September 2009 - Haskell-Cafe

Too much strictness in binary-0.5.0.2
by Khudyakov Alexey 18 Oct '09

18 Oct '09

Hello I run into problems with new binary package. Following function reads a list of elements one by one until end of stream. List is very long (won't fit into memory). In binary-0.5.0.1 and earlier it read list lazily. Now it seems that it tries to read whole list to memory. Program does not produce any output and memory usage steadily grows. > getStream :: Get a -> Get [a] > getStream getter = do > empty <- isEmpty > if empty > then return [] > else do x <- getter > xs <- getStream getter > return (x:xs) How could I add laziness to this function to revert to old behavior. -- Khudyakov Alexey

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(no subject)
by Stavenga, G.C. 15 Oct '09

15 Oct '09

Hi, I'm just started to learn Haskell. Coming from a programming contest background (where it is important to be able to solve problems in a small amount of code) I'm wondering what the best way is for simple IO. A typical input file (in a programming contest) is just a bunch of numbers which you want to read one by one (sometimes interspersed with strings). In C/C++ this is easily done with either scanf or cin which reads data separated by spaces. In Haskell I have not found an equally satisfactionary method. The methods I know of 1) Stay in the IO monad and write your own readInt readString functions. A lot of code for something easy. 2) Use interact together with words and put the list of lexemes in a State monad and define getInt where at least you can use read. 3) Use ByteString.Char8 which has readInt (but I couldn't find a readString). But one has to put it also in a State monad. I think that there must be standard function that can do this. What do experienced Haskellers use? Thanks in advance

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Re: ANN: CmdArgs - easy command line argument processing
by Ben Franksen 13 Oct '09

13 Oct '09

Neil Mitchell wrote: > I am pleased to announce CmdArgs v0.1. CmdArgs is a library for easy > command line argument processing - taking the arguments passed into > your program from getArgs and converting them into a structured value > for use in your program. Compared to the System.Console.GetOpts > library there are two key advantages: > > 1) The command line argument processors are shorter - in a typical > case they are about 1/3 the size, 90 lines with getopts vs 30 simpler > lines with CmdArgs (for HLint). > > 2) CmdArgs can support multiple mode command lines, such as those > found in hpc, darcs and cabal. > > The guiding principle of CmdArgs is to try and write each piece of > information only once, which makes the command line processing > shorter, and eliminates many of the accidental copy-and-paste style > bugs that are easy with getopts. CmdArgs requires GHC, but other than > this restriction, I do not foresee any reason for anyone to use > getopts over CmdArgs. I very much like the general idea of this thing, but have a question regarding the interface. This is from the docs: (&=) :: a -> Attrib -> a Add attributes to a value. Always returns the first argument, but has a non-pure effect on the environment. Take care when performing program transformations. value &= attrib1 & attrib2 To expose an impure function (!) in an API, I don't know... I mean, couldn't one just wrap the value like this data Attributed a -- opaque (&=) :: a -> Attrib -> Attributed a mode :: Data a => Attributed a -> Mode a and thus retain a purely functional interface? Cheers Ben

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Network.Curl and posting XML data
by Erik de Castro Lopo 07 Oct '09

07 Oct '09

Hi all, I need to do a HTTP post of XML data (Content-type == "text/xml") to a HTTP (eventually HTTPS) server and to retrieve both the response code and some returned XML data. I have had an extensive read of the haskell-curl docs: http://hackage.haskell.org/packages/archive/curl/1.3.5/doc/html/Network-Cur… as well as playing around with some code, but haskell-curl doesn't seem to support this. I've tried curlPost which along with not returning a response also insists on encoding my XML data as application/x-www-form-urlencoded which simply doesn't work. Anyone ben able to get something like this to work? Are there any alternatives to haskell-curl which will work for my application? Cheers, Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

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Fwd: [Haskell-cafe] Haskell for Physicists
by Alberto G. Corona 03 Oct '09

