[GHC] #13788: TypeInType fails to compile old code

5 Jun 2017


      #13788: TypeInType fails to compile old code
-------------------------------------+-------------------------------------
           Reporter:  br1            |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1-rc2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 {{{
 {-#  LANGUAGE TypeInType              #-}

 module PP where

 newtype Field l v   =   Field { value :: v }

 label  ::  Field l v -> l
 label  =   undefined

 class HEq x
 hEq :: HEq x => x -> Int
 hEq = undefined

 class HListGet r where
     hListGet :: r -> Int
 instance
     HEq l => HListGet (Field l v) where
     hListGet f = hEq (label f)
 }}}

 fails with

 {{{
 mini.hs:18:29: error:
     • Couldn't match type ‘k’ with ‘*’
       ‘k’ is a rigid type variable bound by
         the instance declaration at mini.hs:17:5-33
       Expected type: Field * l v
         Actual type: Field k l v
     • In the first argument of ‘label’, namely ‘f’
       In the first argument of ‘hEq’, namely ‘(label f)’
       In the expression: hEq (label f)
     • Relevant bindings include
         f :: Field k l v (bound at mini.hs:18:14)
         hListGet :: Field k l v -> Int (bound at mini.hs:18:5)
    |
 18 |     hListGet f = hEq (label f)
    |
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13788
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