June 2004 - Haskell-Cafe

Haskell and concurrency
by Graham Klyne 25 Jun '04

25 Jun '04

[Reminiscing mode] Recent discussions about Haskell concurrency remind me of a design I attempted some years ago for a real-time web content analyzer proxy (the sort of thing that companies would deploy to prevent their employees from downloading viruses from the web, etc.). The design was based on my (imperfect) understanding of lazy functional languages and their implementation (with some help from, SPJ's book), in particular using a graph-reduction model to evaluate a function like isSafe :: [octet] -> Bool The intent was that there could be an arbitrary and potentially very large number of concurrent web accesses passing through the proxy, so a thread-per-access could be unacceptably expensive, especially when each thread would most of the time be waiting for data. The advantage that I saw in the graph-reduction approach was that the current state of each evaluation would be entirely in the current expression graph, and there would be no per-transaction overhead other than the essential state information. The graph-reduction engine itself would have a number of worker threads, each of which would perform any graph reductions for which available incoming data was available -- a kind of demand-and-availability driven evaluation model. (The design was too ambitious to be implemented in the actual product, though it did spawn a company joke about Schonfinkel's device, and I understand that a number of ideas did eventually find their way into the implementation over time, but that's another story.) Anyway, I was wondering if any of the work on concurrent Haskell or concurrent functional programming addresses similar systems or ideas, in which implementation/hardware concurrency is decoupled from application concurrency requirements, and particularly with a view to applications requiring very low-overhead fine-grained concurrency? #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

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how to get started: a text application
by Max Ischenko 25 Jun '04

25 Jun '04

Hi all, I'm going to try to implement a version of Markdown tool[1] in Haskell. The application is rather simple: take a text file with some (simple) mark-up embedded in it and turn it into another text file, this time with XHTML markup. I need some guidelines on how to get started. I'll have to struggle with both the language itself (as I am a newcomer) plus with libraries that I may need. If I to do this in some convenient language, like Python, I'd use a lexer to parse the input, a state machine to build some internal representation and some serializer to write this into XHTML. Which approaches could you suggest for that kind of application in Haskell? I'd especially glad to hear about some "idiomatic" or "native" ways to solve the problem. I've made a quick search and found tools like Parsec, HaXML, Happy and WASH/HTML. Not sure which ones I'll need. [1] - http://daringfireball.net/projects/markdown/ TIA, max.

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RE: [Haskell-cafe] readEitherChan
by Simon Marlow 24 Jun '04

24 Jun '04

On 19 June 2004 03:06, Thomas Hallgren wrote: > Tomasz Zielonka wrote: > >> On Tue, Jun 15, 2004 at 06:29:59PM -0400, S. Alexander Jacobson >> wrote: >> >> >>> readEitherChan::Chan a -> Chan b -> Either a b >>> >>> eitherChan a b left right = readEitherChan a b >>= either left >>> right >>> >>> >> ... I think it would be safer to create once new Chan (Either a b), >> and then read from it. >> >> createEitherChan :: Chan a -> Chan b -> IO (Chan (Either a b)) >> >> > Although createEitherChan might be the best solution for some > application, it would be interesting to know what the recommended way > to implement the function eitherChan above is, if that is what you > really need? > > The eitherChan function corresponds to guarded nondeterministic > choice, usually written (a?P)+(b?Q) process calculi, i.e., a rather > fundamental concurrency operator. > > Section 5 of [1] enumerates some reasons why guarded nondeterministic > choice is not available as a primitive in Concurrent Haskell. Section > 5.2 describes a function select for nondeterministic choice, but I > don't see how to use that to implement readEitherChan (or > takeEitherMVar)... It's a hard problem. Trying to code up takeEitherMVar by hand always seems to lead to solutions with race conditions - I can't see a way to do it. Perhaps takeEitherMVar should be provided as a primitive, but I'm concerned that the same problems will crop up if we tried to implement takeEitherMVar in a multi-threaded runtime (GHC doesn't currently have a true multi-threaded runtime, but we'd like to do it someday). So I think in order to do this you have to build another abstraction around MVars. I've attached some code I just hacked up to do just that; it's untested (but compiles), and has a couple of known deficiencies, but it might be along the right lines. Cheers, Simon

