[GHC] #7658: Support empty record update syntax

[GHC]

#7658: Support empty record update syntax
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Reporter:  glguy             |          Owner:                  
    Type:  feature request   |         Status:  new             
Priority:  normal            |      Component:  Compiler        
 Version:  7.6.2             |       Keywords:                  
      Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
 Failure:  None/Unknown      |      Blockedby:                  
Blocking:                    |        Related:                  
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 Empty record update syntax would be useful for when you are changing
 phantom type parameters. It is currently explicitly disallowed, but it
 isn't clear if that is absolutely necessary to me.

 {{{
 data R phantom = C { a,b :: Int }

 r ::  R Int
 r = C 1 2

 --This is OK
 r1 :: R Char
 r1 = r { a = a r }

 --Why not this?
 r2 :: R Char
 r2 = r {}
 }}}

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7658>
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