August 2013 - Beginners

cabal install relative to $topdir
by harry 11 Aug '13

11 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 09 Aug '13

09 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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Showing a value, when there are many approaches?
by Chris Schneider 07 Aug '13

07 Aug '13

Hi, I'm writing a module that contains a data type for a MacAddress. Given that, I can do things like validate and format. But there are several ways you can display a MacAddress: 'aabbccddeeff' or '00:11:22:33:AA:BB' or 'aa:bb:cc:00:11:22'. Different external systems want the mac addresses in different formats, and I'd like to centralize that logic. ** So the design question: I can't just use a `show` function directly to do this, since there are options I need to pass. I could use separate functions that return a string. I could use newtype to wrap each display style of mac address in a type that has a specific `show` instance that spits out the right format. So a `Pretty` and `Lower` and similar types. --- Is there a best-practice here? Am I totally missing a better way to model this idea?

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Enterprise-quality XML library?
by Mike Meyer 07 Aug '13

07 Aug '13

I've been looking for an enterprise-grade XML library. My standard for comparison is Python's lxml library (a wrapper around libxml2 and libxslt). The only thing I've found that's even close is hxt, and it falls a bit short. Specifically, when I feed it invalid XML, I get this error: error: The content of element "school" must match ( "author"* ). Element "author" expected, but element "auther" found. error: Element "auther" not declared in DTD. Whereas from lxml I get: lxml.etree.XMLSyntaxError: No declaration for attribute name of element auther, line 24, column 19 This is shorter and clearer, and identifies the location of the error. It does have better error messages for ill-formed XML. Is there a library with a validating parser that provides better quality error messages? It also seems like mapping an XML Schema into a collection of Haskell types should be a function of the library. Is there on that does that, or is this completely untenable?

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Looking for judges for haskell competition
by Mike Meyer 07 Aug '13

07 Aug '13

In order to foster commercial use of Haskell, FP Complete is running a competition for monthly cash prizes (details https://www.fpcomplete.com/blog/2013/07/competition-announcement) We are looking for judges from the community. People helping beginners would seem to be a good fit for this. If you're interested, could you please drop me a note along with pertinent information? Thanks, Mike

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Why is my GHC bigger than the distribution
by vold 07 Aug '13

07 Aug '13

I've built GHC 7.6.3 from source using the default "perf" settings, and all my programs are about 4 Mb bigger after linking than if I used the pre-built binary. What could be causing this? (x64 Linux with gold linker.)

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Threading (lack of) interleaving
by Chris Schneider 05 Aug '13

05 Aug '13

Hi, I'm working on a program that should launch, spawn two threads, each with a zeromq connection, where those threads ping the zmq channel repeatedly. The main thread just sits and waits. The code is here: https://gist.github.com/cschneid/3862b33c29a803be7848 If I launch one thread, or the other (lines 13 & 14), and comment the other one out, it runs as expected. But if I attempt to launch both, both build themselves, and run one call of themselves, but then it seems that one or the other thread "wins", and the other never gets to run again. Most of the time it's the first one launched that wins, but not always. See the output for an example. Adding explicit yield calls, using forkOS vs forkIO, and moving around of where I make the ZMQ context & socket (into and out of the threads) don't seem to help. What am I missing? How can I get these threads to play nice with each other? Thanks!

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