[GHC] #11310: Surprising accepted constructor for GADT instance of data family

28 Dec 2015

      #11310: Surprising accepted constructor for GADT instance of data family
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           Reporter:  dfeuer         |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.3
           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
 -- Accepted:
 data family Foo (x :: *) :: * -> *
 data instance Foo Int Char where
   Foo :: Foo Int Char

 -- Not accepted
 data Bar Char where
   Bar :: Bar Char
 }}}

 It seems the second example, using a simple GADT, is syntactically barred
 from having a specific type constructor in its "head". That syntactic
 restriction is relaxed, however, for ''all'' arguments of the data
 instance, even though I'd expect it only for the first argument.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11310
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