April 2003 - Haskell-Cafe

Why no findM ? simple Cat revisited
by Ahn Ki-yung 11 Nov '03

11 Nov '03

Simple Cat (revisitied) \begin{code} import IO findM f [] = return Nothing findM f (x:xs) = do { v <- x; if f v then return (Just v) else findM f xs } isLeft (Left _) = True isLeft _ = False main = findM (isLeft) (hCat stdin) where hCat h = try (hGetLine h) : hCat h \end{code} This is my answer for the question of my own, which is posted a couple There are mapM, filterM in the Haskell 98 Standard Library. But why no findM there ? As you can see from simple cat, it seems quite useful. I think fildM should be added to the module Monad. -- Ahn Ki-yung

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L
by e1rJulie＠email.com 30 Apr '03

30 Apr '03

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Is Haskell usefull for me?
by Ron de Bruijn 29 Apr '03

29 Apr '03

Hi there, I have developed a new timetablingalgorithm, at least I think it is.. I already tested it in C++, but it's not quite complete and before it's a really great application the program will be tens of times bigger. I used Helium(subsetlanguage of Haskell) in the past few months and I think it's working better dan C++(especially because there's a lot to know about C++, before you really "know" C++). I want to use Haskell for creating a GUI. Furtermore I need some speed(code can't be more than 1.5 times slower than C++). And I want to use large array's(>60000 elements) where each element had an type in it with another 5 elements. data Thing=ConFunction Int Int Int Int Int Thus an array of Thing. I use Windows 98 Se (It seems that some libs. are only for UNIX.) Is it usefull for me to use Haskell, are there enough libraries so I can easily recreate and improve my program? Could you give some examples of possible functions or libraries I should use? Regards, Ron __________________________________ Do you Yahoo!? The New Yahoo! Search - Faster. Easier. Bingo. http://search.yahoo.com

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Re: seeking lore of the QuickCheck masters
by Lauri Alanko 29 Apr '03

29 Apr '03

On Tue, Apr 15, 2003 at 11:40:25AM +0100, Malcolm Wallace wrote: > You want a generator that can produce an arbitrary code fragment, > *given* what type you want it to have. So first, generate an arbitrary > type, then generate an arbitrary expression for it. "Generate an arbitrary proposition, then generate an arbitrary proof for it." Boy, it's _that_ simple? :) Lauri Alanko la(a)iki.fi

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Online judge and Haskell?
by Steffen Mazanek 28 Apr '03

28 Apr '03

Hello, it's me again and I have a new mad idea. There are a lot of programming contests out there and unfortunately most (all?) of them do not allow Haskell code till now! I think, this is hard to accept. Why not ask the people which run acm.uva.es/problemset to provide their problem sets, input and output files and the basic judging software and install it at haskell.org? Alternatively this may be a nice project for a student. I think an adaption of this software to permit Haskell code can not be that difficult! Do you think there is a need for such a project? Maybe it could help Haskell to become a part of official programming contests and hence to gain more popularity. I'm sorry to say, that due to another project my free time is very limited so I can not get it going by only myself. Ciao, Steffen -- Haskell: that's where I just curry until fail, unwords any error, drop all undefined, maybe break, otherwise in sequence span isControl and take max $, id: (d:[])

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mmap and IArray slices
by David Roundy 28 Apr '03

28 Apr '03

I had a couple ideas, and I was wondering if they were good ones. One is the idea of a slice function on an IArray: a' = slice a (i,j) giving an array with the elements i..j of array a. The nifty idea would be that this could be done without any memory copy. The catch, of course, would be that you might end up having to retain the entire array a just because you need a little slice of it. However, I think this risk (which should be documented) would be overwhelmed by the increased efficiency (both in space and time) gains that would be possible. The most obvious (to me at least, since I've been working a lot with PackedStrings recently) application would be in substrPS. Using slice, this would be an O(1) operation, rather than an O(n) operation, and would use essentially no memory (at the cost of retaining the mother string). I'm thinking that it may even be possible to make the GC smart enough to get rid of mother strings which are needed by very few children (at least if it's desperate), if array slices were supported at a low enough level... I imagine they'd have to be supported at a very low level. The second (and related) idea is that it would be nice to be able to use mmap on a readonly file to read it into an immutable array. Of course, this works really nicely with array slices, since with the slices the mmapped array wouldn't need to be copied, and could remain backed by the file itself. I imagine being able to read a 100M file into a PackedString, then run linesPS (or possibly a special slice version of linesPS) on it, and have a [PackedString], all of which is backed by the file, so when memory gets tight it can be just thrown away (by the VM system) and then get read in again when I use it. The mmap idea seems even tougher than the slice idea, as it would require (preferably) keeping track of all mmapped regions so when the file is closed they could all be copied over into normal memory and unmapped. Technically they could remain mmapped after the file is closed, but I think that would cause problems if the program then wants to reopen the file for writing. Anyhow, those are just a couple of ideas I had, and I have nowhere near the knowledge if haskell internals to tell if they are workable or not. -- David Roundy http://www.abridgegame.org

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RE: Is this a bug in multithreading code?
by Simon Marlow 28 Apr '03

28 Apr '03

> Nick Name <nick.name(a)inwind.it> > > You are right, [...] > > I further conjecture that more than half of all space leaks can be > identified not by profiling your code but by running the following > command: > > grep '+1' *.hs *.lhs > > I'm half joking but I've found that this one is very common in certain > kinds of code. Good idea! Here's another space-leak-avoiding trick: whenever you write Int as a constructor field, consider writing !Int instead. Same goes for other basic types and tuples. This pre-emptive space leak avoidance helps us in GHC quite a bit. Cheers, Simon

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Arkadasa yardimci olalim
by Dr. Ahmet Yalcin 27 Apr '03

27 Apr '03

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Sell:Industrial Permanent Magnets
by L.Z.Mou 25 Apr '03

25 Apr '03

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Re: Is this a bug in multithreading code?
by Nick Name 25 Apr '03

25 Apr '03

Il 22:54, mercoledì 23 aprile 2003, hai scritto: > I conjecture that the problem would go away if you added appropriate > uses of seq or $! to your program so that the value stored in the MVar > was always fully evaluated You are right, the following code works well strictIncM x = return $! x + 1 ... forkIO (loopM (modifyMVar_ v strictIncM)) ... Thanks for your help Vincenzo

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