03 Oct '09

I´m a physicist, so I think they would be attracted by something like Haskell: high level physics modelling at Fortran speeds Haskell: mathematics beyond numerical calculus 2009/9/30 <edgar(a)ymonad.com> Hi, > > I will give a seminar to physicists at USP (Universidade de São Paulo, > Brazil) university and they asked me for a good title, something that can > attract physicists. Anyone has some suggestions? (Will be > a seminar about the use of Haskell to substitute C or Fortran > in a lot of tasks, and how it can be used in some problems instead of > Matlab, Mathematica, etc.) > > Thanks, > Edgar > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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Comments requested: succ Java
by Curt Sampson 02 Oct '09

02 Oct '09

No, it's not quite what it sounds like. :-) Stuart Halloway recently posted a series of blog entries entitled "Java.next"[1], discussing the benefits of four other languages that compile to JVM bytecode and interoperate with Java: Clojure, Groovy, JRuby, and Scala. I thought I'd put my oar in and write a parallel series comparing Haskell to these. I've finished a draft of the first posting, started on the third, and made a couple of notes on the second and fourth, and I thought I'd post the drafts[2] and solicit comments here. If you have time to read and comment, I'd greatly appreciate the help; feel free either to e-mail me privately or post here. Also feel free to forward this to anybody else you feel might be interested in commenting. I'll probably be posting these about one per week, starting some time next week. [1]: http://blog.thinkrelevance.com/2008/9/24/java-next-overview [2]: http://www.starling-software.com/en/blog/drafts/2009/09/27.succ-java-summar… cjs -- Curt Sampson <cjs(a)starling-software.com> +81 90 7737 2974 Functional programming in all senses of the word: http://www.starling-software.com

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Haskell for Physicists
by edgar＠ymonad.com 02 Oct '09

02 Oct '09

Hi, I will give a seminar to physicists at USP (Universidade de São Paulo, Brazil) university and they asked me for a good title, something that can attract physicists. Anyone has some suggestions? (Will be a seminar about the use of Haskell to substitute C or Fortran in a lot of tasks, and how it can be used in some problems instead of Matlab, Mathematica, etc.) Thanks, Edgar

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I read somewhere that for 90% of a wide class of computing problems, you only need 10% of the source code in Haskell, that you would in an imperative language.
by Casey Hawthorne 01 Oct '09

01 Oct '09

I read somewhere that for 90% of a wide class of computing problems, you only need 10% of the source code in Haskell, that you would in an imperative language. If this is true, it needs to be pushed. And if by changing a few lines of source code one can develop a whole family of similar applications, that needs to be pushed, also. :) -- Regards, Casey

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Can this be done?
by Cristiano Paris 01 Oct '09

01 Oct '09

I wonder whether this can be done in Haskell (see muleherd's comment): http://www.reddit.com/r/programming/comments/7wi7s/how_continuationbased_we… Cristiano

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Splitting data and function declarations over multiple files
by Peter Verswyvelen 01 Oct '09

01 Oct '09

I guess this is related to the expression problem. Suppose I have a datatype *data Actor = Ball ... | Paddle ... | Wall ...* and a function *move (Ball ...) = * *move (Paddle ...) = * *move (Wall ...) = * in Haskell one must put *Actor* and *move* into a single file. This is rather cumbersome if you work with multiple people or want to keep the files small and readable. Surely it is possible to use type classes, existentials, etc to split the data type into multiple ones, but that's already advanced stuff in a sense. But wouldn't it be possible to allow these to be put into multiple files, and let the compiler merge them back into one? A bit like C#'s partial keyword: in file Ball.hs: *partial data Actor = Ball ...* *move (Ball ...) =* in Paddle.hs *partial data Actor = Paddle ...* *move (Paddle ...) =* The compiler would then merge all partial data types and functions into one. As far as no overlap exists in the pattern matches in move, so that the order of the pattern matches does not matter at all, the partial trick should be possible no? Cheers, Peter

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