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Processing of large files
by Alexander N. Kogan 04 Nov '04

04 Nov '04

Hi! I'm newbie and I don't understand how to process large files in haskell with constant memory requirements. For example, to count numbers of different words in text file I wrote following program: -- start import System.Environment merge [] x = [(x,1)] merge (e@(a,b):xs) x | x == a = (a,b+1):xs | otherwise = e : merge xs x procFile = putStrLn . show . foldl merge [] . words main = do args <- getArgs readFile (head args) >>= procFile -- end How should I modify it to make it useful on large file? It eats too much memory... -- Alexander Kogan Auto Wave Processes Group Institute of Applied Physics RAS

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Re: [Haskell-cafe] different arities
by Henning Sato Rosen 03 Nov '04

03 Nov '04

> You have met up with one of the rules for function bindings in standard > Haskell (http://www.haskell.org/onlinereport/decls.html) The use of "arity" > in the error message is a poor choice of words. The rule is that each clause > in the function binding must contain the same number of argument patterns. Oh, I see, thanks Scott and Marco for answering! I am beginning to realise that everything worth knowing about Haskell really is in the report:-) Regards/Henning -------------------------------------------------------------------------------------------------- BTW, what might the reason for that be, is it for the sake of ease of translation/implementation, or for the sake of some principle? I mean, as soon as a person realises that there are multiple syntaxes for declaring a function, he/she will unify them to one abstract function in the head (where the resulting type matters, not the syntax). So, even if one probably should avoid mixing as in the example, for readability reasons, one might still argue that the restriction goes against PoLS. Just a thought.

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Fwd: [Haskell-cafe] A simple question
by Marc Charpentier 03 Nov '04

03 Nov '04

Thank you all for the friendly and helpful explanations - and for your patience. The solution to my problem is finally f :: Double -> Double f i = (-1)**i/(2**(10*i)) * (-2^5/(4*i+1)-1/(4*i+3)+2^8/(10*i+1) -2^6/(10*i+3)-2^2/(10*i+5)-2^2/(10*i+7)+1/(10*i+9)) I had confused (^) and (**) due the fact that 1.0/2^16 or (-1.0)^2/2^16 are accepted. The important sentence in the Gentle Tutorial is " It is easy to forget at times that numerals are overloaded, and not implicitly coerced to the various numeric types," (Finding my errors was made somewhat difficult due to the following messages I got while testing my conjectures: *Main> let i=1 in 1/2^i 0.5 *Main> let i=1 in i/2^i Ambiguous type variable `a' in these top-level constraints: `Fractional a' arising from use of `/' at <interactive>:1 `Integral a' arising from use of `^' at <interactive>:1 :-) ) Thanks to all. Marc Charpentier

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A simple question
by Marc Charpentier 03 Nov '04

03 Nov '04

Good evening, Doing some calculations in ghci, I encountered a difficulty which I cannot resolve. Here's a distilled version, prepared for the command line: let i = 2 in (-1)^i/(2^(10*i)) * (-2^5/(4*i+1)) gives the following error messages ghci: Ambiguous type variable `b' in these top-level constraints: `Fractional b' arising from use of `/' at <interactive>:1 `Integral b' arising from use of `^' at <interactive>:1 hugs: ERROR - Unresolved overloading *** Type : (Integral a, Fractional a) => a *** Expression : let {...} in (-1) ^ i / 2 ^ (10 * i) * negate (2 ^ 5 / (4 * i + 1)) However, if I substitute i = 2 in the last fraction, then the transformed expression let i = 2 in (-1)^i/(2^(10*i)) * (-2^5/(4*2+1)) is accepted. Note that both let i=2 in (-1)^i/(2^(10*i)) and let i=2 in (-2^5/(4*i+1)) are accepted. Why? Thank you in advance. Marc Charpentier

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Examples using STRef
by John Velman 02 Nov '04

02 Nov '04

I want to label items on a canvas (using HTk for gui) with a sequence of labels. It looks like STRef would provide the right capability. (Needless to say, at this point, I'm new at using Haskell and am trying to swallow everything at once.) Are there some examples of programs using STRef? I'd like examples that are not too complicated, but not too simple, either. :-) Thanks, John Velman

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different arities
by Henning Sato Rosen 02 Nov '04

02 Nov '04

These two functions have the same type. Is this behaviour intended? (Can functions of the same type have different arities?) ------------------------------------------- data Fun = Increment | Square fun:: Fun -> Int -> Int fun Increment n = n+1 fun Square = (\n -> n*n) ------------------------------------------- Error message in HUGS: ERROR "arity.hs":17 - Equations give different arities for "fun GHC: arity.hs:3: Varying number of arguments for function `fun' /Regards Henning

