July 2010 - Beginners

Enforcing Monad Laws
by Jorden M 19 Jul '10

19 Jul '10

Hello, C++ `Concepts', which almost made it into the C++0x standard, are roughly similar to Haskell type classes. The proposal for concepts in C++ had a feature called axioms, which allow the programmer to specify semantics on the functions the concept contains. This allows for enforcing things such as the Monad Laws, as well as letting the compiler make certain optimizations it may not have been able to make without axiomatic guarantees. Why does Haskell not have a similar functionality in its type classes? Was there not time, desire, etc.? Or are there technical limitations? Thanks, ~j

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Parsing Ambiguous Languages
by Alex Rozenshteyn 19 Jul '10

19 Jul '10

I was just wondering, is there a way to use parsec get all valid parses of an ambiguous language? A quick google said no, but I may have missed something. If not, is there another, similar library which does? If not, why not? -- Alex R

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Hints on how to remove unsafePerformIO from my function?
by David McBride 19 Jul '10

19 Jul '10

I am writing a voip server to take advantage of haskells awesome threading and parsing ability. At first everything was going great, but my thread that fetches udp makes use of unsafePerformIO: fetchUDPSIP :: TChan B.ByteString -> IO () fetchUDPSIP chan = do sock <- getUDPSocket 5060 let results = (unstrict . repeat . getUDP) sock mapM_ (atomically . writeTChan chan) results where unstrict [] = [] unstrict (x:xs) = unsafePerformIO x:unstrict xs It fetches it from a socket, and then writes it to a TChan. This results in a stream of bytestrings that another thread can read from. If I don't use unsafePerformIO, then it tries to read all possible packets before returning anything, and so it never writes to the TChan at all. The problem with doing it this way is that unsafePerformIO apparently stops every other thread from doing anything while it is waiting for a packet. But I can't think of a way to rewrite this function to do what I want. I'm kind of new to this, does anyone have any hints that could help me out? Here's a simple version without any of the implementation details: fetchLine = do let results = (unstrict . repeat) getLine mapM_ putStrLn results where unstrict [] = [] unstrict (x:xs) = unsafePerformIO x:unstrict xs

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general comments requested
by Michael Mossey 18 Jul '10

18 Jul '10

I'm wondering if I could get really general comments on my program... like notes about language features I'm failing to take advantage of. This is part of a program to convert MusicXML to a simpler representation which I then manipulate. Most of the data is defined in MusDoc.Data: <http://hpaste.org/fastcgi/hpaste.fcgi/view?id=27679#a27679> and the functions are defined in MusDoc.FromXml: <http://hpaste.org/fastcgi/hpaste.fcgi/view?id=27678#a27678> A module called XmlAccess can access various interesting parts of the MusicXML document in a way that lets me ignore most of its complexity: <http://hpaste.org/fastcgi/hpaste.fcgi/view?id=27680#a27680> MusicXML Haskell module can be found here: <http://hackage.haskell.org/package/musicxml> Thanks, Mike

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Template Haskell confusion
by Alex Rozenshteyn 17 Jul '10

17 Jul '10

I'm trying to learn Template Haskell. I decided to try to write a mapTup function which takes function and a tuple (e.g. mapTup id (1,"hi",'a') ) and, well, applies the function to each cell. The closest I've gotten so far is this: > {-# LANGUAGE TemplateHaskell #-} > module MapTup > where > import Language.Haskell.TH > import Control.Monad > > mapTup fun tup = do > funE <- runQ $ [| $fun |] > TupE tupContents <- runQ $ [| $tup |] > return $ TupE $ map (AppE funE) tupContents The problem is that I can't figure out how to get the representation of the values bound to fun and tup. Sorry if I'm wording this badly. My head hurts from meta-programming ;) -- Alex R

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Re: Re: A difficult package (Daniel Fischer)
by Ben Wise 17 Jul '10

17 Jul '10

Thanks for the pointers to hayoo and hoogle! >From the way they referenced things, I was able to figure out that I'd been too specific in my Import: 'Import Text.Regex.Posix' --> 'Import Text.Regex' fixed the problem. Thanks again! > Message: 1 > Date: Thu, 15 Jul 2010 00:19:03 +0200 > From: Daniel Fischer <daniel.is.fischer(a)web.de> > Subject: Re: [Haskell-beginners] A difficult package > To: beginners(a)haskell.org, bwise(a)oceanofstones.net > Message-ID: <201007150019.03984.daniel.is.fischer(a)web.de> > Content-Type: text/plain; charset="iso-8859-1" > > On Wednesday 14 July 2010 19:20:09, Ben Wise wrote: >> Folks, >> >> I'm trying to work through Conrad Barski's Haskell tutorial, >> http://www.lisperati.com/haskell/, and I've run into a difficult package >> problem. I've googled around for two days, but nothing seems to work. >> >> Basically, I can not get it recognize matchRegex, mkRegex, and similar. >> They are all in the Text.Regex.Posix package, > > hoogle and hayoo think they're not in regex-posix but in regex-compat > ... SNIP ... -- Ben Wise, PhD 0xCAF514E1

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Having fun with yesod, and a few questions came up.
by Michael Litchard 17 Jul '10

