[GHC] #11485: Very unhelpful message resulting from kind mismatch

23 Jan 2016

      #11485: Very unhelpful message resulting from kind mismatch
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           Reporter:  RyanGlScott    |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1-rc1
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Other
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 The following code:

 {{{#!hs
 module Foo where

 import Data.Typeable

 tyConOf :: Typeable a => Proxy a -> TyCon
 tyConOf = typeRepTyCon . typeRep

 tcList :: TyCon
 tcList = tyConOf (Proxy :: Proxy [])
 }}}

 fails because `-XPolyKinds` is not enabled. But the error message that you
 get is quite different on GHC 7.10 and 8.0.

 On GHC 7.10.3:

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

 Foo.hs:9:19:
     Couldn't match kind ‘* -> *’ with ‘*’
     Expected type: Proxy a0
       Actual type: Proxy []
     In the first argument of ‘tyConOf’, namely ‘(Proxy :: Proxy [])’
     In the expression: tyConOf (Proxy :: Proxy [])
     In an equation for ‘tcList’: tcList = tyConOf (Proxy :: Proxy [])
 }}}

 But on GHC 8.0.1-rc1:

 {{{
 Foo.hs:9:19: error:
     • Expected kind ‘Proxy []’,
         but ‘Data.Proxy.Proxy :: Proxy []’ has kind ‘Proxy []’
     • In the first argument of ‘tyConOf’, namely ‘(Proxy :: Proxy [])’
       In the expression: tyConOf (Proxy :: Proxy [])
       In an equation for ‘tcList’: tcList = tyConOf (Proxy :: Proxy [])
 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11485
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