May 2009 - Haskell-Cafe

setResourceLimit
by brian＠lorf.org 06 May '09

06 May '09

I have a long-lived multithreaded server process that needs to execute programs and interact with them via Handles. The programs could misbehave, like loop or hang, so I need to limit the real and CPU time they can take. I guess System.Posix.Resource.setResourceLimit sets limits on the current process, so I can't use it in the server process. Is it a good approach to write some utility program for the server to call that does setResourceLimit and executes the programs that could misbehave?

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Reply to
by Ashok Gautham 06 May '09

06 May '09

I feel that the haskell mailing lists must have the reply-to field set, so that a person can reply to the list by just clicking reply. I am unsure if I am the only one facing this problem. --- Ashok `ScriptDevil` Gautham

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Haskell Arrows Applications
by Hannousse 06 May '09

06 May '09

Hello, I'm interested to the concept of arrows in Haskell, however, I couldn't find a real application or example using this new technology in a real world application. All what I found are just academic examples and other people developing new specific libraries using arrows.Could someone, please, give me a reference to one of the real applications that uses arrows. Thank you for all Hannousse. -- View this message in context: http://www.nabble.com/Haskell-Arrows-Applications-tp23388417p23388417.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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ST.Lazy vs ST.Strict
by Tobias Olausson 06 May '09

06 May '09

Hello! I have a program that is using ST.Strict, which works fine. However, the program needs to be extended, and to do that, lazy evaluation is needed. As a result of that, I have switched to ST.Lazy to be able to do stuff like foo y = do x <- something xs <- foo (y+1) return (x:xs) However, when running the program compiled with ST.Lazy, the following is outputted: [tobsi@wobsi]$ ./runnerLazy looper.hex runnerLazy: <<loop>> The very same program compiled with ST.Strict outputs: [tobsi@wobsi]$ ./runner looper.hex 83298556 The code that is actually computing stuff is this: loopSys :: Int -> CPU s Int loopSys cc = do instr <- fetch if instr == 0xEA --NOP then return cc else do c <- execute instr loopSys $! (cc+c) The CPU type looks as follows: type CPU s a = ReaderT (SysEnv s) (ST s) a The program is run like runReaderT (loopSys 0) which in turn is being runST'd and then printed Does anyone know why the program just outputs <<loop>> when compiled under ghc 6.10.2, and runs perfectly fine under ghc 6.8.2? The program is compiled with --make and -O2 Tobias Olausson tobsan(a)gmail.com

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Writing a compiler in Hakell (continued 1)
by Rouan van Dalen 06 May '09

06 May '09

Hi everyone. Thanks for the speedy replies. Let me elaborate on my language and situation. The language I have in mind is a statically typed, hybrid (OOP + functional), strict language. It contains both functional and imperitive features (much the same as OCaml/F#) with ideas from haskell, scheme, smalltalk, mozart/oz, F#, C#. The syntax is kept as succint as possible, adding things that I think would be useful that are not defined in any of the inspirational languages. As for the target language, im not quite sure yet. I am doing a lot of work in .NET/C# at the moment, but I would eventually like to use my own programming language, instead of C#. I would also like to use my language for linux programming, so I will eventually support both Windows and Linux environments. My aim with this project is to further explore haskell and deepen my understanding of it, while at the same time creating something useful where I can explore my own ideas about programmnig languages and concepts. I don't really intend to write "the next programming language" used by billions of people. If others find it useful, that's awsome. I definitely intend to use my language for my own projects and to make it freely availble on the net. So it is not just for learning or fun, it is more focused on implementing a decent programming language that can be used for commercial applications (and fun). The compiler for my language should have a decent compile time, comparing to compilers written in C. The approach that I have in mind is to piggy back on existing languages/compilers until their implementation is no longer adequite or I have time to write my own code generator. So the first stage would be something like translate to C# and then use the C# compiler to obtain an executable. I want the first steps to go as quick as possible so that I can refine the language and test ideas quickly, and once the language has stabalized, concentrarte on my own full fledged compiler (if it is necessary). I think I will try Parsec as a starting point for creating a parser. Thanks again everyone. Regards Rouan

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List comprehension
by applebiz89 06 May '09

