June 2005 - Haskell-Cafe

Garbage collection and finalizer thread priority
by Gracjan Polak 13 Jun '05

13 Jun '05

Hi, My space problems continued... :) I have some ForeignPtr's, with finalizers (finalizerFree only). And I have lazyly produced list of those (yes, there is some unsafePerformIO magic behind, but I think I got this right). The problem is I get out-of-memory condition because elements get produced faster than those consumed are garbage collected. Example: list = ... mapM_ writeOut list writeOut :: Ptr Word8 writeOut dat = do hPutBuh handle dat 1024 -- Control.Concurrent.threadDelay 1000 Uncommenting this line allows gc thread to run finalizers, memory gets freed, everything runs smoothly... As far as I know finalizers are run in separate thread. How do I increase priority of this thread so it runs faster? -- Gracjan

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class Ref...
by Gracjan Polak 13 Jun '05

13 Jun '05

Hi, I the paper of Magnu Carlsson I noticed small, interesting class: class Monad m => Ref m r | m -> r where newRef :: a -> m (r a) readRef :: r a -> m a writeRef :: r a -> a -> m () He defined it locally, but it seems to be very useful generalization of IORef and STRef. Is there something like this in standard libraries? I couldn't find it... :( Is there any reason why isn't it included? Another question: priority queue. In libraries bundled with ghc we have Data.Queue, but I couldn't find PriorityQueue. Is there somewhere an implementation that everybody uses, but is not in the library? Thanks! -- Gracjan

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Control.Monad.Error
by Andrew Pimlott 12 Jun '05

12 Jun '05

I just discovered Control.Monad.Error, and all sorts of questions and ideas came to mind. I would be happy for any comments or pointers. - Monad and MonadError are uncannily similar, especially if you ignore that ugly duckling fail: class (Monad m) => MonadError e m | m -> e where throwError :: e -> m a catchError :: m a -> (e -> m a) -> m a The instances for Either are exactly symmetrical. Is there some more general way to look at this that makes the symmetry stand out? - The instance MonadError IOError IO is problematic, because one might have preferred to use Control.Exception.Exception as the error type. I've been told that the latter is generally preferred, and it's what I usually want. Any chance of the instance being changed? - It's a shame that the utility functions in Control.Exception aren't defined in terms of MonadError. Maybe they should at least be duplicated in Control.Monad.Error. - Why not instance MonadError () Maybe where throwError x = Nothing Nothing `catchError` f = f () Just x `catchError` f = Just x instance Error () where noMsg = () strMsg s = () ? This would seem to facilitate a variation of the "NotJustMaybe" pattern, using MonadError instead of Monad. What I'm getting at is that it might be nice to see MonadError used more in preference to Monad. Then we can get rid of fail. ;-) - I found this function terribly useful: liftError :: (MonadError e m, MonadTrans t, MonadError e (t m)) => m a -> t m a liftError m = join (lift (liftM return m `catchError` (return . throwError))) However, I'm not sure exactly how to describe it (or what to call it--maybe liftTry). It basically pulls errors from the inner monad to the outer monad. One application is try :: MonadError e m => m a -> m (Either e a) try m = runErrorT (liftError m) The really nice thing for me is that I can use it with StateT to prevent errors from disturbing the state, as normally happens when you mix StateT with, say, IO: test1 = execStateT test 0 >>= print where test :: StateT Int IO () test = do put 2 lift (fail "foo") `catchError` \e -> modify (+1) This prints 1, because the IOError obliterates all state changes in the first argument to catchError. However, test2 = execStateT (runErrorT test) 0 >>= print where test :: ErrorT IOError (StateT Int IO) () test = do put 2 liftError (fail "foo") `catchError` \e -> modify (+1) prints 3, because the IOError was caught by liftError, then passed down along with the state change. Oh, and test2 = execStateT (runErrorT test) 0 >>= print where test :: ErrorT IOError (StateT Int IO) () test = do put 2 lift (fail "foo") `catchError` \e -> modify (+1) raises an IOError, because the error is never seen by the outer monad (and catchError). Andrew

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Combining Haskell and Software Verification
by Gerd M 11 Jun '05

11 Jun '05

Hello, I have to do a larger project about Software Verification for university. The methods and systems I've seen so far only concern imperative/OOP languages while there seems to be little concerning functional languages. Since the exact topic can be choosen freely and I've already spent too many years messing around with OOP languages, I would like to do a project related to (or using a) functional language, especially Haskell. The problem is, up to now I wasn't successful in finding it. Suggestions are _very_ welcome! regards, Gerd _________________________________________________________________ Don't just search. Find. Check out the new MSN Search! http://search.msn.com/

