instance {{{#!hs instance forall x k (a :: k). (...) => C x (D k a) }}} No instantiation is needed to get `D k a` to have kind `Type`. But I'm not terribly bothered here -- I don't think the general argument is wrong, just the example. In the next example (with `C2`), I'm not sure the kind of `C2`. Perhaps it's `forall k -> k -> Type -> Constraint`? But even so, we can get {{{#!hs instance ... => C2 k a (D k a) }}} without getting hurt. Regardless, I don't see how this argument refutes comment:27. The example I was considering was {{{#!hs class C a b data D a = D deriving (C (a :: k)) }}} where the `k` in the `deriving` clause is ''not'' mentioned in the data declaration. Thus the concerns in comment:28 don't apply. The `a` ''is'' mentioned, but it needs no unification. I do agree that this is hard to nail down. It seems, though, that one way forward is to say that any variables explicitly mentioned in the `deriving` clause are ''not'' free for unification. Other variables in the data decl but ''not'' in the deriving clause can be unified. This is a little strange perhaps, but not hard to articulate or understand. I do think it's worth asking if anyone ever really needs variables shared between the data decl and the `deriving` (Simon's last point in comment:28). -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14331#comment:29 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler