Re: [GHC] #14331: Overzealous free-floating kind check causes deriving clause to be rejected

17 Oct 2017

      #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:                    |
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Comment (by simonpj):

 Ah!  I disagee with comment:20.  Let's start with this, after renaming:
 {{{
 class C a b
 data D c = D
   deriving (forall k. C (c :: k))
 }}}
 Now we typecheck the class and `data D` decls, but not yet the `deriving`
 part, yielding
 {{{
 class C {k1} {k2} (a:k1) (b:k2)
 data D {k3} (c:k3) = D
   deriving (forall k. C (c :: k))
 }}}
 Now we move on to typechecking the `deriving` clause.  Here we fail,
 becuase
 `c` has kind `k3` but its occurrence in the deriving clause claims it has
 kind `k`,
 where those two binders are quite different.  And that's just what the
 error message says.

 To make it work, write
 {{{
 data D (c::k) = D deriving( D c )
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14331#comment:26
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