#14331: Overzealous free-floating kind check causes deriving clause to be rejected -------------------------------------+------------------------------------- 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 rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): Replying to [comment:2 goldfire]:
I assume you really meant
{{{#!hs data D = D deriving (forall k a. C (a :: k)) }}}
but that GHC doesn't quantify `k` the right way.
I most certainly didn't. (In fact, I've opened a [https://ghc.haskell.org/trac/ghc/ticket/14332 separate bug] about the fact that you //can// put `forall`s in `deriving` clauses, which horrifies me.) It's somewhat surprising, but `deriving` clauses can bind type variables themselves. Note that this is currently accepted: {{{#!hs data D = D deriving (C a) }}} Here, `a` isn't bound by the data type `D`, so it is in fact bound in the `deriving` clause. By the same principle, `deriving (C (a :: k))` should be allowed, and the free-floating kind check is mistaken to reject it. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14331#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler