id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
7493	STM and TVar report incorrect results	parcs		"On Haskell Cafe, I posted:

I'm getting strange behavior when using the 'many' combinator to read zero or more items off of a TQueue with readTQueue. The script that exhibits this behavior is as follows:

{{{
import Control.Concurrent.STM
import Control.Concurrent
import Control.Monad
import Control.Applicative

main = do
    q <- newTQueueIO
    atomically $ writeTQueue q True
    atomically $ writeTQueue q False
    forever $ do
        xs <- atomically $ many $ readTQueue q
        print xs
        threadDelay 500000
}}}

I'd expect the output of the script to be:
{{{
[True,False]
[]
[]
...
}}}

However, that is not the case: the actual output of the script is:
{{{
[True,False]
[True,False]
[True,False]
...
}}}

If 1 element (say, True) is written into the TQueue instead of 2, then the output of the script is:
{{{
[True]
[]
[]
...
}}}

Which is expected behavior, but inconsistent with the behavior when the TQueue has 2 or more elements in it.

Is this considered a bug, or undocumented behavior of TQueue?

----

Bas vas Dijk noted that this may be a bug in STM, and provided a condensed test case which reproduces the behavior of my original script:

{{{
$ cat stmTest.hs
import Control.Concurrent.STM
main = do
  x <- atomically $ do
         t <- newTVar 1
         writeTVar t 2
         ((readTVar t >> retry) `orElse` return ()) `orElse` return ()
         readTVar t
  print x

$ ghc --make stmTest.hs -fforce-recomp -threaded -o stmTest && ./stmTest
[1 of 1] Compiling Main             ( stmTest.hs, stmTest.o )
Linking stmTest ...
1
}}}

The program prints 1 when it should print 2."	bug	closed	highest	7.6.2	Runtime System	7.6.1	fixed		v.dijk.bas@… felipe.lessa@…	Unknown/Multiple	Unknown/Multiple	Incorrect result at runtime	Unknown