#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 RyanGlScott): Thanks Richard, this looks wonderful. I have some questions/comments about your algorithm: 3. What happens if the user writes an explicit `forall` in a `deriving` clause but leaves out a variable, such as in `data Foo a = ... deriving (forall b. C b c)`? Also, in what sense is this not a "full-blooded `forall`"? 5. c. I'm not sure I understand this business about `kappa2` (or why you'd choose `kappa2 := Type`). Can you give an example of where this would arise? 5. e. Some more validity checks that would need to be performed for data family instances are documented [http://git.haskell.org/ghc.git/blob/7ac22b73b38b60bc26ad2508f57732aa17532a80... here]. 5. h. I was hoping for a little more detail on this skolem business. For instance, we'd want to reject `data D = D deriving (forall k. C k)` (where we have `class C k (a :: k)`) since the `k` is skolem, yes? (Disclaimer: due to my lack of experience with typechecker terminology, I'm guessing that "skolem" is a fancy way of saying "can't unify with other types". Please correct me if I'm off-track here.) Currently, this just says "replacing any unfilled unification variable with fresh skolems", which doesn't give me a sense of //where// the skolems are coming from. Also, does the skolemicity only kick in if you write an explicit `forall`? Or would the `k` in `data D = D deriving (C k)` also get skolemized? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14331#comment:32 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler