[GHC] #2595: Implement record update for existential and GADT data types

[GHC]

#2595: Implement record update for existential and GADT data types
--------------------------------+-------------------------------------------
    Reporter:  simonpj          |       Owner:             
        Type:  feature request  |      Status:  new        
    Priority:  normal           |   Milestone:  6.12 branch
   Component:  Compiler         |     Version:  6.8.3      
    Severity:  normal           |    Keywords:             
  Difficulty:  Unknown          |    Testcase:             
Architecture:  Unknown          |          Os:  Unknown    
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 Ganesh writes: The most important thing for me is supporting record update
 for
 existentially quantified data records, as in the error below.

 In general I also find working with code that involves existential
 type variables quite hard work - for example I can't use foo as a
 record selector either, even if I immediately do something that seals
 the existential type back up again.

 I don't understand this stuff well enough to be sure whether it's an
 impredicativity issue or not, though.

 {{{
 Foo.hs:11:8:
     Record update for the non-Haskell-98 data type `Foo' is not (yet)
 supported
     Use pattern-matching instead
     In the expression: rec {foo = id}
     In the definition of `f': f rec = rec {foo = id}
 }}}
 Program is:
 {{{
 {-# LANGUAGE Rank2Types #-}
 module Foo where

 data Foo = forall a . Foo { foo :: a -> a, bar :: Int }

 x :: Foo
 x = Foo { foo = id, bar = 3 }

 f :: Foo -> Foo
 f rec = rec { foo = id }

 g :: Foo -> Foo
 g rec = rec { bar = 3 }
 }}}

 Simon says: Ah now I see.  The relevant comment, `TcExpr` line 465, says
 {{{
 -- Doing record updates on
 -- GADTs and/or existentials is more than my tiny
 -- brain can cope with today
 }}}
 Should be fixable, even with a tiny brain.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2595>
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