[GHC] #12089: :kind command allows unsaturated type family,

19 May 2016


      #12089: :kind command allows unsaturated type family,
-------------------------------------+-------------------------------------
           Reporter:  Iceland_jack   |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 {{{#!hs
 data A = B

 type family
   F a where
   F B = String

 infixr 9 `Compose`
 class    (f (g x)) => (f `Compose` g) x
 instance (f (g x)) => (f `Compose` g) x
 }}}

 GHCi accepts

 {{{
 ghci> :kind Compose Eq F
 Compose Eq F :: A -> Constraint
 }}}

 but when used in code it gets rejected

 {{{
 -- tvI6.hs:13:8-33: error: …
 --     • The type family ‘F’ should have 1 argument, but has been given
 none
 --     • In the type signature:
 --         foo :: (Eq `Compose` F) a => String
 -- Compilation failed.
 foo :: (Eq`Compose`F) a => String
 foo = undefined
 }}}

 Bug? Version 8.0.0.20160511

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12089
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