17 Jul '10

I'm playing with yesod http://docs.yesodweb.com/yesod/, and I have a few questions: Here's an excerpt from yesod/tutorial/i18n.lhs **NOTE: This tutorial requires the development version of Yesod (version 0.4.0). The [tutorial main page]($root/yesod/tutorial/) has instructions on setting up your environment.** Where is $root? I thought it was where yesod-examples-0.4.0 was installed. But if this is the case, I'm not finding these instructions. This makes me think I am confused about the directory $root represents. I was playing around with the code and made some changes. Here is the line in question, with the complete code below for context. > instance Yesod I18N where > approot _ = "" This does what I expect it to do, it runs the program when I open up http://my.blog.server/ however, I wanted to see what would happen if I played around with it a little bit. I want the same program to run when I point my browser to http://my.blog.server/blog so I made this change > approot _ = "/blog" but now when I point my browser to http://my.blog.server/blog it gets re-written to http:/my.blog.server/blog/blog and then this error message Not Found /blog/blog Not sure what is going on here, could someone enlighten me? --- title: Multi-lingual -- Tutorials -- Yesod --- **NOTE: This tutorial requires the development version of Yesod (version 0.4.0). The [tutorial main page]($root/yesod/tutorial/) has instructions on setting up your environment.** > {-# LANGUAGE QuasiQuotes #-} > {-# LANGUAGE TemplateHaskell #-} > {-# LANGUAGE TypeFamilies #-} > import Yesod > import Data.Monoid (mempty) > data I18N = I18N > mkYesod "I18N" [$parseRoutes| > / HomepageR GET > /set/#String SetLangR GET > |] > instance Yesod I18N where > approot _ = "/blog" > getHomepageR :: Handler I18N RepHtml > getHomepageR = do > ls <- languages > let hello = chooseHello ls > let choices = > [ ("en", "English") > , ("es", "Spanish") > , ("he", "Hebrew") > ] > applyLayout "I18N Homepage" mempty [$hamlet| > %h1 $hello$ > %p In other languages: > %ul > $forall choices choice > %li > %a!href=@SetLangR.fst.choice@ $snd.choice$ > |] > chooseHello :: [String] -> String > chooseHello [] = "Hello" > chooseHello ("he":_) = "×©×~\×~U×~]" > chooseHello ("es":_) = "Hola" > chooseHello (_:rest) = chooseHello rest > getSetLangR :: String -> Handler I18N () > getSetLangR lang = do > setLanguage lang > redirect RedirectTemporary HomepageR > main :: IO () > main = basicHandler 3000 I18N

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Learning about channels
by Benjamin Edwards 17 Jul '10

17 Jul '10

NB: This was posted in fa.haskell first, I guess it was the wrong forum for this kind of question as it was left unanswered :) Hi, I'm having a few issues getting some toy programs to work whilst I try to get a better understanding of how to model processes and channels. I am just trying to use the real base blocks and failing miserably. Here is an example (yes this is utterly contrived and sill, but I lack imagination... sue me): I want my main thread to do the following: 1. make a channel 2. spawn a thread (producer) that will write a series of lists of integers to the the channel, then exit. 3. spawn another thread that will read from the channel and sum all of the input. It should exit when both the channel is empty and and the producer thread has finished writing to it. 4. Main thread should print the sum. My current code should uses a trick I have seen else where which is to have the result of "task" running in the thread put into an MVar. So my condition for the reading thread exiting is to check if the MVar of the producer thread is not empty and if the channel is empty. If those two things are true, exit the thread. Unfortunately if somehow seems able to to get to a stage where the produce thread has finished and the channel is empty, but is blocking on a read. I have the following code, but it always blocks indefinitely on a read. I am sure there is something obviously deficient with it, but I can't work out what it is. Any help would be greatly appreciated. Of course, if I'm doing it all wrong, please tell me that too :) module Main where import Control.Concurrent import Control.Concurrent.STM import Control.Monad (forever) import Data.Map as M main :: IO () main = do oc <- newChan counter <- newTVarIO (0 :: Integer) p <- forkJoin $ produce oc [1..1000] c <- forkJoin $ loop oc p counter takeMVar c >>= print produce :: Chan [Integer] -> [Integer] -> IO () produce ch [] = return () produce ch xs = do let (hs,ts) = splitAt 100 xs writeChan ch hs produce ch ts loop :: Chan [Integer] -> MVar () -> TVar Integer -> IO Integer loop ch p n = do f <- isEmptyMVar p e <- isEmptyChan ch if e && (not f) then atomically (readTVar n) else do xs <- readChan ch atomically $ do x <- readTVar n writeTVar n (x + sum xs) loop ch p n forkJoin :: IO a -> IO (MVar a) forkJoin task = do mv <- newEmptyMVar forkIO (task >>= putMVar mv) return mv

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Applicative instance for Either String
by Michael Mossey 17 Jul '10

17 Jul '10

Why does the following give me an error unless I enable FlexibleInstances? instance Applicative (Either String) where pure x = Right x Right g <*> Right x = Right (g x) Right _ <*> Left s = Left s Left s <*> _ = Left s

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question about fclabels - libnova port project
by Michael Litchard 15 Jul '10

15 Jul '10

I'm porting libnova http://libnova.sourceforge.net/ to haskell, and another question has arisen. I'm starting with header files, ln_types.h to begin with. I'm turning C structs into fclabels. struct lnh_lnlat_posn { struct ln_dms lng; /*!< longitude. Object longitude.*/ struct ln_dms lat; /*!< latitude. Object latitude */ }; becomes data LNH_lnlat_posn = LHN_lnlat_posn { _lng :: LN_dms ,_lat :: LN_dms } $(mkLabels [''LNH_lnlat_posn]) lng :: LNH_lnlat_posn :-> LN_dms lat :: LNH_lnlat_posn :-> LN_dms I have many, many structs to change. What would it take to write an abstraction that encompassed all steps? Michael

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