[Haskell-cafe] BlockedIndefinitelyOnMVar

[Massimo Zaniboni]

Hi,

I were discussing the problem on #haskell-it , and they noted that the 
correct exception is thrown changing from

```
   wait c
   wait p
```

to

```
   wait p
   wait c
```

More importantly, this code

```
   () <$ (waitAny [c, p])
```

and this

```
   () <$ (waitAny [p, c])
```

works in the expected way. So it seems that ``waitAny`` is a lot more 
robust, and extensible to 2 or more threads.

I'm not interested to the final result of threads, so ``waitAny`` it is 
a good solution, but in the real code (not this example) I have many 
nested threads, so I need to create a unique ``waitAny`` on the main 
caller, that is not optimal, but probably viable.

So I'm still interested, if there are better solutions, not requring a 
`waitAny` with all launched threads.

Regards,
Massimo

Il 13/07/2018 16:51, Massimo Zaniboni ha scritto:
> Hi,
> 
> I have a code like this
> 
> ```
> producer :: Int -> MVar Int -> IO ()
> producer c mvar
>    = case c > 10 of
>        True -> throwIO HighNumberException
>        False -> do putMVar mvar (c + 1)
>                    producer (c + 1) mvar
> 
> consumer :: MVar Int -> IO ()
> consumer mvar = do
>    _ <- takeMVar mvar
>    consumer mvar
> 
> coordinator :: MVar Int -> IO ()
> coordinator mvar = do
>    withAsync (producer 0 mvar) $ \p ->
>      withAsync (consumer mvar) $ \c -> do
>        wait c
>        wait p
> 
> main :: IO ()
> main = do
>    mvar <- newEmptyMVar
>    withAsync (coordinator mvar) (wait)
>    putStrLn "done"
> ```
> 
> The producer send the informative exception HighNumberException, but the 
> coordinator job receives a generic BlockedIndefinitelyOnMVar, because 
> the producer stop working.
> 
> How can I discard the generic exception, and re-throw/catch only 
> HighNumberException?
> 
> In case the complete source code is on 
> https://github.com/massimo-zaniboni/blocked-on-mvar
> 
> Thanks in any case,
> Massimo