April 2008 - Haskell-Cafe

Re: HXT 7.4/7.5 leaking TCP connections
by Uwe Schmidt 08 Apr '08

08 Apr '08

Daniel McAllansmith wrote: > I'm having some problems with connections leaking when using 7.4. I've > just checked and it happens with 7.5 off of hackage as well. > > It appears that readDocument doesn't close the connection when using the > haskell http library, it works ok when curl is used though. > > Following is a program which demonstrates the problem. /tmp/list.txt just > has the name of files available from my local webserver, more than 1024 > files is enough to exhaust my open file limit. > > I think the garbage collector does collect the connections, but they don't > end up fully closed... that's another issue I'm planning on looking at. > > If you need any more information please ask. I may get a chance to try and > write a patch but using curl is a work around while I'm busy. > .... this is a known problem with HTTP package (version 3001.0.4). Paul Brown has described this somewere in his blog. (http://mult.ifario.us/t/haskell) but my firefox only shows an incomplete page of this blog, the solution is missing. Paul promissed in his blog to send a patch to Björn Bringert. Here is the solution: edit the close function in HTTP/TCP.hs as follows: ---------------------------------------------------------------- close ref = do { c <- readIORef (getRef ref) ; closeConn c ; writeIORef (getRef ref) ConnClosed } where -- Be kind to peer & close gracefully. closeConn (ConnClosed) = return () closeConn (MkConn sk addr [] _) = mapM_ (flip Exception.catch $ \_ -> return ()) [ shutdown sk ShutdownSend , suck ref , shutdown sk ShutdownReceive , sClose sk ] suck :: Connection -> IO () suck cn = readLine cn >>= either (\_ -> return ()) -- catch errors & ignore (\x -> if null x then return () else suck cn) --------------------------------------------------------------------- I hope this will help. Cheers Uwe P.S. to Björn: It would be nice, if you could include this change into the HTTP module and upload a new version to hackage.

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ANN: The Monad.Reader Issue 10
by Wouter Swierstra 08 Apr '08

08 Apr '08

I am pleased to announce that a new issue of The Monad.Reader is now available: http://www.haskell.org/haskellwiki/The_Monad.Reader Issue 10 consists of the following two articles: * Bernie Pope Step inside the GHCi debugger * Matthew Naylor Evaluating Haskell in Haskell The Monad.Reader is a quarterly magazine about functional programming. It is less formal than a journal, but somehow more enduring than a wiki page or blog post. If you'd like to write something for the next issue of The Monad.Reader, please get in touch. I haven't fixed the deadline for the next issue just yet. I hope to send out an official call for copy fairly soon. Wouter This message has been checked for viruses but the contents of an attachment may still contain software viruses, which could damage your computer system: you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

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Quick check?
by Galchin, Vasili 08 Apr '08

08 Apr '08

Hello, What are some examples of libraries in HackageDB that use either Quick Check (pure side) or Monad testing? Is there a way to use "darcs" to answer my question? Thanks, vasili

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Re: [Haskell-cafe] deriving
by PR Stanley 07 Apr '08

07 Apr '08

Hi, Thank you very much for your helpful replies. So, we define (or is it declare) a class with a polymorphic value(s which will be used in defining function types or functions associated with that class. The data types are then used for deriving instances of that class specific to each data type. The function defined in the general class can be redefined if necessary. It sort of reminds me of specialisation in OO. Get thee behind me beast. :-) Any comments? Cheers, Paul At 20:05 07/04/2008, you wrote: >Paul, > > > No, sorry. I'm not sure how this differs from my definition. > > Could you elaborate please? > >Gladly. I haven't tested any of the code below, so pardon any typos >and thinkos. > >It sounded like you were trying to say that Bool derives from Eq, Ord, >Show, and Read, and thus the keyword "from" would apply. Of course, >I may have misunderstood your intent. > >I was countering by saying that the "deriving" keyword indicates something >different. I'll try to describe what I meant in a little more detail. > >A type class definition defines properties of the type class. Eg., > >class Example a where > foo :: a > bar :: a -> a > >The class is then related to specific types by giving them instance >declarations. The instance declarations can be explicit, as in > >data Xyzzy = Xy | Zzy >instance Example Xyzzy where > foo = Xy > bar = const Zzy > >Sometimes the instance declaration can be automatically derived instead, >which looks like: > >data Xyzzy = Xy | Zzy > deriving (Example) > >This lets the compiler derive the "obvious" class instance declaration. >But it won't work for just any class, because the compiler needs to know >how to derive "obvious" instances. A few classes have deriving >machinery "built-in": Eq, Ord, Enum, Bounded, Show, Read. > >"deriving (Example)" won't work, since that machinery doesn't exist. > >With effort, you could implement this "automatic derivation" machinery >for your own types, at least in GHC. I've never needed it, so I've >never learned exactly how to do it. > >In short, so "data Foo [...] deriving (Eq)" means: "define a new type Foo, >and automatically derive its class instance for Eq". > >Does this help? > >Regards, >John

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(forw) Re: [Haskell-cafe] deriving
by John Dorsey 07 Apr '08