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Re: [Haskell-cafe] Polymorphic algebraic type constructors
by MR K P SCHUPKE 24 Jun '04

24 Jun '04

What you are suggesting - whilst it seems reasonable requires a fundamental change to the type system. At the moment [] signifies an empty list of type 'a' ... what you are suggesting requires [] to have a different type from [a]... This means a simple case statement : case a of (a:as) -> [] -> would now not type because everything on the LHS of the cases must be the same type. Infact I don't see how it could be any other way... 'a' has a type call it [a]... how could [] be any other type? Keean.

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RE: atomic MVar overwrite (was RE: [Haskell-cafe] How to use QSem?)
by Simon Marlow 24 Jun '04

24 Jun '04

On 23 June 2004 17:46, S. Alexander Jacobson wrote: > Basically, what I want is a tryReadSampleVar > function. > > Or, if we stick with MVars, I don't want another > thread to operate between the tryTakeMVar and the > putMVar. In particular, I don't want another > thread to believe (mistakenly) that the mvar is > empty when it is simply being updated. > > I suppose I could define a new abstraction where I > have a second mvar that acts as an exclusive lock > on the the first MVar. e.g. > > data XMVar a = XMVar (MVar ()) MVar a > > overWriteXMVar (XMVar lock mvar) val = > withMVar lock (\_->tryTakeMVar mvar >> putMVar mvar val) > tryTakeXMVar (XMVar lock mvar) val = > withMVar lock (\_->tryTakeMVar mvar) > > But, I think it would be more sane simply to > have a tryReadSampleVar somewhere. If you send me code for tryReadSampleVar, I'll incorporate it. Unfortunately SampleVar is abstract, so you'll need to take the source for that library (libraries/base/Control/Concurrent/SampleVar.hs in a GHC distribution) and modify it to create your own version. Cheers, Simon

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Polymorphic algebraic type constructors
by Graham Klyne 23 Jun '04

23 Jun '04

If I have a polymorphic algebraic type (T a) with several type constructors, only one of which actually references the type parameter, is there any way to express type conversion for the type-parameter-independent constructors without actually mentioning all the constructors? Here's a simple example based on Either: [[ data A = A String deriving (Show, Eq) data B = B String deriving (Show, Eq) f :: (a->b) -> Either String a -> Either String b f g (Right a) = (Right $ g a) f g (Left s) = (Left s) -- f g (s) = (s) -- doesn't work a2b (A s) = (B s) t1 = f a2b (Left "x") t2 = f a2b (Right (A "y")) ]] The second case for 'f' throws a type error when the constructor 'Left' is omitted, because the type of 's' is fixed to be Either String A when the required result (in this case, because of a2b) is Either String B. #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

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Re: [Haskell-cafe] Polymorphic algebraic type constructors
by MR K P SCHUPKE 23 Jun '04

