[GHC] #8554: kindFunResult panic when type checking wrongly-kinded function

[GHC]

#8554: kindFunResult panic when type checking wrongly-kinded function
-------------------------------------+-------------------------------------
       Reporter:  patrick_thomson    |             Owner:
           Type:  bug                |            Status:  new
       Priority:  normal             |         Milestone:
      Component:  Compiler (Type     |           Version:  7.6.3
  checker)                           |  Operating System:  MacOS X
       Keywords:                     |   Type of failure:  Compile-time
   Architecture:  x86_64 (amd64)     |  crash
     Difficulty:  Unknown            |         Test Case:
     Blocked By:                     |          Blocking:
Related Tickets:                     |
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 While working on some writing about recursion schemes and morphisms, I
 wrote the following well-known catamorphism:

 {{{
 import Control.Arrow
 newtype Fix f = Fix { unFix :: f (Fix f) }
 bottomUp f = unFix >>> fmap (bottomUp f) >>> Fix >>> f
 }}}

 On a lark, I decided to reverse the direction of the arrows in the above
 statement and see what the typechecker had to say about it:


 {{{
 nonsense f = unFix <<< fmap (nonsense f) <<< Fix <<< f
 }}}

 This function is (I think) not well-kinded, but during type checking (both
 from ghci and ghc) I get the following error:


 {{{
 ghc: panic! (the 'impossible' happened)
   (GHC version 7.6.3 for i386-apple-darwin):
         kindFunResult
 <<details unavailable>>

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

 I am running 32-bit GHC 7.6.3 (from the Haskell Platform) on an x86_64
 machine running OS X 10.9. I have attached the output of ghc -v.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8554>
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