- Haskell-Cafe - Haskell.org

Network module problem
by David Roundy 03 Mar '03

03 Mar '03

Hello. I'm running into a problem with the Network module, which I suspect is pretty easy to fix, but am not sure how to best do so. The problem is that "accept" fails when the reverse DNS fails, with the following error: Fail: does not exist Action: getHostByAddr Reason: no such host entry I'm not sure how to get around this. I don't actually need the hostname of the client, and would be happy to just substitute its IP address in that field, but I'm not sure how to do that. If I understand correctly (which I may not), if I were to catch this connection with code like: (myh,_,_) <- accept s `catch` ... there would be no way to actually get the handle itself from the connection, which seems to imply that I'll have to go into the Network.Socket module and use the lower level code (which looks like it would be straightforward). However, it seems that this is a simple enough (and presumably common enough) problem that there should be a more elegant solution which would allow me to use the high level interface. Is there any way to `catch` the exception within getHostByAddr, and substitute the IP address for the output of that function? (I'll admit, I've never thoroughly understood exceptions in any language, in the sense of understanding how really to use them right.) Also, what if I just didn't care about the hostname? That seems like a common enough case that you wouldn't always want to do a reverse DNS lookup on every accept. Does that just mean you would have to use the low level interface? -- David Roundy http://civet.berkeley.edu/droundy/

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modeling out of memory
by Cagdas Ozgenc 03 Mar '03

03 Mar '03

Greetings, 1) How does one model "out of memory" condition in Haskell, perhaps using a Maybe type? 2) Could you give an intutive description of data construction, and how it relates to lamda calculus? Thanks

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is identity the only polymorphic function without typeclasses?
by Cagdas Ozgenc 03 Mar '03

03 Mar '03

Greetings, Is identity function the only meaningful function one can write without constraining the type variable using a typeclass? If not, could you please give a counter-example? Thanks

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Re:
by wr 28 Feb '03

28 Feb '03

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Hugs' graphics lib
by Daniel Jian 27 Feb '03

27 Feb '03

Hi all, I have just installed Hugs to my system. And then I also compiled its drawing library. My question is how can I include that library in my code? I know this is rather silly a question, but I have tried to play around with the environment variable in .bashrc, and I have added:-P$HOME/local/graphics-2.0.4/lib/x11 to the HUGSFLAGS. But Hugs still complains that it cannot include the lib. I am just wondering, can I make an executable file using hugs? How can I do that (if I can)? Many thanks! Blessings, Daniel Jian __________________________________________________ Do you Yahoo!? Yahoo! Tax Center - forms, calculators, tips, more http://taxes.yahoo.com/

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Buenas noches !!!
by Olga Lucia Quintero Fonseca 24 Feb '03

24 Feb '03

Buenas noches, quisiera consultarles sobre lo siguiente, tengo estas dos funciones -- Calcula Raiz Wn wn n = cos c :+ (-1*sin c) where c=2*pi/n twiddlefactor r k = concat [(map (^i) [(wn rs)^i | i<-[0..s-1]])| i<-[0..r-1]] where rs = (r^k) ; s = (r^(k-1)) la dificultad que tengo es que twiddlefactor no me acepta enteros compila pero no me acepta los parametros, sin embargo si la defino como --Calcula vector de Twiddle Factor twiddlefactor r s rs= concat [(map (^i) diag) | i<-[0..r-1]] where diag = [(wn rs)^i | i<-[0..s-1]] no tengo problema, lo que sucede es que posteriormente esta definicion me es inconveniente pues el valor de rs es r^k y lo necesito en un programa recursivo, que me esta dando inconvenientes precisamente por que debo estar reenviando el valor r^k. Agradezco la ayuda que puedan darme

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Re: Simple problem from Hudak's SOE
by C.Reinke 24 Feb '03

24 Feb '03

> I checked out the type of "snd" in hugs and basically > figured out what it does, but I was wondering where I could find the > definition of this function. Is it possible to look at it in hugs? If your hugs is configured correctly to use an editor, simply type ":find snd". This underappreciated feature is one reason why you want to set the editor in hugs!-) http://cvs.haskell.org/Hugs/pages/hugsman/started.html hth, Claus

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Debugging haskell
by Sengan.Baring-Gould＠nsc.com 24 Feb '03

24 Feb '03

http://www.catb.org/~esr/writings/taoup/html/ch01s06.html states that "debugging often occupies three-quarters or more of development time". I don't think that is my experience in Haskell... more like 1/4 at most. I was wondering what others felt. Sengan

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Simple problem from Hudak's SOE
by M. Parker 24 Feb '03

24 Feb '03

I'm a real newbie to Haskell, and I'm having trouble with a particular problem dealing with higher-order functions. Exercise 5.9 in Hudak's "School of Expression" asks us to write a function, "makeChange," s.t. it makes change for a given amount using coins in a coin supply (represented by a list of decreasing integers). For example, make Change 99 [5,1] ==> [19,4] This chapter is about higher order functions, so I'm assuming he wants us to compute the result using the higher order functions defined in the chapter (map, foldl, foldr). I devised two solutions: {-Solution 1-} makeChange money coinList = zipWith div (scanl mod money coinList) coinList {-Solution 2-} makeChange' money (coin:coins) = let money' = money `mod` coin numCoins = money `div` coin in (numCoins: makeChange' money' coins) makeChange' 0 _ = [] makeChange' _ [] = [] However, my problem is that neither solution uses the higher-order functions defined in the chapter. So is it possible to solve this problem using map and fold? Furthermore, Hudak makes the case that we should strive to find the higher-order solutions instead of the recursive ones because the latter leads to clearer and more concise solutions. Although solution 1 is more concise, I feel like Solution 2 is clearer to me than Solution 1, but maybe this is just because I'm new to haskell and higher order functions. It just seems like its easier to understand the actual algorithm in solution 2 than in solution 1. Thanks, Matt Parker University of North Texas undergrad http://www.cs.unt.edu

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by Domain Monitor 23 Feb '03

23 Feb '03

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