[Haskell-cafe] multi-thread and lazy evaluation
Hi all,
I have a program where the user can submit his own little programs, which
are interpreted using Hint. The user-submitted programs are used to modify
a state held in a TVar.
As of course those user-submitted programs can't be trusted, I'm trying to
protect them, like in Mueval.
I installed a watchdog to monitor and kill the user's thread if it doesn't
finish. However it doesn't work properly, due to lazy evaluation I believe.
I made a little exemple to illustrate the problem.
- The following program doesn't terminate, but if you uncomment the
putStrLn at the end, it will.
Could someone explain me this and how to do it properly??
Merry Christmas to all
Corentin
*
{-# LANGUAGE ScopedTypeVariables #-}
import Control.Concurrent.STM.TVar
import Control.Concurrent.MVar
import Control.Concurrent
import Control.Monad.STM
data MyData = MyData { a :: String, b :: String } deriving (Show)
main = do
tv - atomically $ newTVar $ MyData a b
protectedExecCommand tv
myNewData - atomically $ readTVar tv
putStrLn $ show myNewData
protectedExecCommand :: (TVar MyData) - IO ()
protectedExecCommand tv = do
mv - newEmptyMVar
before - atomically $ readTVar tv
id - forkIO $ execBlocking mv
forkIO $ watchDog' 5 id mv
res - takeMVar mv
case res of
Nothing - (atomically $ writeTVar tv before)
Just after - (atomically $ writeTVar tv after)
watchDog' :: Int - ThreadId - MVar (Maybe x) - IO ()
watchDog' t tid mv = do
threadDelay $ t * 100
killThread tid
putStrLn $ process timeout
tryPutMVar mv Nothing
return ()
execBlocking :: MVar (Maybe MyData) - IO ()
execBlocking mv = do
let (a::String) = a
--If you uncomment the next line, it will work
--putStrLn $ show a
putMVar mv (Just $ MyData a toto)*
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Re: [Haskell-cafe] multi-thread and lazy evaluation
Sorry, I'm thinking my example program wasn't maybe too explicit.
In it, the line *let (a::String) = a* represents the program submitted by
the user, that is faulty.
The objective is to stop it after some time, and set the (TVar MyData) to
its previous value.
As you can see, it works only if I put a putStrLn in the same thread to
force the evaluation
*{-# LANGUAGE ScopedTypeVariables #-}
import Control.Concurrent.STM.TVar
import Control.Concurrent.MVar
import Control.Concurrent
import Control.Monad.STM
data MyData = MyData { a :: String, b :: String } deriving (Show)
main = do
tv - atomically $ newTVar $ MyData a b
protectedExecCommand tv
myNewData - atomically $ readTVar tv
putStrLn $ show myNewData
protectedExecCommand :: (TVar MyData) - IO ()
protectedExecCommand tv = do
mv - newEmptyMVar
before - atomically $ readTVar tv
id - forkIO $ execBlocking mv
forkIO $ watchDog' 5 id mv
res - takeMVar mv
case res of
Nothing - (atomically $ writeTVar tv before)
Just after - (atomically $ writeTVar tv after)
watchDog' :: Int - ThreadId - MVar (Maybe x) - IO ()
watchDog' t tid mv = do
threadDelay $ t * 100
killThread tid
putStrLn $ process timeout
tryPutMVar mv Nothing
return ()
execBlocking :: MVar (Maybe MyData) - IO ()
execBlocking mv = do
let (a::String) = a
--If you uncomment the next line, it will work
--putStrLn $ show a
putMVar mv (Just $ MyData a toto)*
On Mon, Dec 24, 2012 at 1:17 PM, Corentin Dupont
corentin.dup...@gmail.comwrote:
Hi all,
I have a program where the user can submit his own little programs, which
are interpreted using Hint. The user-submitted programs are used to modify
a state held in a TVar.
As of course those user-submitted programs can't be trusted, I'm trying to
protect them, like in Mueval.
I installed a watchdog to monitor and kill the user's thread if it doesn't
finish. However it doesn't work properly, due to lazy evaluation I believe.
I made a little exemple to illustrate the problem.
- The following program doesn't terminate, but if you uncomment the
putStrLn at the end, it will.
Could someone explain me this and how to do it properly??