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CGI module almost useless
by John Goerzen 11 Jun '05

11 Jun '05

My apologies if this sounds like a bit of a rant; I know people put good effort into this, but.... The Network.CGI module in fptools (and GHC) is not very useful. I think that it should be removed or re-tooled. Here are the main problems with it: 1. It does not permit custom generation of output headers. Thus the CGI script cannot do things like set cookies, manage HTTP auth, etc. 2. It does not permit generation of anything other than text/html documents. Many CGI scripts are used to manage other types of documents. Notably this makes it incompatible with serving up even basic things like stylesheets and JPEGs. 3. It does not permit the use of any custom design to serve up HTML, forcing *everything* to go through Text.Html. This makes it impossible to do things like serving up HTML files from disk. 4. There is documentation in the code, but it is as comments only, and doesn't show up in the Haddock-generated GHC library reference. (Should be an easy fix) 5. It does not appear to support file uploads in any sane fashion Is there a better CGI module out there somewhere that I'm missing, or should I just set about writing my own?

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Re: [Haskell] how can i learn haskell?
by Cale Gibbard 10 Jun '05

10 Jun '05

On 10/06/05, mehmet oguz <moguz21(a)hotmail.com> wrote: > Hi, > > I need help all about Haskell. > > How can i study?Which programs i need to write codes? etc. > > Thank you for your attention Though Haskell.org will provide good pointers, I'll try to answer this more directly. There are a number of good resources. One thing which I'd recommend is Hal Daume's "Yet Another Haskell Tutorial" at http://www.isi.edu/~hdaume/htut/ There is an IRC channel, #haskell on irc.freenode.org, where you can come to ask questions and talk about Haskell, and computer science in general. There is also a wiki where you can post questions and read articles on various aspects of the language: http://www.haskell.org/hawiki/ The programs you might want for executing Haskell code include GHC/GHCi at http://www.haskell.or/ghc/. GHC is a full featured Haskell compiler, and GHCi is an interactive interpreter, they come bundled together, and are quite good for most purposes. These are the ones I'd recommend to most people, but there are a few other choices as well. Hugs, at http://www.haskell.org/hugs/ is a fairly popular interpreter written in C, it's faster than GHCi, but the error messages could use some more work in places. There's also NHC, another Haskell compiler, at http://www.haskell.org/nhc98/ which aims at low memory consumption of produced executables. Hope this helps, - Cale

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Re: [Haskell] how can i learn haskell?
by padiolea＠irisa.fr 10 Jun '05

10 Jun '05

go to www.haskell.org You may not be aware of an extraodinary tool called google. Go to www.google.com, type in the box "haskell", click enter, and boom you have got your answer.

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Re: [Haskell-cafe] Looking for lost library
by Samuel Bronson 10 Jun '05

10 Jun '05

On 05/06/05, Dean Herington <heringtonlacey(a)mindspring.com> wrote: > I believe you're describing: > http://portal.acm.org/citation.cfm?doid=581478.581482 . I don't suppose you know of any place to get it *free*?

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A Tool To Show Functions Relationship?
by Dimitry Golubovsky 09 Jun '05

09 Jun '05

Does there exist a tool which given a Haskell source, shows functions that are mutually recursive (i. e. call each other, even via calling third, etc. functions)? Knowledge of that would help to split the module into smaller modules without risk to create recursive modules. For example (slightly artificial, from a parsec-based C syntax parser): declarator = try (do ps <- pointer id <- idd cp <- many cpi return (Declarator ps id cp)) <?> "declarator" idd = try (do s <- anyIdString return (Left s)) <|> try (do tkOp "(" d <- declarator tkOp ")" return (Right d)) <?> "idd" `declarator' and `idd' are mutually recursive, so placing them into different modules would create recursive modules. -- Dimitry Golubovsky Anywhere on the Web

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Re: [Haskell-cafe] A reference manual for the Haskell monad > functions
by Frank-Andre Riess 09 Jun '05

09 Jun '05

> L.S., > > I have written a reference manual for the most common Haskell monad > functions, in the > style of "A Tour of the Haskell Prelude". It can be found at: > http://members.chello.nl/hjgtuyl/tourdemonad.html > > > Known bug: > Not all keywords in the "See also" sections, that could be links, are > links. Nice :) Reading over it once, I noticed that you confused the types of (>>) and (>>=). Didn't see any other mistake, though. Greets, Frank-Andre Riess

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