[GHC] #933: Separate compilation fails with existential records

[GHC]

#933: Separate compilation fails with existential records
-----------------------+----------------------------------------------------
  Reporter:  guest     |          Owner:         
      Type:  bug       |         Status:  new    
  Priority:  normal    |      Milestone:         
 Component:  Compiler  |        Version:  6.5    
  Severity:  normal    |     Resolution:         
  Keywords:            |             Os:  Unknown
Difficulty:  Unknown   |   Architecture:  Unknown
-----------------------+----------------------------------------------------
Old description:

> Consider M.hs:
>
> > {-# OPTIONS_GHC -fglasgow-exts #-}
> > module M where data D = forall a. C { l :: a }
>
> Then consider Main.hs:
>
> > import M
> > main = print () where x = C { l = () }
>
> A normal compilation works:
>
>     $ ghc --make Main.hs
>     [1 of 2] Compiling M                ( M.hs, M.o )
>     [2 of 2] Compiling Main             ( Main.hs, Main.o )
>     Linking Main ...
>
> However, separate compilation does not:
>
>     $ touch Main.hs
>     $ ghc --make Main.hs
>     [2 of 2] Compiling Main             ( Main.hs, Main.o )
>
>     Main.hs:2:26:
>         Constructor `C' does not have field `l'
>         In the expression: C {l = ()}
>         In the definition of `x': x = C {l = ()}
>         In the definition of `main':
>             main = print ()
>                  where
>                      x = C {l = ()}
>
> Where should I start looking to fix this? :-)
>
> Thanks,
> Audrey

New description:

 Consider M.hs:
 {{{
 > {-# OPTIONS_GHC -fglasgow-exts #-}
 > module M where data D = forall a. C { l :: a }
 }}}
 Then consider Main.hs:
 {{{
 > import M
 > main = print () where x = C { l = () }
 }}}
 A normal compilation works:
 {{{
     $ ghc --make Main.hs
     [1 of 2] Compiling M                ( M.hs, M.o )
     [2 of 2] Compiling Main             ( Main.hs, Main.o )
     Linking Main ...
 }}}
 However, separate compilation does not:
 {{{
     $ touch Main.hs
     $ ghc --make Main.hs
     [2 of 2] Compiling Main             ( Main.hs, Main.o )

     Main.hs:2:26:
         Constructor `C' does not have field `l'
         In the expression: C {l = ()}
         In the definition of `x': x = C {l = ()}
         In the definition of `main':
             main = print ()
                  where
                      x = C {l = ()}
 }}}
 Where should I start looking to fix this? :-)

 Thanks,
 Audrey

Comment (by simonpj):

 Audrey, when you added the "record fields in existentials" feature, you
 didn't arrange that the info is recorded in interface files.

 I discovered and fixed this when cleaning up IfaceSyn to add FC to GHC;
 but the fix is only in the HEAD.  The patch is not easy to incorporate in
 the branch,
 because the FC patch was very big.

 Fixing the 6.6 branch is possible, but would mean a change in interface
 file format, which we strongly discourage between patch-level releases.

 So I'm inclined to leave it alone in the branch, and encourage you to try
 the HEAD.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/933>
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