July 2009 - Haskell-Cafe

Re: cabal: : openFile: does not exist (No such file or directory)
by Thomas Hartman 03 Sep '09

03 Sep '09

cabal -v3 update will give you a more verbose version of what is going wrong. cabal --help regrettably, cabal --help doesn't tell you this but there is always the man page I suppose. 2009/7/16 Tony Hannan <tonyhannan2(a)gmail.com>: > Hello, > > I'm on Ubuntu 8.10. > I installed ghc 6.10.4 (from binary package: > ghc-6.10.4-i386-unknown-linux-n.tar.bz2). > I installed haskell-platform-2009.2.0.1 (from source package: > haskell-platform-2009.2.0.1.tar.gz). It contains cabal-install-0.6.2. > > Then when I run "cabal update", I get the following error: > cabal: : openFile: does not exist (No such file or directory) > > Any ideas? > > Thanks, > Tony > > _______________________________________________ > Libraries mailing list > Libraries(a)haskell.org > http://www.haskell.org/mailman/listinfo/libraries > >

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generalize RecordPuns and RecordWildCards to work with qualified names?
by Evan Laforge 20 Aug '09

20 Aug '09

Record punning is not all that useful with qualified module names. If I write '(M.Record { M.rec_x })' it says " Qualified variable in pattern" and if I write '(M.Record { rec_x })' it says 'Not in scope: `rec_x''. Could it be this extension be further extended slightly so that 'f (M.Record { M.rec_x })' will desugar to 'f (M.Record { M.rec_x = rec_x })'? Similarly, RecordWildCards could support this too. It seems simple and useful to me... am I missing anything fatally problematic about this? Would anyone else use it?

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Hugs used in circuit simulations code
by Fernan Bolando 19 Aug '09

19 Aug '09

Hi all thanks to everyone that reviewed my code. The good news 1. I happy to say that it has become useful enough for me to use it in some matlab type caluculations. includes transient and dc op 2. The simple pivtoing code I added into the DSP Lu appears to be useable for this application. The bad news 1. If you dont use some strategy in simplifying circuits and use the simulator only, it would take a considerable amount of time to converge. A simple 10x10 non linear matrix will take 13 minutes. In a high-end circuit simulator this would have taken less than a second. What is everybodies expereience in speed difference between C and interpreted haskell? I am hoping to achieve at least 10x an equivalent C code. So if a 10x10 matrix takes 1 second for C I want it to take 10seconds for hugs. regards fernan -- http://www.fernski.com

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why does the binary library require so much memory?
by Jeremy Shaw 16 Aug '09

16 Aug '09

Hello, Using encode/decode from Binary seems to permamently increase my memory consumption by 60x fold. I am wonder if I am doing something wrong, or if this is an issue with Binary. If I run the following program, it uses sensible amounts of memory (1MB) (note that the bin and list' thunks won't actully be evaluated): import Data.Binary main :: IO () main = let list = [1..1000000] :: [Int] bin = encode list list' = decode bin :: [Int] in putStrLn (show . length $ takeWhile (< 10000000) list) >> getLine >> return () /tmp $ ghc --make -O2 Bin.hs -o bin /tmp $ ./bin +RTS -s /tmp/bin +RTS -s 1000000 68,308,156 bytes allocated in the heap 6,700 bytes copied during GC 18,032 bytes maximum residency (1 sample(s)) 22,476 bytes maximum slop 1 MB total memory in use (0 MB lost due to fragmentation) Generation 0: 130 collections, 0 parallel, 0.00s, 0.00s elapsed Generation 1: 1 collections, 0 parallel, 0.00s, 0.00s elapsed INIT time 0.00s ( 0.00s elapsed) MUT time 0.05s ( 0.92s elapsed) GC time 0.00s ( 0.00s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 0.05s ( 0.92s elapsed) %GC time 0.0% (0.1% elapsed) Alloc rate 1,313,542,603 bytes per MUT second Productivity 100.0% of total user, 5.7% of total elapsed According to top: VIRT RSS SHR 3880 1548 804 Now, if I change *list* in the last line to *list'* so that the encode/decode stuff actually happens: /tmp $ ./bin +RTS -s /tmp/bin +RTS -s 1000000 617,573,932 bytes allocated in the heap 262,281,412 bytes copied during GC 20,035,672 bytes maximum residency (10 sample(s)) 2,187,296 bytes maximum slop 63 MB total memory in use (0 MB lost due to fragmentation) Generation 0: 1151 collections, 0 parallel, 0.47s, 0.48s elapsed Generation 1: 10 collections, 0 parallel, 0.36s, 0.40s elapsed INIT time 0.00s ( 0.00s elapsed) MUT time 0.47s ( 20.32s elapsed) GC time 0.84s ( 0.88s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 1.30s ( 21.19s elapsed) %GC time 64.1% (4.1% elapsed) Alloc rate 1,319,520,653 bytes per MUT second Productivity 35.9% of total user, 2.2% of total elapsed And top reports: VIRT RSS SHR 67368 64m 896 63 times as much total memory in use. And, this is while the program is waiting around at 'getLine' after it is 'done' with the data. I am using GHC 6.10.4 on GNU/Linux. Thanks! - jeremy