06 May '09

Hi, I think I need to use a list comprehension for this function but Im not good with list comprehension. this is what I have so at the moment? filmsInGivenYear :: Int -> [Film] -> [String] filmsInGivenYear filmYear ?= [ title | year <- (Film title director year fans) , year == filmYear] (this code wont compile - error given '?Syntax error in expression (unexpected `;', possibly due to bad layout)') Is there an alternative solution instead of comprehension? I want the function to be given a release date and then return all films with that date. Thanks -- View this message in context: http://www.nabble.com/List-comprehension-tp23389686p23389686.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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A brief note on n_k patterns and Hackage
by Richard O'Keefe 05 May '09

05 May '09

I never really understood why it was thought to be relevant, but I was challenged to show that n+k patterns occurred in Hackage. Last week I downloaded it. I had to miss a couple of days looking after my daughter, but today I got back to it. Guess what? I found one. Having found one I stopped looking. I don't want to start another thread, but having promised that I'd look I thought I should follow that up.

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Thread on scala ml: Usefulness of OOP
by Paolo Losi 05 May '09

05 May '09

Hi all, I'd like to mention an interesting thread on the scala mailing list about usefulness of OOP. http://www.nabble.com/-scala--usefulness-of-OOP-td23268250.html Martin Odersky, scala creator, advocates the use of OOP using an interesting toy problem. I would be very interested about the haskell point of view on the topic. Is there any haskell guru that wants to take a stub at the problem or give his opinion? Thanks Paolo

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applicative challenge
by Thomas Hartman 05 May '09

05 May '09

{-# LANGUAGE NoMonomorphismRestriction #-} import Data.List import Control.Monad import Control.Applicative -- Can the function below be tweaked to quit on blank input, provisioned in the applicative style? -- which function(s) needs to be rewritten to make it so? -- Can you tell/guess which function(s) is the problem just by looking at the code below? -- If so, can you explain what the strategy for doing so is? notQuiteRight = takeWhile (not . blank) <$> ( sequence . repeat $ echo ) echo = do l <- getLine putStrLn l return l -- this seems to work... is there a way to make it work Applicatively, with lifted takeWhile? seemsToWork = sequenceWhile_ (not . blank) (repeat echo) sequenceWhile_ p [] = return () sequenceWhile_ p (mx:mxs) = do x <- mx if p x then do sequenceWhile_ p mxs else return () blank x = "" == x

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Re: Tests
by John Goerzen 05 May '09

05 May '09

Guenther Schmidt wrote: > let me first of all thank you for providing the HDBC package. Haskell > would be a much, much less usefull language without a working database > interface. I could certainly not have written the app in Haskell > without it and in any other language I know writing this app would have > been much more difficult. Thanks! I'm glad you found it (and Real World Haskell) helpful. > The problem is what's in the database. > > You'd think there'd be a "Günni" in the database, right? > > Wrong! > > At least this is where your library and Sqlite disagree. Sqlite with any > GUI client doesn't show a "Günni", it shows a "G!$%§$§%nni". So do MS > Access and MySql btw. And now that is REALLY weird, because I can't duplicate it here. I wrote this little Haskell program: import Database.HDBC import Database.HDBC.Sqlite3 main = do dbh <- connectSqlite3 "foo.db" run dbh "CREATE TABLE foo (bar text)" [] run dbh "INSERT INTO foo VALUES (?)" [toSql "Günther"] run dbh "INSERT INTO foo VALUES (?)" [toSql "2-G\252nther"] commit dbh disconnect dbh And when I inspect foo.db with the sqlite3 command-line tool: /tmp$ sqlite3 foo.db SQLite version 3.5.9 Enter ".help" for instructions sqlite> select * from foo; Günther 2-Günther Exactly correct, as expected. I can read it back correctly from Haskell, too: import Database.HDBC import Database.HDBC.Sqlite3 import qualified System.IO.UTF8 as U main = do dbh <- connectSqlite3 "foo.db" results <- quickQuery' dbh "SELECT * from foo" [] mapM_ ((print :: String -> IO ()) . fromSql . head) results mapM_ (U.putStrLn . fromSql . head) results disconnect dbh and when I run this: /tmp$ ./foo3a "G\252nther" "2-G\252nther" Günther 2-Günther I wonder if there is something weird about your environment: non-unicode terminals, databases, editors, or something? For me, it Just Works as it should. > > For now I managed to rollback the UTF8 code in the HDBC-2.1 and got my > app to work as needed. > > I hope you find this info useful, thanks once more > > Günther > >

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