[Haskell-cafe] wanted: Function to break circular data dependencies
joeyadams3.14159 at gmail.com
Sun May 4 18:46:39 UTC 2014
Job isn't trying to solve the halting problem, but to catch a specific type
of infinite loop--an expression that demands itself in its own evaluation.
fix (\n -> n+1) :: Int
This produces an infinite loop (or throws NonTermination) because to
evaluate n, you have to evaluate n and then do something to the result.
On Sun, May 4, 2014 at 12:36 PM, Corentin Dupont
<corentin.dupont at gmail.com>wrote:
> Not all recursive functions are infinite loops... In general it's
> impossible to detect an infinite loop: it's the "Halting Problem".
> IMO, the only way to do that is with a watchdog. You launch the evaluation
> in a separate thread, watch it, and if it doesn't finish you kill it and
> return a default value.
> But maybe it's also possible to forbid any recursive program by analysing
> the AST?
> I've used a watchdog in Nomyx (inspired from Mueval):
> --Sets a watchdog to kill the evaluation thread if it doesn't finishes.
> -- The function starts both the evaluation thread and the watchdog thread,
> and blocks awaiting the result.
> -- Option 1: the evaluation thread finishes before the watchdog. The MVar
> is filled with the result,
> -- which unblocks the main thread. The watchdog then finishes latter, and
> fills the MVar with Nothing.
> -- Option 2: the watchdog finishes before the evaluation thread. The eval
> thread is killed, and the
> -- MVar is filled with Nothing, which unblocks the main thread. The
> watchdog finishes.
> evalWithWatchdog' :: NFData a => IO a -> IO (Maybe a)
> evalWithWatchdog' s = do
> mvar <- newEmptyMVar
> hSetBuffering stdout NoBuffering
> --start evaluation thread
> id <- forkOS $ do
> s' <- s
> let s'' = force s'
> putMVar mvar (Just s'')
> --start watchdog thread
> forkIO $ watchDog 3 id mvar
> takeMVar mvar
> -- | Fork off a thread which will sleep and then kill off the specified
> watchDog :: Int -> ThreadId -> MVar (Maybe a) -> IO ()
> watchDog tout tid mvar = do
> threadDelay (tout * 1000000)
> killThread tid
> putMVar mvar Nothing
> On Sun, May 4, 2014 at 6:00 PM, Job Vranish <job.vranish at gmail.com> wrote:
>> Is a function like the following possible?:
>> avoidCircularDataDependency :: a -> a -> a
>> avoidCircularDataDependency a b = ?
>> I want avoidCircularDataDependency to evaluate 'a', but if in the process
>> of evaluating 'a' its own result is demanded (which normally would result
>> in an infinite loop) it returns 'b' otherwise it returns 'a' .
>> I've often found myself wanting a function like this. It would make
>> certain kinds of knot-tying/cycle detection _much_ easier.
>> Is there any reason why this function can't/shouldn't exist?
>> - Job
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>> Haskell-Cafe at haskell.org
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