#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): Replying to [comment:33 goldfire]:
{{{#!hs class C a data D a = D deriving C }}}
We see that `C :: forall k. k -> Constraint`. So we have `ki` = `forall k. k -> Constraint`. We then instantiate (note new step, above) to get `ki` = `kappa3 -> Constraint`, where `kappa3` is a fresh unification variable. Unifying with `kappa -> Constraint` simply sets `kappa := kappa3`. So, `ki2` is really just `kappa3`. This is the special case in (d).
OK. My question is: why do we need to special-case this at all? After all, in step (i) we unify the kind of `ty` with `ki2`, and the kind of `ty` will always be of the form `... -> Type` by virtue of the kind of `ty` coming from a data type. So we achieve the same effect without needing a special case at all. (In fact, this is currently what the implementation of `TcDeriv` does.) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14331#comment:34 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler