August 2014 - Beginners

Haskeline and forkIO
by Jeff C. Britton 04 Sep '14

04 Sep '14

I am trying to modify an example in RealWorldHaskell from Chapter 24. The example is the first code snippet labeled -- file: ch24/Compressor.hs I am trying to replace the use of Readline with Haskeline. In my code the forkIO thread does not run. I guessed that since the result of the worker thread was thrown away that perhaps laziness was the problem. So, I attempted to use `seq`, but that does not work either. I am able to run the RealWorldHaskell example. I am using GHC 7.8.3. I have tried runhaskell with and without the -threaded option and on both Linux and Windows 7. import Control.Concurrent (forkIO) import Control.Exception import qualified Data.ByteString.Lazy as L import System.Console.Haskeline hiding (handle) -- Provided by the 'zlib' package on http://hackage.haskell.org/ import Codec.Compression.GZip (compress) -- Read the file, compress the data, write the compressed data worker :: FilePath -> IO () worker path = L.readFile path >>= L.writeFile (path ++ ".gz") . compress -- Run the worker on a new thread runWorker :: FilePath -> IO() runWorker path = handle (print :: SomeException -> IO ()) $ do forkIO (worker path) return () loop :: InputT IO () loop = do maybeLine <- getInputLine "Enter a file to compress> " case maybeLine of Nothing -> return () -- user entered EOF Just "" -> return () -- treat no name as "want to quit" Just path -> let f = runWorker path in f `seq` do return f loop main = runInputT defaultSettings loop

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Feature request: anonymous arguments
by Julius Gedvilas 30 Aug '14

30 Aug '14

Anonymous argument (AA) is an argument without a name. w/o AA: func :: a -> a -> a -> a func aa0 aa1 aa2 = aa0 `op` aa1 `op` aa2 \aa0 aa1 aa2 -> aa0 `op` aa1 `op` aa2 w/ AA: func :: a -> a -> a -> a func = \0 `op` \1 `op` \2 \-> \0 `op` \1 `op` \2 AA syntax: '\n', where n is one of the (0 1 2 3 4 5 6 7 8 9). 'n' corresponds to the position of a parameter. I think a language shouldn't force you to invent names for things that are used only once, same philosophy as with an anonymous function. Is this feature needed? Does something like this already exist?

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Can't compile Diagrams tutorial example
by Roger Mason 30 Aug '14

30 Aug '14

Hello, I have the following code from the Diagrams package tutorial: > {-# LANGUAGE NoMonomorphismRestriction #-} > > import Diagrams.Prelude > import Diagrams.Backend.SVG.CmdLine > main = mainWith (example :: Diagram B R2) > example = circle 1 # fc blue > # lw veryThick > # lc purple > # dashing [0.2,0.05] 0 I get the following error on compilation using GHC 7.6.3: ghc --make DiagramsTutorial-2.lhs [1 of 1] Compiling Main ( DiagramsTutorial-2.lhs, DiagramsTutorial-2.o ) DiagramsTutorial-2.lhs:10:33: Could not deduce (Fractional (Measure R2)) arising from the literal `0.2' from the context (TrailLike b, Transformable b, HasStyle b, V b ~ R2) bound by the inferred type of example :: (TrailLike b, Transformable b, HasStyle b, V b ~ R2) => b at DiagramsTutorial-2.lhs:(7,3)-(10,43) Possible fix: add an instance declaration for (Fractional (Measure R2)) In the expression: 0.2 In the first argument of `dashing', namely `[0.2, 0.05]' In the second argument of `(#)', namely `dashing [0.2, 0.05] 0' DiagramsTutorial-2.lhs:10:43: Could not deduce (Num (Measure R2)) arising from the literal `0' from the context (TrailLike b, Transformable b, HasStyle b, V b ~ R2) bound by the inferred type of example :: (TrailLike b, Transformable b, HasStyle b, V b ~ R2) => b at DiagramsTutorial-2.lhs:(7,3)-(10,43) Possible fix: add an instance declaration for (Num (Measure R2)) In the second argument of `dashing', namely `0' In the second argument of `(#)', namely `dashing [0.2, 0.05] 0' In the expression: circle 1 # fc blue # lw veryThick # lc purple # dashing [0.2, 0.05] 0 Thanks for any help. Roger

