Re: [Haskell-cafe] How to catch exception within the Get monad (the Binary package)
Henning, On Tue, Sep 7, 2010 at 12:51 PM, Henning Thielemann wrote: > This solution looks very ugly to me. Catching 'error's is debugging, but > parser failure is kind of exception handling. I guess, the errors you > want to catch are caused by non-supported fail method, right? Can't you > use a monad transformer like explicit-exception:Synchronous.Exception or > transformers:ErrorT around the Binary parser? Alexey already replied, but after looking at the way Binary processes parser errors I came to the same conclusion: it is by design that it falls hard when data cannot be deserialized (btw makes sense in many cases, but not in mine). I am beginning to realize that probably (de)serializing lazy structures is not a good idea at all since such structure may be consumed in pure code, and such parser errors are hidden inside, and may "fire" at any moment. Any complications to the binary parser will bring performance penalty. Or weird solutions like mine are needed. Thanks. -- Dimitry Golubovsky Anywhere on the Web ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to catch exception within the Get monad (the Binary package)
On Tue, Sep 7, 2010 at 2:45 PM, Dimitry Golubovsky wrote: > unThrow a = unsafePerformIO $ (E.evaluate a >>= return . Right) `E.catch` > (\e -> return $ Left e) > > -- or perhaps the right argument of catch could be just (return . Left)? > > bm2mb :: a -> Maybe a > > bm2mb a = case unThrow a of > Left (e::SomeException) -> Nothing > Right a -> Just a Philosophically these functions are Nasty because they violate referential transparency. In particular it's possible for the same expression to throw different exceptions each time it's run depending on how it's optimised, what other threads are doing, if the user presses ctrl-C, etc. etc. See the spoon package: http://hackage.haskell.org/package/spoon which alleviates this a little by only catching some kinds of exception, and not telling you which it caught. It still violates monotonicity (I believe), so purists will be upset, but practically it can be useful for when editing the source code to provide explicit exceptions (which is ideally what you'd do) is not an option. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to catch exception within the Get monad (the Binary package)
On 07.09.2010 20:51, Henning Thielemann wrote: This solution looks very ugly to me. Catching 'error's is debugging, but parser failure is kind of exception handling. I guess, the errors you want to catch are caused by non-supported fail method, right? Can't you use a monad transformer like explicit-exception:Synchronous.Exception or transformers:ErrorT around the Binary parser? ErrorT is useless here since it cannot intercept calls to 'error'. Same should be the case with ExcepcionalT. There is also performance penalty ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to catch exception within the Get monad (the Binary package)
Dimitry Golubovsky schrieb: > Hi, > > Thanks to everybody who replied. > > I see another solution: are there any hidden problems? > > I found an interesting package, ChasingBottoms which contains a > function testing a value to be bottom and returning a Boolean (of > course it cannot be done without unsafePerformIO). > > I borrowed the idea from that package, and wrote two functions: > > unThrow :: (Exception e) => a -> Either e a > > unThrow a = unsafePerformIO $ (E.evaluate a >>= return . Right) `E.catch` >(\e -> return $ Left e) > > -- or perhaps the right argument of catch could be just (return . Left)? > > bm2mb :: a -> Maybe a > > bm2mb a = case unThrow a of >Left (e::SomeException) -> Nothing >Right a -> Just a > > So, if there is a value inside the lazy list which is a bottom (binary > parse failure of the last received object in this case, catching any > possible exception) then the value can be converted to Nothing within > pure code, and then excluded from the result using catMaybes. > > This solution seems to be working for me. This solution looks very ugly to me. Catching 'error's is debugging, but parser failure is kind of exception handling. I guess, the errors you want to catch are caused by non-supported fail method, right? Can't you use a monad transformer like explicit-exception:Synchronous.Exception or transformers:ErrorT around the Binary parser? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to catch exception within the Get monad (the Binary package)
Hi, Thanks to everybody who replied. I see another solution: are there any hidden problems? I found an interesting package, ChasingBottoms which contains a function testing a value to be bottom and returning a Boolean (of course it cannot be done without unsafePerformIO). I borrowed the idea from that package, and wrote two functions: unThrow :: (Exception e) => a -> Either e a unThrow a = unsafePerformIO $ (E.evaluate a >>= return . Right) `E.catch` (\e -> return $ Left e) -- or perhaps the right argument of catch could be just (return . Left)? bm2mb :: a -> Maybe a bm2mb a = case unThrow a of Left (e::SomeException) -> Nothing Right a -> Just a So, if there is a value inside the lazy list which is a bottom (binary parse failure of the last received object in this case, catching any possible exception) then the value can be converted to Nothing within pure code, and then excluded from the result using catMaybes. This solution seems to be working for me. PS Maybe the way I am using the serialized data needs to be changed by implementing some kind of an iterator over the binary stream, and then by taking one object at a time and consuming it right there the problem might be eliminated entirely... Thanks again. -- Dimitry Golubovsky Anywhere on the Web ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to catch exception within the Get monad (the Binary package)
> If however something goes wrong, and prs fails, the whole function > fails (error is thrown). Since [a] (result of decoding) is a lazy > list, actual exception may be thrown at any moment the list is being > processed, and exception handler may not be properly set. > True -> return (reverse a) > False -> many' (s:a) Lazy lists are built in left to right order, but this one is right to left. > Is there any way to catch/detect failures inside the Get monad? lookAhead, lookAheadM, etc ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to catch exception within the Get monad (the Binary package)
Also any half decent binary format should tell you how long the list is *before* you parse it, either: 1) How many elements it has - for this you just need a counting version of the many combinator. 2) The length of bytes that the flattened list takes. In this case the repeating combinator has to test length remaining before deciding whether to parse the next element. Neither should need exceptions. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to catch exception within the Get monad (the Binary package)
On Sun, 5 Sep 2010, Dimitry Golubovsky wrote: Hi, The following function* is supposed to decode a list of some serialized objects following each other in a lazy Bytestring: many :: Get a -> Get [a] many prs = many' [] where many' a = do s <- prs r <- isEmpty case r of True -> return (reverse a) False -> many' (s:a) prs is a "parser" to decode a single object. It is more efficient to call fmap (s:) (many prs) in the recursion in order to avoid the final 'reverse'. The way you have implemented the loop, the complete list must be hold in memory, even if it is consumed lazily. The trick to catch exceptions lazily is to make them explicit, either by using http://hackage.haskell.org/packages/archive/capped-list/1.2/doc/html/Data-CappedList.html or http://hackage.haskell.org/packages/archive/explicit-exception/0.1.5/doc/html/Control-Monad-Exception-Asynchronous.html with an enclosed list. The latter solution is the more general one, but unfortunately it causes a space leak in GHC. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to catch exception within the Get monad (the Binary package)
> If however something goes wrong, and prs fails, the whole function > fails (error is thrown). Since [a] (result of decoding) is a lazy > list, actual exception may be thrown at any moment the list is being > processed, and exception handler may not be properly set. > True -> return (reverse a) > False -> many' (s:a) Lazy lists are built in left to right order, but Yours is right to left. > Is there any way to catch/detect failures inside the Get monad? lookAhead, lookAheadM, etc ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
