September 2012 - Beginners

space leak processing multiple compressed files
by Ian Knopke 04 Sep '12

04 Sep '12

Hi everyone, I have a collection of bzipped files. Each file has a different number of items per line, with a separator between them. What I want to do is count the items in each file. I'm trying to read the files lazily but I seem to be running out of memory. I'm assuming I'm holding onto resources longer than I need to. Does anyone have any advice on how to improve this? Here's the basic program, slightly sanitized: main = do -- get a list of file names filelist <- getFileList "testsetdir" -- process each compressed file files <- mapM (\x -> do thisfile <- B.readFile x return (Z.decompress thisfile) ) filelist display $ processEntries files putStrLn "finished" -- processEntries -- processEntries is defined elsewhere, but basically does some string processing per line, -- counts the number of resulting elements and sums them per file processEntries :: [B.ByteString] -> Int processEntries xs = foldl' (\x y -> x + processEntries (B.lines y)) 0 xs -- display a field that returns a number display :: Int -> IO () display = putStrLn . show

4 4

programming with Error monad
by Dennis Raddle 04 Sep '12

04 Sep '12

I recently completed a large project and started another one. The first project was about processing music, in particular taking MusicXML and/or Sibelius files (Sibelius is a music notation program) and producing MIDI files according to the specifications I wanted. The second project is about algorithmic music composition. To handle errors in the first project, I used "throw" to throw exceptions. Now I am using the Error monad... actually a combination of ErrorT and State. (In the state I hold a random generator for use in algorithms that require pseudo-randomness.) I notice two things now. The first thing is that I can catch and rethrow errors to provide annotation about the context. It's like getting a stack trace automatically every time there is an error. I realize now this makes it easier to debug, and during my first project I spent a lot of time debugging that might have been avoidable. (I confess I never could figure out the ghci debugger, which might have helped.) The second thing is that there seems to be less laziness. Every statement in a do statement seems to execute even if I don't use result. I think. I would have to do some more experiments, but that is what it looks like right now. That is another good thing for debugging... less laziness (and maybe guaranteed order of execution too?) means it's easier to understand what the program is attempting to do. Does this seem correct? Dennis

1 1

TagLib
by linuxlinux2006 03 Sep '12

03 Sep '12

Hello Please tell me why this program does not work: http://hpaste.org/74171 Tags in audio file are recorded correctly, other programs see them. Thanks for the help

1 0

(no subject)
by Dennis Raddle 02 Sep '12

02 Sep '12

I wanted to run the map function from Data.Map, let's call it M.map, but inside a monad transformer stack including the Error monad. M.map has this type: M.map :: (Ord k) => (a -> b) -> Map k a -> Map k b However, I want to use a mapping function that has type (Monad m) => a -> m b (i.e. errors could be thrown during the computation, a log could be written, etc) I wrote the following. Any comments on this way of doing things? mapMapM :: (Monad m, Ord k) => (a -> m b) -> Map k a -> m (Map k b) mapMapM g mapIn = do let h (k,a) = do b <- g a return (k,b) y <- mapM h (M.toAscList mapIn) return $ M.fromAscList y

2 1

literate programming
by Dennis Raddle 01 Sep '12

01 Sep '12

Does literate programming in haskell (.lhs) support the idea in Knuth's WEB of using aliases for blocks of code? In other words doing something like main = do <part 1> <part 2> <part 1> === putStrLn "foo" <part 2> === putStrLn "bar" (except I don't know what the actual syntax is) -Dennis

2 1

<-
by Patrick Redmond 01 Sep '12

01 Sep '12

I'm reading "Learn You a Haskell for Great Good!", chapter 9, "Input and Output" <http://learnyouahaskell.com/input-and-output>. IO actions are given liberal coverage throughout the chapter, however it is never mentioned whether the value-extractor syntax (<-) has a type or not. main = do x <- getLine putStrLn $ reverse x In this little program, getLine has type "IO String" and x has type "String". This implies to me that (<-) has type "IO a -> a". However, GHCI chokes on ":t (<-)" and Hoogle says it's just a syntactic element <http://www.haskell.org/haskellwiki/Keywords#.3C->. I guess I don't have a specific question, but I was kind of expecting it to be a function with a type because everything seems to be a function with a type in Haskell... Thanks for listening!

5 5

Warp and Yesod benchmark puzzle
by Lorenzo Bolla 01 Sep '12

01 Sep '12

Hi all, This is a question specific to the Yesod framework, but simple enough (I hope) to be considered a beginner question... I am puzzled by the performance of these two very simple web-servers, one written in Warp and another written in Yesod: === YESOD === {-# LANGUAGE TypeFamilies, QuasiQuotes, MultiParamTypeClasses, TemplateHaskell #-} import Yesod data HelloWorld = HelloWorld mkYesod "HelloWorld" [parseRoutes| / HomeR GET |] instance Yesod HelloWorld getHomeR :: Handler RepHtml getHomeR = defaultLayout [whamlet|$newline always Hello World! |] main :: IO () -- main = warpDebug 3000 HelloWorld main = warp 3000 HelloWorld === WARP === {-# LANGUAGE OverloadedStrings #-} import Network.Wai import Network.HTTP.Types import Network.Wai.Handler.Warp (run) import Data.ByteString.Lazy.Char8 () app :: Application app _ = return $ responseLBS status200 [("Content-Type", "text/html")] "Hello, Warp!" main :: IO () main = do putStrLn "http://localhost:8080/" run 8080 app === I've tested both using httperf: $> httperf --hog --client=0/1 --server=localhost --port=3000 --uri=/ --rate=1000 --send-buffer=4096 --recv-buffer=16384 --num-conns=100 --num-calls=100 --burst-length=20 and I got very different results: YESOD: Request rate: 4048.0 req/s (0.2 ms/req) WARP: Request rate: 33656.2 req/s (0.0 ms/req) Now, I understand that Yesod is expected to be slower than the "raw" Warp, but I wasn't expecting a 10x slowdown, especially for such a trivial Yesod app (no db, no auth, etc.). [ Compilation command was: ghc -Wall -O2 --make yesod.hs $ yesod version yesod-core version:1.1.0 ] What is going on? Thanks, L.

4 8

Haskell POP3S
by Franco 01 Sep '12

01 Sep '12

Hello everyone, I am trying to write a simple script in Haskell. Such script is meant to: - contact my email (pop3) - ask the server wheter there are new messages - report the result back to me This was trivial to write, using the "pop3-client" library, but I soon realized the whole thing is pretty unsecure. User and password are sent in plain text and I would like to encrypt those. So I asked Hayoo: it seems there is only one package which supports pop3s, curlhs; still, I didn't manage to get it. So I am asking you: - do you know wheter curlhs is fit for my goal? pop3-client had a nice 'getNumberOfMessages' function. I don't see anything similar in there - do you have examples of querying mail via curl? I found very few of those Thanks in advance. I should mention this thing was meant to be fed to xmobar, to check at intervals of, say, 15 minutes, wheter I got new mail without having a mail client always open -Franco -- Franco <franco00(a)gmx.com>

2 1