[GHC] #12842: DatatypeContexts in data types and data families

[GHC]

#12842: DatatypeContexts in data types and data families
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           Reporter:  Iceland_jack   |             Owner:
               Type:  task           |            Status:  new
           Priority:  lowest         |         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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 Does datatype contexts only apply for non-phantom variables? Figured I'd
 ask

 {{{#!hs
 {-# Language DatatypeContexts, TypeFamilies #-}

 -- F :: Foo a b
 data (a ~ b) => Foo a b = F
 }}}

 and

 {{{#!hs
 {-# Language DatatypeContexts, KindSignatures, DatatypeContexts,
 DataKinds, TypeFamilies #-}

 data N = O | S N

 -- FO :: Fin (S n) n' O
 data family   Fin (n::N) (a::N) (b::N)
 data instance (n ~ n') => Fin (S n)  n' O =  FO
 }}}

 It surprised me to see the first two of these (yes I know, datatype
 contexts):

 {{{#!hs
 -- F :: x -> Foo x x
 data (a ~ b) => Foo a b = F a

 -- F :: x -> Foo x x
 data (a ~ b) => Foo a b = F b

 -- F :: x -> x -> Foo x x
 data (a ~ b) => Foo a b = F a b
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12842>
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