[GHC] #1204: Associated types don't work with record updates

[GHC]

#1204: Associated types don't work with record updates
-----------------------------------+----------------------------------------
    Reporter:  nominolo at gmail.com  |       Owner:         
        Type:  bug                 |      Status:  new    
    Priority:  normal              |   Milestone:         
   Component:  Compiler            |     Version:  6.7    
    Severity:  normal              |    Keywords:         
  Difficulty:  Unknown             |    Testcase:         
Architecture:  Unknown             |          Os:  Unknown
-----------------------------------+----------------------------------------
(I couldn't find a more descriptive title, feel free to change it)
 When instantiating associated types with records, record update doesn't
 seem to work (at least in indirect uses).  I think it should.  If this is
 supposed to fail, then a more helpful error message might be very useful.

 Here's the code:
 {{{
 {-# OPTIONS -findexed-types #-}
 module Bug where

 ------------------------------------------------------------------------------

 class C c where
   data D1 c

 class C c => D c where
   works :: Int -> D1 c -> D1 c
   buggy :: Int -> D1 c -> D1 c
   buggy2 :: Int -> D1 c -> D1 c

 ------------------------------------------------------------------------------

 data FooC = FooC

 instance C FooC where
   data D1 FooC = D1F { moo :: Int }

 instance D FooC where
   works x d = d  -- d unchanged, so OK

   buggy x d@(D1F { moo = k }) =
     d { moo = k + x }  -- d is updated wrong type--why?

   buggy2 x d@(D1F { moo = k }) =
     (d :: D1 FooC) { moo = k + x } -- type annotation doesn't work
 }}}

 GHC Output:
 {{{
 $ ghc67 -v -c Bug67.hs
 Glasgow Haskell Compiler, Version 6.7.20070303, for Haskell 98, compiled
 by GHC version 6.7.20070303
 Using package config file: /usr/local/lib/ghc-6.7.20070303/package.conf
 wired-in package base mapped to base-2.0
 wired-in package rts mapped to rts-1.0
 wired-in package haskell98 mapped to haskell98-1.0
 wired-in package template-haskell mapped to template-haskell-2.0
 Hsc static flags: -static
 Created temporary directory: /tmp/ghc422_0
 *** Checking old interface for main:Bug:
 *** Parser:
 *** Renamer/typechecker:

 Bug67.hs:25:4:
     Couldn't match expected type `D1 FooC'
            against inferred type `(Bug.:R2D1)'
     In the expression: d {moo = k + x}
     In the definition of `buggy':
         buggy x (d@(D1F {moo = k})) = d {moo = k + x}
     In the definition for method `buggy'

 Bug67.hs:28:4:
     Couldn't match expected type `D1 FooC'
            against inferred type `(Bug.:R2D1)'
     In the expression: (d :: D1 FooC) {moo = k + x}
     In the definition of `buggy2':
         buggy2 x (d@(D1F {moo = k})) = (d :: D1 FooC) {moo = k + x}
     In the definition for method `buggy2'
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1204>
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