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Re: Adding an ignore function to Control.Monad
by Jeff Wheeler 14 Aug '09

14 Aug '09

On Fri, Jul 10, 2009 at 10:10 PM, Don Stewart<dons(a)galois.com> wrote: >> ## Control.Monad.void m a -> m () >> Don Stewart >> Iavor Diatchki For whatever it's worth, I prefer void as well, for the exact reason Don said. Indeed, 'ignore' indicates to me that the argument won't even be evaluated: it'll be ignored, and skipped. But it is, and only part --- the result --- is ignored. What about 'void' with functors? Jeff Wheeler

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[ANNOUNCE] hgettext 0.1.10 - last major release
by Vasyl Pasternak 10 Aug '09

10 Aug '09

Hello, I've uploaded last (and latest) significant version on hgettext module. Currently it works fine, and has bindings to all gettext functions (from libintl.h). Next versions will be only bug fixes of this version. GNU GetText allows you only to create simple, standalone desktop applications for Linux, to make something complex you should use different trick, fortunately Haskell could do these tricks much easier than other languages (if you want to see, how to build Gtk application, which supports run time language switching, and shows information on two languages simultaneously you could download example from http://hgettext.googlecode.com/files/gtk-hello-0.0.2.tar.gz , or read about it from http://progandprog.blogspot.com/2009/04/multilingual-ui-and-dynamic-languag…) I don't see any strong reasons to write any combinators over this basic bindings. Haskell needs more powerful internationalization library, and I am plan to design it, but it will be completely different from gettext principles, so this library will be released with another name. CHANGELOG: * Added '--version' command line parameter to hgettext tool * Added support for literate haskell files to hgettext tool * Changed all functions to throw IOError when they fails instead of return Nothing * Added bindings for: ngettext, dgettext, dngettext, dcgettext and dcngettext functions -- Best regards, Vasyl Pasternak

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n00b question: defining datatype
by Iain Barnett 06 Aug '09

06 Aug '09

Hi, I'm trying to get my head around datatypes, and wondering how I might define a simple "Task" datatype in Haskell. data Task = Task { title :: String, completed :: Bool } Ok, that's straightforward, but sometimes tasks become a list of tasks themselves data Task = Task { title :: String, completed :: Bool, subtasks :: [Task] } But that's not really right, because obviously, some tasks don't have subtasks. So I try this: data Task = Task { title :: String, completed :: Bool } | TaskWithSubtasks { title :: String, completed :: Bool, subtasks :: [Task] } It's a bit more accurate, but it's repeating things, which is ok with a simple type. Could anyone suggest a better way to define this? If I was using C#, which I'm far more familiar with, I could overload the constructor and refer to the smaller constructor. Is there a way to do that in Haskell, or am I still thinking too OOP? Iain

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Efficient functional idiom for histogram
by Paul Moore 05 Aug '09

05 Aug '09

How would I efficiently write a function in Haskell to count occurrences of unique elements in a (potentially very large) list? For example, given the list [1,2,3,4,5,3,4,2,4] I would like the output [[1,1], [2,2], [3,2], [4,3], [5,1]] (or some equivalent representation). Clearly, this won't be possible for infinite input, but I would like it to be as efficient as possible for (lazy) input lists containing many millions of elements. So an implementation based on group . sort is not going to work. In an imperative language like Python, I'd use a dictionary as an accumulator - something like for el in input: accums[i] = accums.get(i, 0) + 1 That's about as efficient as you can hope for (assuming an efficient dictionary implementation). How would I code something equivalent in Haskell? Thanks, Paul.

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powerSet = filterM (const [True, False]) ... is this obfuscated haskell?
by Thomas Hartman 05 Aug '09

05 Aug '09

on haskell reddit today powerSet = filterM (const [True, False]) is said to be beautiful / mind blowing. I just don't get it. I can play with transformations until I get powerSet [] = [[]] powerSet (x:xs) = let pxs = powerSet xs in map (x:) pxs ++ pxs which is understandable to me, but no matter how long I look at the original filterM definition it just doesn't click. Is this a uniquely haskell obfu, or is there a way of reading this definition that makes sense? If anybody agrees with me, care to throw out other examples of "obfuscated haskell considered harmful"?

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Re: Simple quirk in behavior of `mod`
by gladstein＠gladstein.com 04 Aug '09

04 Aug '09

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