[GHC] #12086: Allow omitting type family signature

[GHC]

#12086: Allow omitting type family signature
-------------------------------------+-------------------------------------
           Reporter:  Iceland_jack   |             Owner:
               Type:  feature        |            Status:  new
  request                            |
           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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 Syntactic issue.

 I find myself writing

 {{{#!hs
 type family
   Not a where
   Not False = True
   Not True  = False
 }}}

 quite often, where the signature can be inferred it would be nice to be
 able to write ... to mean the same thing

 {{{#!hs
 type family
   Not False = True
   Not True  = False
 }}}

 I know Richard has
 [https://www.reddit.com/r/haskell/comments/4amov2/the_future_of_dependent_haskell/d14xtjs
 a lot of things planned], I don't know how they mesh together

 ----

 The desugaring is straightforward, for a type family with n-indexes, add a
 signature with n fresh type variables.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12086>
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