January 2008 - Haskell-Cafe

Re: [Haskell-cafe] [newbie question] Memoization automatic in Haskell?
by Hugh Perkins 14 Jan '08

14 Jan '08

On Jan 12, 2008 10:54 PM, Henning Thielemann <lemming(a)henning-thielemann.de> wrote: > > On Sat, 12 Jan 2008, Hugh Perkins wrote: > > > I guess that Haskell's referential transparence means the answers to > > the isPerfectSquare will be cached, ie automatically memoized? (not > > sure if is correct term?) > > http://www.haskell.org/haskellwiki/Memoization > Interesting... but I dont understand... I thought that referential transparence meant that once the answer to a function has been calculated once, it will always be the same, and that the interpreter can, and will, cache this answer? So, if I call f( 20 ) once, for some, arbitrary, f, it will have to go away and calculate f(20), but if I call it multiple times, it will just return the value it already calculated?

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How to add ENV variable in runhaskell shell script
by Steve Lihn 14 Jan '08

14 Jan '08

Hi, In perl scripts (unix), one can do == #!/usr/local/bin/perl BEGIN { $ENV{LD_LIBRARY_PATH} = ...; } do my perl stuff == The setting of ENV variable before the program runs allows me to fix the shell environments. In this case, I need to specify where to look for the shared library. How do I do this in Haskell if I am writing the same script using: == #!/path/to/runhaskell {- how to set LD_LIBRARY_PATH = ... ? -} do my haskell stuff == Thanks, Steve

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Possibility to port vshaskell to VS2008?
by Philipp Riemer 14 Jan '08

14 Jan '08

Because my university is in the MSNDAA-program I could download the new Visual Studio 2008 Professional, what I've done today. Now I tried to install the vshaskell-extension to start learning Haskell but the installer only quits with the message that (of course) VS2005 is not installed. Now is my question if I have to download the "old" VS2005 or if it is somehow possible to get this extension running with VS9.0? I already tried to use the EclipseFP-plugin so I already have (win)hugs, ghc and haddoc installed (if it helps for the problem above). The problem with this plugin was that Ctrl+Space does apparently not work for code hints and because I'm just started to learn the language, I don't know every keyword... So I have to look it up everytime somewhere, I want to implement something new. The fact that Prelude.hs and others are plain text are a great help, but what I have seen in the screenshot at http://www.haskell.org/visualhaskell/screenshots.html would be much better...

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Solving a geometry problem with Haskell
by Rafael Almeida 14 Jan '08

14 Jan '08

Hello, I have been browsing through the problems at projecteuler.net and I found one that seemed interesting. It's the problem 176, I'll state it here: The four rectangular triangles with sides (9,12,15), (12,16,20), (5,12,13) and (12,35,37) all have one of the shorter sides (catheti) equal to 12. It can be shown that no other integer sided rectangular triangle exists with one of the catheti equal to 12. Find the smallest integer that can be the length of a cathetus of exactly 47547 different integer sided rectangular triangles. I thought I've solved it nicely, only to find later on that my solution was too slow (it has been running for almost three days now). While I was creating my solution I used literate haskell, which I think helped me a bunch to think about the problem. I wrote it originally in portuguese -- the portuguese literate version has all the attempts I've made until getting to the final one. For posting it here I've translated the reasoning around the attempt that solved the problem -- although it does it very slow -- into a new .lhs file: http://www.dcc.ufmg.br/~rafaelc/problem176.lhs After some profiling I found out that about 94% of the execution time is spent in the ``isPerfectSquare'' function. So I began researching about better ways to write that functio. Someone pointed to me on #haskell that the C library gmp has such a function. So I went ahead and wrote a C version of the program using gmp: http://www.dcc.ufmg.br/~rafaelc/problem176.c It's not the prettiest C code, as I did it really quickly, simply translating haskell code into C, but I believe it works. That C solution has been running for more than one hour now and no solution has come up yet. So I don't think it's worthed even writing a faster isPerfectSquare in haskell. As I was translating from Portuguese to English I revisited my logic and I can't see any problems with it. I think the problem is only of slowness, really. Does anyone have a better idea for how I should try to solve this problem? I'm all out of ideas. []'s Rafael

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Re: All equations must have the same arity - why?
by Jonathan Cast 13 Jan '08

13 Jan '08

On 13 Jan 2008, at 4:56 PM, jerzy.karczmarczuk(a)info.unicaen.fr wrote: > Jonathan Cast writes: >> On 13 Jan 2008, at 4:15 PM, Achim Schneider wrote: > ... never mind... >> I'm glad you are capable of ever so nobly assigning the purest >> imaginable motives to the designers of Haskell. You are an >> inspiration to us all. jcc > > Jonathan, please... > You see as well as anybody else what is the point of this discussion. > Perhaps it is not as bad as discussing with Cristian Baboi, but still. Very well, I'll be good. jcc (Meant to send this to the list first time...)

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Add "number of downloads" to hackageDB page?
by Hugh Perkins 13 Jan '08

13 Jan '08

I seem to remember a thread about this a while back actually, but... Any chance of adding the number of downloads to the hackageDB page? For those packages that are included in ghc, hugs etc, perhaps add a green tick with "included in ghc", "included in hugs", etc? That way it should be relatively easier to know for newbies which packages are more or less mature/stable. Optional: perhaps include a tick-box/dropdown to only show packages with more than a certain (configurable) number of downloads?

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Possibilities for website construction using Haskell?
by Hugh Perkins 13 Jan '08

13 Jan '08

What are the possibilities for website construction using Haskell?

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hash-cons
by Tony Finch 13 Jan '08

13 Jan '08

I've been thinking about hashlife recently. I've written part of a fairly didactic C implementation <http://dotat.at/prog/misc/hashlife.c> but I there are more interesting things that can be done with it. In particular, the core calc2() function is essentially implementing lazy evaluation, so it ought to be possible to write it even more nicely in Haskell. However it relies crucially on a hash-cons aka intern aka memoized constructor find(). How can one implement this in Haskell? Google doesn't turn up anything very helpful. If I can solve the hash cons problem, then it ought to be fairly straightforward to use Parallel Haskell to speed up the algorithm on multicore machines. Tony. -- f.a.n.finch <dot(a)dotat.at> http://dotat.at/ PORTLAND: SOUTH 7 TO SEVERE GALE 9 VEERING SOUTHWEST 5 TO 7. ROUGH OR VERY ROUGH. RAIN AT FIRST, THEN SQUALLY SHOWERS. MODERATE OR GOOD.

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Patterns overlapped?
by Fernando Rodriguez 13 Jan '08

13 Jan '08

Hi, When I compile this code, ghc complains about some overlapped patterns in function depth. What on Earth is ghc talking about? O:-) data BinTree a = EmptyTree | NodeBT a (BinTree a) (BinTree a) deriving Show emptyBT = EmptyTree depth emptyBT = 0 depth (NodeBT _ left right) = max (1 + depth left) (1 + depth right)

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Access to list
by Fernando Rodriguez 13 Jan '08

13 Jan '08

Hi, If I define the follwoing functions: car (x:_) = x car [] = [] cdr (_:xs) = xs cdr [] = [] and try to apply them to some list, such as car [1,2,3] I get this odd error: <interactive>:1:9: No instance for (Num [a]) arising from the literal `3' at <interactive>:1:9 Possible fix: add an instance declaration for (Num [a]) In the expression: 3 In the first argument of `car', namely `[1, 2, 3]' In the expression: car [1, 2, 3] What am I doing wrong this time? :-P Thanks

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