[GHC] #1683: strange MVar / UnsafePerformIO (and type checker...) behaviour

[GHC]

#1683: strange MVar / UnsafePerformIO (and type checker...) behaviour
-------------------------------+--------------------------------------------
  Reporter:  komuves at renyi.hu  |          Owner:                                    
      Type:  bug               |         Status:  new                               
  Priority:  normal            |      Milestone:                                    
 Component:  Compiler          |        Version:  6.6.1                             
  Severity:  normal            |       Keywords:  MVar, UnsafePerformIO, typechecker
Difficulty:  Unknown           |             Os:  Unknown                           
  Testcase:                    |   Architecture:  Unknown                           
-------------------------------+--------------------------------------------
Compare the following two code snippets:

 {{{
 import Data.Word
 import Control.Concurrent.MVar
 import System.IO.Unsafe

 a :: MVar Int
 a = unsafePerformIO newEmptyMVar

 b :: MVar Word8
 b = unsafePerformIO newEmptyMVar

 main1 = do
   putMVar a (-5)
   putMVar b (-127)
   x <- readMVar a
   y <- readMVar b
   print (x,y)
 }}}

 and (I omitted the imports here for brevity):

 {{{
 gvar :: MVar a
 gvar = unsafePerformIO newEmptyMVar

 a :: MVar Int
 a = gvar

 b :: MVar Word8
 b = gvar

 main = do
   putMVar a (-5)
   x <- readMVar a
   y <- readMVar b
   print (x,y)
 }}}

 The first version works as intended (and prints `(-5,129)`). However, what
 happens in the second version, that it compiles without any warnings, but
 `a` and `b` becomes the same thing (prints `(-5,-5)`), even though I
 declared them to have ''different types''! I'm not sure what would be the
 correct behaviour, but I reckon this is not the correct one.

 The effect is the same on 6.6.1 and 6.7.20070830, and was tested on an x86
 machine running Mac OS X.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1683>
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