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LYAHFGG, chapter 11 question
by Frank 30 Aug '14

30 Aug '14

About "Learn You a Haskell ...", is it My imagination or is chapter 11 absurdly long and/or thick? I can (and have) read a 100+ page U.S. Supreme Court ruling, readily understand it, and be able to explain it in plain English, with next to zero trouble. I spend every work day reading, parsing, interpreting, and using, the ISO C++ standard. I *taught* My undergraduate Physics IV class while simultaneously taking it. Yet, chapter 11 feels as if it goes on and on to the point I easily forget what I read just a few lines before, rendering comprehension of the same an almost Sisyphean task. Is it just Me? Am I just tired? Is there an alternative resource for understanding the concepts that particular chapter presents? Sincerely, Frank D. Martinez -- P.S.: I prefer to be reached on BitMessage at BM-2D8txNiU7b84d2tgqvJQdgBog6A69oDAx6

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Why is cabal trying to install a particular dependency?
by Peter 29 Aug '14

29 Aug '14

Sometimes I want to know why installing package X will require package Y. After a lot of recursive manual inspection of .cabal files and --dry-run, I discover that X depends on A, which depends on B, which depends on C, which requires Y unless some flag is specified. Is there a way to ask cabal to tell me this?

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Re: [Haskell-beginners] [Haskell-cafe] 2014 haskell cabal update hangs on mac
by Madhu Babu 27 Aug '14

27 Aug '14

You are right. It worked for me now. Earlier, i deleted only /Library/Hashell, ~/.cabal & one more similar dir. Looks like there are lot of directories ( & more importantly broken symlinks which were still pointing to old Library ). Once i deleted all of them, it worked. Thanks again for the advice. On Tue, Aug 26, 2014 at 4:23 PM, S D Swierstra <doaitse.swierstra(a)gmail.com> wrote: > I suffered from the same phenomenon. I carefully removed everything which > smelled of ghc, including .ghc in my home directory and many spurious > aliases. That cured the problem, > > Doaitse > > > On 26 Aug 2014, at 6:51 , Madhu Babu <madhub.bits(a)gmail.com> wrote: > > I initially installed haskell platform ( 2013 version; 7.6.3 ghc ) on my > mac. Everything was working great. Just now saw the haskell platform > website again and found new version was released ( Haskell Platform > 2014.2.0.0 for Mac OS X, 64bit ). I installed it, and un-installed the > older version using uninstall-hs. > > Now when i type "cabal" or "cabal update" on my terminal, it hangs. > Actually when i look into Activity Monitor, i can see that it is invoking > some "sh script & possibly some find command" infinitely. I initially guess > may be it is building some indexes. but it has been running for an hr or so. > > I have Xcode 5, gcc and other command line tools installed properly > > Please advice. I cannot install any other package using cabal. > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > >

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2014 haskell cabal update hangs on mac
by Madhu Babu 27 Aug '14

27 Aug '14

I initially installed haskell platform ( 2013 version; 7.6.3 ghc ) on my mac. Everything was working great. Just now saw the haskell platform website again and found new version was released ( Haskell Platform 2014.2.0.0 for Mac OS X, 64bit ). I installed it, and un-installed the older version using uninstall-hs. Now when i type "cabal" or "cabal update" on my terminal, it hangs. Actually when i look into Activity Monitor, i can see that it is invoking some "sh script & possibly some find command" infinitely. I initially guess may be it is building some indexes. but it has been running for an hr or so. I have Xcode 5, gcc and other command line tools installed properly Please advice. I cannot install any other package using cabal.