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installing HaXml
by Tom Spencer 02 Nov '04

02 Nov '04

Hi, I thought i had successfully installed HaXml, ghci loads up the package successfully etc. but then it complains that it can't find XmlLib. Using -v i get the following: Prelude> :load iChing.hs *** Chasing dependencies: iChing.hs: Can't find module `XmlLib' locations searched: XmlLib.hs XmlLib.lhs /usr/lib/ghc-6.2.1/imports/XmlLib.hi /usr/lib/ghc-6.2.1/imports/HaXml/XmlLib.hi /usr/lib/ghc-6.2.1/hslibs-imports/data/XmlLib.hi /usr/lib/ghc-6.2.1/hslibs-imports/util/XmlLib.hi /usr/lib/ghc-6.2.1/hslibs-imports/posix/XmlLib.hi /usr/lib/ghc-6.2.1/hslibs-imports/concurrent/XmlLib.hi /usr/lib/ghc-6.2.1/hslibs-imports/lang/XmlLib.hi i ran the install script again to see what was happening and noticed this line at the end of the install script: warning: can't find GHCi lib `HSHaXml.o' Saving old package config file... done. Writing new package config file... done. make[1]: Leaving directory `/usr/local/bin/HaXml-1.12/obj/ghc' it doesn't seem to work in ghc or hugs either any ideas? Thanks in advance Tom Spencer

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RE: [Haskell-cafe] Re: www.haskell.org -> applications -> HWS-WP
by Bayley, Alistair 02 Nov '04

02 Nov '04

> From: Benjamin Franksen [mailto:benjamin.franksen@bessy.de] > > The tar ball seems to have vanished already. Do you guys have > any idea where > to get the sources for the HWS (with or without plugins)? http://cvs.sf.net/viewcvs.py/haskell-libs/libs/hws-wp/ I think Shae is mirroring this at: http://www.scannedinavian.org/repos/hlibs/hws-wp/ ... but I don't know if one is more up-to-date than the other. ----------------------------------------- ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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Re: www.haskell.org -> applications -> HWS-WP
by Benjamin Franksen 02 Nov '04

02 Nov '04

On Tuesday 02 November 2004 11:53, you wrote: > Benjamin Franksen wrote: > >The Haskell Web Server with Plugins is not easily found if one only has > > the link to the sourceforge announcement. Maybe a link to the home page > > http://www.mdstud.chalmers.se/~md9ms/hws-wp/ would be nice? > > > >Cheers, > >Ben > > Cheers, > > I added the link (although I wonder for how long it will survive, > belonging to a student login). The tar ball seems to have vanished already. Do you guys have any idea where to get the sources for the HWS (with or without plugins)? Ben

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RE: [Haskell-cafe] Newbie Question on type constructors
by Brian Beckman 02 Nov '04

02 Nov '04

Most interesting discussion -- in reading it, I realized that I had a 'hidden agenda' in asking my question (hidden even from myself), and that is: can I put interesting functionality, like precondition checks & data validation, in data constructors? I suspect not, and that's why I tend to write something like the following: data Shape = Circle Float | Square Float deriving (Eq, Show) circle :: Float -> Shape circle x = if (x <= 0) then error "Bad radius!" else Circle x That's fine, but I don't know how to prevent users from calling "Circle" directly (in some other languages, I could declare the raw constructor to be "private", and, in Haskell, there may be some way for me to hack module exports to hide the raw constructor, but I haven't seen a way to do that). In any event, it might be useful if I could have some pattern to prevent circumvention of initialization code. Advice? -----Original Message----- From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Ben Rudiak-Gould Sent: Monday, November 01, 2004 11:33 AM To: finnw(a)iname.com Cc: haskell-cafe(a)haskell.org Subject: Re: [Haskell-cafe] Newbie Question on type constructors Finn Wilcox wrote: >On Mon, 1 Nov 2004, Ben Rudiak-Gould wrote: > >>In particular, one cannot write an invert :: (a->b) -> Maybe >>(b->a) which never returns a wrong answer, except for invert = const >>Nothing > >How about: > >invert = undefined > >This never returns an answer at all, so it can't return a wrong one! Sorry, I should have been clearer: my Nothing return was intended to mean "I don't know an answer", not "there is no answer". So my const Nothing is like your undefined, a function which never returns an answer (and thus never returns a wrong one). -- Ben _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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