- Beginners - Haskell.org

Sorting Bank Accounts problem in Haskell
by Nadav Chernin 14 Aug '13

14 Aug '13

Hi all, I learn Haskell and i try to solve questions from SPOJ. Currently, i try to solve problem *Sorting Bank Accounts*<http://www.spoj.com/problems/SBANK/> Here is my code: *import qualified Data.Map as M* * * *extractTest []=[]* *extractTest (x:xs)=(take n xs):(extractTest (drop (n+1) xs)) where* * n=read x* * * *emp=M.empty* * * *collect m []=m* *collect m (x:xs)* * |M.member x m =collect (M.insert x (amount+1) m) xs* * |otherwise=collect (M.insert x 1 m) xs* * where* * Just amount=M.lookup x m* * * *getList m=map (\(x,k)->x++" "++(show k)) (M.toList m)* *f x=unlines$(getList (collect emp x))++[""]* *main=getLine>>=(\x->interact$unlines.map f.take (read x).extractTest.lines) * * * The problem is that i can't finish it during 7 sec. So i want to know is there more quick solution/method for this problem. Thanks, Nadav

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cabal binary packages
by harry 14 Aug '13

14 Aug '13

I need to build some software on a server, but it doesn't have enough spare memory for compiling all the dependencies. I would like to create a binary distribution of e.g. snap including all of its dependencies (other than those which come with ghc), so that the server can just install them. I found http://www.haskell.org/cabal/users-guide/installing-packages.html#creating-…, but this seems to have two deficiencies - the destination prefix needs to be know in advance (which might be OK if it can be a relative path), and it has to be done individually for each package.

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Text.XML.writeFile question
by Alan Buxton 13 Aug '13

13 Aug '13

Hi I am trying to write an XML file where the filename is created based on a timestamp. Simplified version below. This won't compile - I get this error in doWrite2 filepathtest.hs|24 col 17 error| Couldn't match expected type `system-filepath-0.4.7:Filesystem.Path.Internal.FilePath' || with actual type `String' || In the second argument of `writeFile', namely `t1' || In a stmt of a 'do' block: writeFile def t1 doc || In the expression: || do { t1 <- tsString; || writeFile def t1 doc } Somehow the String "text.xml" in doWrite1 is converted into a FilePath, but not the String t1 in doWrite2. What am I doing wrong? {-# LANGUAGE OverloadedStrings #-} module Filepathtest where import Text.XML import Data.Time.Clock.POSIX (utcTimeToPOSIXSeconds) import Data.Time.Clock (getCurrentTime) import Prelude hiding (writeFile, FilePath) tsString :: IO String tsString = do x <- getCurrentTime let x' = show $ floor $ utcTimeToPOSIXSeconds x return x' doWrite1 :: Document -> IO () doWrite1 doc = writeFile def "test1.xml" doc doWrite2 :: Document -> IO () doWrite2 doc = do t1 <- tsString writeFile def t1 doc

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cabal logs
by vold 12 Aug '13

12 Aug '13

If I set build-summary in cabal's config file, cabal will create duplicate logs - the one specified in build-summary and the default build-reports.log in the packages directory. There's also a log-dir option in the config file which doesn't seem to do anything. Are these bugs, or intended behaviour?

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HTTP Download -> Save File - non-strict
by Dan Krol 12 Aug '13

12 Aug '13

Hi, I'm working on an rss file getter. I was wondering if I could get some help getting files to download and save without holding the entire file in memory in between. I chose Conduit's version of SimpleHttp only because it was recommended, and it was the quickest thing I could get to work correctly because I was eager to get started on this project, so I'd be happy to switch. Here's where I define the download and save functions: https://github.com/orblivion/feedGetter/blob/master/rss.hs#L107 And here's where I use them, getting multiple at a time with async: https://github.com/orblivion/feedGetter/blob/master/rss.hs#L208 What happens when I run this is that it outputs that it's "Getting" the file, waits a while (presumably to download the whole thing), then says it's "Saving". And I checked the file system, it's not there during the pause. I'm not entirely sure why. Is it my choice of libraries, or the way I'm using them? Perhaps something to do with async? I just tried content <- simpleHttp "http://google.com" in ghci, and it does pause for a second, so I'm guessing this is strict from the getgo. But I've done almost no I/O before. Is there a straightforward, canonical option? It seems like there perhaps should be. But if it comes down to using pipes or conduit, what the heck I'll try it out, I'd like to learn pipes eventually. Thanks a lot, Dan

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cabal install relative to $topdir
by harry 12 Aug '13

12 Aug '13

I would like cabal install to create paths in the package database relative to $topdir, so that I can mount it on a different machine (where the absolute path will obviously be different). Is there any way of automating this?

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How to construct complex string
by martin 10 Aug '13

10 Aug '13

Hello all, recently I was working on a haskell program, which does some calculations and the produces a "plot", i.e. a set of Tikz (a LaTeX thing) commands. While this was not a problem in itself, it lacked composability. While all my plots show a grid, a caption and some data points, some of them additionally show a line with a legend, some of them show two. Now I clearly do not want to duplicate the entire function just to add another line. I'd rather construct the Tikz commands (which at the end of the day is just a String) in a stepwise fashion. When constructing the set of Tikz commands, I cannot rely that additional commands are placed at the end. It is typically more an inside-out thing. The last thing I need to do is wrap the whole thing in \begin and \end directives. Additionally there may be "global" values (like the scale) which need to be known at more than one step. I had a brief look at the "Diagrams" package, which MUST have similar issues, and they "do everything with monoids", but I fail to see the light. Could anyone point me in the right direction?

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Re: [Haskell-beginners] alternative sourcegraph
by Brent Yorgey 09 Aug '13

09 Aug '13

On Fri, Aug 09, 2013 at 12:00:42PM +0200, Kees Bleijenberg wrote: > I want to remove 'dead' code and unnecessary exports out of modules. I tried > sourceGraph with sourceGraph tbGlasDll.hs and I got: > > Could not parse source file ./Toetsing.hs > > Could not parse source file ./TestGlas.hs > > Could not parse source file ./TBGlasDll.hs > > Could not parse source file ./Glas.hs > > Could not parse source file ./CalcGlas.hs > > Could not parse source file ./calc.hs > > Could not parse source file ./BelastingFactoren.hs > > sourceGraph: Map.findMin: empty map has no minimal element > > The files above compile without errors with ghc. > > Maybe it has to do with Template Haskell code? Is there an alternative to > sourceGraph? I don't know of any alternatives to sourceGraph. Why don't you file a bug report with the maintainer? -Brent

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alternative sourcegraph
by Kees Bleijenberg 09 Aug '13

09 Aug '13

I want to remove 'dead' code and unnecessary exports out of modules. I tried sourceGraph with sourceGraph tbGlasDll.hs and I got: Could not parse source file ./Toetsing.hs Could not parse source file ./TestGlas.hs Could not parse source file ./TBGlasDll.hs Could not parse source file ./Glas.hs Could not parse source file ./CalcGlas.hs Could not parse source file ./calc.hs Could not parse source file ./BelastingFactoren.hs sourceGraph: Map.findMin: empty map has no minimal element The files above compile without errors with ghc. Maybe it has to do with Template Haskell code? Is there an alternative to sourceGraph? Kees

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maybe in IO
by vold 09 Aug '13

09 Aug '13

I've defined a function similar to check x assoc = let found = lookup x assoc in when (isJust found) $ putStrLn $ "found " ++ fromJust found which I've used several times from within the IO monad. Is there a more compact way of doing this?

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