23 Jun '04

Adrian Hey wrote: I think all occurences of [] have type forall a. [a] The case expression.. case a of (a':as) -> <expr1> [] -> <expr2> Should be typed as if written.. case a of (a':as) -> let a=(a':as) in <expr1> [] -> let a=[] in <expr2> ---------------------------------------------------- Well, we will have to agree to disagree, I find the current system makes perfect sense, and is easy to reason with. If I want a let binding I can write one. The difference is really this: case a of (a:as) -> <expr1> e -> e In this case e represent 'the same []' as on the left hand side. in the let binding this is a new and fresh '[]'... When I pattern match a variable in a case extression I really expect it to get the type of the case... There are circunstances when I go on to use e, and if it is not grounded by the case expression I will get hit by the monomorphoism restriction, and have to add non haskell98 type annotation to ground 'e'... for example If I pass e to a polymorphic function, it won't know what type e has... case a of (a':as) -> let a=(a':as) in <expr1> [] -> let a=[] in (polyfn a) where polyfn a has a type like polyfna :: Ord a -> [a] -> String Now the above won't work and you will have to do: [] -> let a=[] in (polyfn (a :: [SomeListType])) and of course if you are using generics then how can polyfn get the original type of 'a' in the case expression... for example: polyfn :: Typable a -> [a] -> ShowS polyfn a = shows (typeOf a) then: case a of (a:as) -> <expr1> e -> polyfn e is correct, but: case a of (a':as) -> let a=(a':as) in <expr1> [] -> let a=[] in (polyfn a) does what? Keean.

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RE: atomic MVar overwrite (was RE: [Haskell-cafe] How to use QSem?)
by Simon Marlow 23 Jun '04

23 Jun '04

On 23 June 2004 17:13, S. Alexander Jacobson wrote: > Ah, that worked. Thank you. The MVar > documentation is also very brief. > > I would also like to overwrite the contents of an > MVar atomically regardless of whether it is full > or empty. I am current using. > > overWrite mvar val = tryTakeMVar mvar >> putMVar mvar val > > But it is not atomic :-( The lib functions > (modifyMVar and tryPutMVar) are atomic but seem to > require that you know in advance the MVar's state. The question to ask here is "atomic with respect to what?". modifyMVar is atomic only with respect to other threads performing modifyMVar-type operations on the MVar (i.e. take followed by put). If another thread is doing repeated puts into the MVar, then modifyMVar's atomicity is defeated. There isn't really a problem here, you just need to be sure that you use your MVars in a consistent way - you can always define a new abstraction that doesn't permit operations to be performed in the wrong order. So in order to tell you how to write your overWrite function, we need to know what other operations are being concurrently performed on the MVar, and what invariants you expect to hold. Cheers, Simon

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RE: [Haskell-cafe] How to use QSem?
by Simon Marlow 23 Jun '04

23 Jun '04

On 22 June 2004 21:44, S. Alexander Jacobson wrote: > The GHC documentation on QSem is very sparse. I > would like to give a thread exclusive access to a > resource. > > My *guess* based on the documentation is that I > can create an exclusive lock using: > > logSem <- newQSem 1 If the maximum value of your QSem is 1, then an MVar will do. That is, an MVar can be used as a simple semaphore, and a QSem is necessary if you need a quantity semaphore (there are > 1 units of the resource). > And then any thread that wants to lock the > resource uses: > > withLogSem x = do waitQSem logSem; y <- x; signalQSem logSem; > return y > > as follows: > > withLogSem $ rotate curLogPos import Control.Concurrent, create the semaphore like this: m <- newMVar () and use it like this: withMVar m $ \_ -> ... critical section ... If you are using the semaphore to protect a mutable variable, then you can handily merge the two ideas into an MVar, which behaves like a thread-safe mutable variable. Cheers, Simon

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online judge with Haskell
by Tomek Noiński 23 Jun '04

23 Jun '04

I have read a post to this list from about a year ago, considering Haskell support on online judges, like acm.uva.es. I think our online judge (a system for the verification of the correctness of submitted programs, which solve problems selected from a repository) might interest you. The Sphere Online Judge (SPOJ) is available at http://spoj.sphere.pl, and supports solutions written in 20 different programming languages including Haskell. We would like to invite you to solve problems at our server. At present, nearly half of the rapidly expanding problemset (currently more than 60 tasks) can be solved using functional languages easily within the time-limit. It is possible to add tasks dedicated to functional programming and the development team will be glad to cooperate if you are willing to create some new interesting problems. With best wishes, Adrian Kosowski, Michal Malafiejski, Tomasz Noiński, Gdansk University of Technology, Poland

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