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do notation, pattern matching, if, & case
by Vale Cofer-Shabica 27 Aug '14

27 Aug '14

I'm working on a cli program where I'd like to take input from a file or stdin. I've managed to get my program to work but thought my dispatch mechanism could be simplified. I use a scheme along the lines of f in the following minimal example, which compiles. However, if I try to use f' or f'', I get parse errors. Three questions: * Is there a better way of doing this * Are stylistic changes to f really called for? * How can/ought I correct my syntax errors? Many thanks, vale >module MinimalExample where >import System.IO (getContents) >f :: String -> IO () >f "-" = do > s <- getContents > putStrLn s >f file = do > s <- readFile file > putStrLn s >-- f' :: String -> IO () >-- f' file = do >-- if file == "-" >-- then s <- getContents >-- else s <- readFile file >-- putStrLn s >-- f'' :: String -> IO () >-- f'' file = do >-- case file of >-- "-" -> s <- getContents >-- _ -> s <- readFile file >-- putStrLn s -- vale cofer-shabica vale.cofershabica(a)gmail.com

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Re: [Haskell-beginners] Beginners Digest, Vol 74, Issue 22
by felipe zapata 26 Aug '14

26 Aug '14

Dear List, I have a problem with me new smartphone. Sorry for the inconvenient. Best, FelipeZ On 25 August 2014 08:00, <beginners-request(a)haskell.org> wrote: > Send Beginners mailing list submissions to > beginners(a)haskell.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://www.haskell.org/mailman/listinfo/beginners > or, via email, send a message with subject or body 'help' to > beginners-request(a)haskell.org > > You can reach the person managing the list at > beginners-owner(a)haskell.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Beginners digest..." > > > Today's Topics: > > 1. ? Beginners, Felipe left a message for you (Felipe) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Sun, 24 Aug 2014 21:15:00 +0000 > From: Felipe <noreply+1034183911(a)badoo.com> > To: beginners(a)haskell.org > Subject: [Haskell-beginners] ? Beginners, Felipe left a message for > you > Message-ID: <20140824211500.73654BE015F(a)cluster2020.monopost.com> > Content-Type: text/plain; charset="utf-8" > > Felipe left a message for you > > Its sender and content will be shown only to you and you can delete it at > any time. You can instantly reply to it, using the message exchange system. > To find out what was written to you, just follow this link: > > http://us1.badoo.com/01300371749/in/lUW1caPE.5c/?lang_id=106&g=57-0-4&m=21&… > > > If clicking the links in this message does not work, > copy and paste them into the address bar of your browser. > > > This email is a part of delivering a message sent by Felipe on the system. > If you received this email by mistake, please just ignore it. After a short > time the message will be removed from the system. > > > Have fun! > The Badoo Team > > You have received this email from Badoo Trading Limited (postal address > below). > If you do not wish to receive further email communications from Badoo > click here to opt out: > https://us1.badoo.com/impersonation.phtml?lang_id=106&email=beginners%40has… > . > Badoo Trading Limited is a limited company registered in England and Wales > under CRN 7540255 with its registered office at Media Village, 131 - 151 > Great Titchfield Street, London, W1W 5BB. >

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★ Beginners, Felipe left a message for you
by Felipe 25 Aug '14

25 Aug '14

Felipe left a message for you Its sender and content will be shown only to you and you can delete it at any time. You can instantly reply to it, using the message exchange system. To find out what was written to you, just follow this link: http://us1.badoo.com/01300371749/in/lUW1caPE.5c/?lang_id=106&g=57-0-4&m=21&… If clicking the links in this message does not work, copy and paste them into the address bar of your browser. This email is a part of delivering a message sent by Felipe on the system. If you received this email by mistake, please just ignore it. After a short time the message will be removed from the system. Have fun! The Badoo Team You have received this email from Badoo Trading Limited (postal address below). If you do not wish to receive further email communications from Badoo click here to opt out: https://us1.badoo.com/impersonation.phtml?lang_id=106&email=beginners%40has…. Badoo Trading Limited is a limited company registered in England and Wales under CRN 7540255 with its registered office at Media Village, 131 - 151 Great Titchfield Street, London, W1W 5BB.

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