[Haskell-cafe] wanted: Function to break circular data dependencies

[Ben Franksen]

Doesn't ghc detect this with "black holes" or something? Maybe just catch 
http://hackage.haskell.org/package/base-4.7.0.0/docs/Control-Exception.html#t:NonTermination ?

Cheers
Ben

Joey Adams wrote:
> 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. Example:
> 
>     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
>> thread.
>> 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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>>
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-- 
"Make it so they have to reboot after every typo." -- Scott Adams