[GHC] #10114: Kind mismatches with AnyK in rank-2 types

24 Feb 2015

      #10114: Kind mismatches with AnyK in rank-2 types
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              Reporter:  cam         |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.8.4
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
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 The following module:

 {{{#!hs
 {-# LANGUAGE Rank2Types #-}
 module KindBug where

 type T = forall f a. f a

 foo :: T
 foo = undefined

 bar :: forall f a. f a
 bar = foo
 }}}

 ...produces the following error:

 {{{
 [1 of 1] Compiling KindBug          ( KindBug.hs, KindBug.o )

 KindBug.hs:10:7:
     Kind incompatibility when matching types:
       f0 :: AnyK -> *
       f :: * -> *
     Expected type: f a
       Actual type: f0 a0
     Relevant bindings include bar :: f a (bound at KindBug.hs:10:1)
     In the expression: foo
     In an equation for ‘bar’: bar = foo
 }}}

 Note that no extensions are enabled other than rank-2 types.

 Enabling and using kind annotations fixes the error, as does replacing the
 type synonym with its definition in the type signature of foo.

 Similar errors can also be generated with PolyKinds enabled and less
 plausible code, e.g. {{{ (return undefined :: forall f a. f a) :: f a }}}.
 The non-PolyKinds example is simplified from some actual code, where the
 type synonym was similar to the ones used in lens, and is the simplest
 example I could find (with help from Shachaf on IRC) that resembled real
 code.

 I searched for existing tickets and the closest I could find are #7916 and
 #5935, both of which are already fixed.

 Also, according to glguy (Eric Mertens) just now in #haskell-lens, the
 same error occurs in the 7.10 branch and in 7.11.20150120.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10114
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