Merry Christmas to all
Corentin
*
{-# LANGUAGE ScopedTypeVariables #-}
import Control.Concurrent.STM.TVar
import Control.Concurrent.MVar
import Control.Concurrent
import Control.Monad.STM
data MyData = MyData { a :: String, b :: String } deriving (Show)
main = do
tv - atomically $ newTVar $ MyData a b
protectedExecCommand tv
myNewData - atomically $ readTVar tv
putStrLn $ show myNewData
protectedExecCommand :: (TVar MyData) - IO ()
protectedExecCommand tv = do
mv - newEmptyMVar
before - atomically $ readTVar tv
id - forkIO $ execBlocking mv
forkIO $ watchDog' 5 id mv
res - takeMVar mv
case res of
Nothing - (atomically $ writeTVar tv before)
Just after - (atomically $ writeTVar tv after)
watchDog' :: Int - ThreadId - MVar (Maybe x) - IO ()
watchDog' t tid mv = do
threadDelay $ t * 100
killThread tid
putStrLn $ process timeout
tryPutMVar mv Nothing
return ()
execBlocking :: MVar (Maybe MyData) - IO ()
execBlocking mv = do
let (a::String) = a
--If you uncomment the next line, it will work
--putStrLn $ show a
putMVar mv (Just $ MyData a toto)*
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Re: [Haskell-cafe] multi-thread and lazy evaluation
On Mon, Dec 24, 2012 at 8:45 AM, Corentin Dupont corentin.dup...@gmail.comwrote: *execBlocking :: MVar (Maybe MyData) - IO () execBlocking mv = do let (a::String) = a --If you uncomment the next line, it will work --putStrLn $ show a putMVar mv (Just $ MyData a toto)* It's laziness, yes; you need to do something along the lines of let a = length a `seq` a or possibly Control.Exception.evaluate needs to be involved somewhere. -- brandon s allbery kf8nh sine nomine associates allber...@gmail.com ballb...@sinenomine.net unix, openafs, kerberos, infrastructure, xmonadhttp://sinenomine.net ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] multi-thread and lazy evaluation
The real question is, does this mean that GHC is stopping the world every time it puts an MVar? Tim -- Původní zpráva -- Od: Brandon Allbery allber...@gmail.com Datum: 24. 12. 2012 Předmět: Re: [Haskell-cafe] multi-thread and lazy evaluation On Mon, Dec 24, 2012 at 8:45 AM, Corentin Dupont corentin.dup...@gmail.com (mailto:corentin.dup...@gmail.com) wrote: execBlocking :: MVar (Maybe MyData) - IO () execBlocking mv = do let (a::String) = a --If you uncomment the next line, it will work --putStrLn $ show a putMVar mv (Just $ MyData a toto) It's laziness, yes; you need to do something along the lines of let a = length a `seq` a or possibly Control.Exception.evaluate(http://Control.Exception.evaluate) needs to be involved somewhere. -- brandon s allbery kf8nh sine nomine associates allber...@gmail.com(mailto:allber...@gmail.com) ballb...@sinenomine.net(mailto:ballb...@sinenomine.net) unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net (http://sinenomine.net) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] multi-thread and lazy evaluation
On Mon, 2012-12-24 at 16:16 +0100, timothyho...@seznam.cz wrote: The real question is, does this mean that GHC is stopping the world every time it puts an MVar? No, GHC rts only locks the MVar itself. See here: http://hackage.haskell.org/trac/ghc/browser/rts/PrimOps.cmm#L1358 Yuras ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
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Re: [Haskell-cafe] Substituting values
On Sat, Dec 22, 2012 at 12:35 AM, Kim-Ee Yeoh k...@atamo.com wrote: This is reminiscent of the Either (exception) monad where Left values, the exceptions, pass through unaltered, and Right values are transformable, i.e. acted on by functions. Interesting. I hadn't thought of that parallel. But yes, I think it's a similar notion of: transform the value only in one (or more) cases. It's also similar to the Maybe monad in that I'm looking for something akin to fromMaybe, but without having to place the value in a Maybe. But I have no idea what you're trying to achieve in the bigger picture. Help us help you by fleshing out your use case. I'm not sure the bigger picture is helpful, but, for example, the last case was something like: if ch == '\n' then ' ' else ch I think I ended up rewriting it as something like: replace '\n' = ' ' replace ch = ch Which is straightforward enough, though it feels to me like there's too much syntax involved. Thanks, Alvaro ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Substituting values
Yeah, I guess I was wondering whether something like it already existed.
Thinking about it some more, perhaps what I want is `tr` [1]. E.g.
tr [Foo] [Bar]
Incorporating your suggestion would then yield a more general version like:
trBy (== Foo) Bar
[1] http://en.wikipedia.org/wiki/Tr_(Unix)
On Sat, Dec 22, 2012 at 6:46 AM, David Thomas davidleotho...@gmail.comwrote:
Seems like the function is easy to define:
replaceIfEq a b c = if c == a then b else c
Then the above can be written
replaceIfEq Foo Bar value
Or the slightly more general (in exchange for slightly more verbosity at
the call site)
replaceIf p r a = if p a then r else a
replaceIf (== Foo) Bar value
On Fri, Dec 21, 2012 at 6:46 PM, Radical radi...@google.com wrote:
Sometimes I'll need something like:
if value == Foo then Bar else value
Or some syntactic variation thereof:
case value of { Foo - Bar; _ - value }
Is there a better/shorter way to do it? I'm surprised that it's more
complicated to substitute a value on its own than e.g. in a list, using
filter. Or perhaps I'm missing the right abstraction?
Thanks,
Alvaro
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