[GHC] #15545: Forced to enable TypeInType because of (i ~ i)

20 Aug 2018


      #15545: Forced to enable TypeInType because of (i ~ i)
-------------------------------------+-------------------------------------
           Reporter:  Iceland_jack   |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.5
           Keywords:  TypeInType     |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 I don't know if this is a bug but `i ~ i` holds by reflexivity so I would
 not have expected it to require `TypeInType`

 {{{
 $ ghci -ignore-dot-ghci
 GHCi, version 8.5.20180128: http://www.haskell.org/ghc/  :? for help
 Prelude> :set -XPolyKinds -XGADTs
 Prelude> import Data.Kind
 Prelude Data.Kind> data NP :: (k -> Type) -> (i -> Type) where NP :: f a
 -> NP f a

 <interactive>:3:45: error:
     • Data constructor ‘NP’ constrains the choice of kind parameter:
         i ~ i
       Use TypeInType to allow this
     • In the definition of data constructor ‘NP’
       In the data type declaration for ‘NP’
 Prelude Data.Kind>
 }}}

 I would rather expect the warning I get after enabling `TypeInType`

 {{{
 Prelude Data.Kind> :set -XTypeInType
 Prelude Data.Kind> data NP :: (k -> Type) -> (i -> Type) where NP :: f a
 -> NP f a

 <interactive>:8:28: error:
     • Couldn't match ‘k’ with ‘i’
     • In the data declaration for ‘NP’
 }}}

 ----

 '''ps''' making sure this is OK as well; it works after enabling
 `-XTypeInType` and quantifying with `-XRankNTypes` (is this using
 polymorphic recursion?)

 {{{
 Prelude Data.Kind> :set -XTypeInType -XRankNTypes
 Prelude Data.Kind> data NP :: forall i k. (k -> Type) -> (i -> Type) where
 NP :: f a -> NP f a
 Prelude Data.Kind> :t NP
 NP :: forall i (f :: i -> *) (a :: i). f a -> NP f a
 }}}

 it also works for

 {{{#!hs
 data NP :: forall k. (k -> Type) -> (i -> Type) where NP :: f a -> NP f a
 data NP :: forall i. (k -> Type) -> (i -> Type) where NP :: f a -> NP f a
 data NP :: (k -> Type) -> forall i. (i -> Type) where NP :: f a -> NP f a
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15545
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