07 Apr '08

I meant to send this reply to the cafe. ----- Forwarded message ----- Date: Mon, 7 Apr 2008 15:05:00 -0400 To: PR Stanley <prstanley(a)ntlworld.com> Subject: Re: [Haskell-cafe] deriving Paul, > No, sorry. I'm not sure how this differs from my definition. > Could you elaborate please? Gladly. I haven't tested any of the code below, so pardon any typos and thinkos. It sounded like you were trying to say that Bool derives from Eq, Ord, Show, and Read, and thus the keyword "from" would apply. Of course, I may have misunderstood your intent. I was countering by saying that the "deriving" keyword indicates something different. I'll try to describe what I meant in a little more detail. A type class definition defines properties of the type class. Eg., class Example a where foo :: a bar :: a -> a The class is then related to specific types by giving them instance declarations. The instance declarations can be explicit, as in data Xyzzy = Xy | Zzy instance Example Xyzzy where foo = Xy bar = const Zzy Sometimes the instance declaration can be automatically derived instead, which looks like: -- non-working code, see below data Xyzzy = Xy | Zzy deriving (Example) This lets the compiler derive the "obvious" class instance declaration. But it won't work for just any class, because the compiler needs to know how to derive "obvious" instances. A few classes have deriving machinery "built-in": Eq, Ord, Enum, Bounded, Show, Read. "deriving (Example)" won't work, since that machinery doesn't exist. With effort, you could implement this "automatic derivation" machinery for your own types, at least in GHC. I've never needed it, so I've never learned exactly how to do it. In short, so "data Foo [...] deriving (Eq)" means: "define a new type Foo, and automatically derive its class instance for Eq". Does this help? Regards, John ----- End forwarded message -----

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Re: [Haskell-cafe] deriving
by PR Stanley 07 Apr '08

07 Apr '08

Hi >data Bool = False | True > deriving (Eq, Or, Show, Read) > >Bool is an instance of Eq, Ord, Show and Read. It is derived form >these classes. If that is the meaning of the keyword deriving then >wouldn't a different keyword such as from or derivative or even >derivation be closer to the semantics of the concept? >Discuss :-) > >Cheers, >Paul >I'm not getting it. Are you thinking that Bool itself "is derived >from these classes", rather than those four instances for Bool? - Conal >Paul: >I am aware that Bool is an instance of Eq, Ord, ... What I'm not >sure about is the terminology used here. Deriving, deriving what >from Eq, Ord, ...? How about data Bool = False | True from (Eq, Ord, Show, Read) although I have a feeling that "from" is already used somewhere in Haskell. Paul

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ANNOUNCE: Well-Typed LLP - The Haskell Consultants
by Ian Lynagh 07 Apr '08

07 Apr '08

Fellow Haskellers, We (Björn Bringert, Duncan Coutts and Ian Lynagh) are pleased to announce that we have recently set up a Haskell consultancy company, Well-Typed LLP (http://www.well-typed.com/) Our services include application development, library and tool maintenance, project advice, and training. Please see our website or drop us an e-mail for more details. We look forward to hearing from you in the future! -- Björn Bringert, Duncan Coutts, Ian Lynagh http://www.well-typed.com/ info(a)well-typed.com

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[GSoC] WWW::Mechanize-like package for Haskell
by Max Desyatov 07 Apr '08

07 Apr '08

Hi, I'm interested in working on a library for a stateful web browsing in Haskell during Google Summer of Code. The basic idea is described at http://hackage.haskell.org/trac/summer-of-code/ticket/1107. WWW::Mechanize is a ready to use library written in Perl, though I used python's mechanize when I wrote some simple scripts (http://wwwsearch.sourceforge.net/mechanize/) which provides much cleaner interface. Anyway, it gives simple and convenient way to retreive web-sites, to handle cookies, history and to process retrieved content and forms. There are basics of it Network.Browser module from Haskell's HTTP library (http://hackage.haskell.org/packages/archive/HTTP/3001.0.4/doc/html/Network-…) but it's ugly (uses unsafePerformIO for error reporting) and lacks a greater part of needed functionality. My aim is to greatly improve Network.Browser module and to make coding small scripts with it in more functional way possible. At this moment it uses BrowserAction state monad. Though, the deadline is approaching, I still seek some ways to improve my proposal. So here are the questions: are there any other data structures that will make programming with this library more convenient, besides simple state monad? Should we contrive more sophisticated system with other other separate data structures? What other improvements you'd like to see? Thanks in advance for any advice. -- Max.

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Parsec Expected Type
by Paul Keir 07 Apr '08

07 Apr '08

Hi, Does anyone know why this reduced Parsec production stops compilation, and how I can fix it? tester = reserved "parameter" <|> do { reserved "dimension"; symbol ":" } Thanks, Paul

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Shouldn't this loop indefinitely => take (last [0..]) [0..]
by Olivier Boudry 07 Apr '08

07 Apr '08

Hi all, If you compile and run this: main = do putStrLn $ show $ take (last [0..]) [0..] or simply run: take (last [0..]) [0..] in ghci, it first hang for about one minute and then starts to generate an infinite list. I was expecting "last [0..]" to never produce a value and the "take" function to never take from the [0..] list. I found that line of code in a friend's "Skype Message", lauched it in ghci and forgot it. When I came back to my ghci window a couple minutes later it was listing numbers, which was really unexpected. If I just run "last [0..]" it doesn't return a value and my computer will quickly start to paginate to death. Am I missing something? Some laziness magic? Rewrite rules? Thanks, Olivier.

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