July 2009 - Beginners

GHC compiling for different platforms
by Bernhard Lehnert 04 Jul '09

04 Jul '09

Hi folks! When I compile a program under windows - will the .exe run under any windows? Do I have to distribute only the .exe or more files? When I compile a program under a 64bit-Linux on an AMD-CPU, will the executable run under 32bit and/or Intel-CPU as well? Are there any, say, compiler flags, to improve portability? Thanks, Bernhard

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help with types and composition
by Britt Anderson 03 Jul '09

03 Jul '09

I would like to write a function that takes two lists of numbers and returns a number. For simplicity, let's assume an integer, i.e. myfunc:: [Integer]->[Integer]->Integer But I can't quite get things right when trying to define myfunc = ... and making the arguments implicit. My difficulty might be seen with the following two functions, let, f1 = (zipWith ($)) . (map (*)) f2 = sum if I set the types such that f1 returns [Integer] and sum accepts [Integer] it seems that somehow I should be able to compose them in to a single function that takes two lists and returns the number, but f1 . f2 and numerous variations don't work. I can curry some list l and f1 . (f2 l) works, but is there a way to get f1 and f2 combined into a function that accepts two lists? My problem is not really about writing the function but about composition. I can write the function with arguments or a local lambda that suits my practical purpose. But the fact that I think I should be able to compose f1 . f2 somehow reveals a conceptual misunderstanding that I hope someone will help me clear up. Thank you, Britt Anderson

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Fwd: [Haskell-beginners] help with types and composition
by Alexander Dunlap 03 Jul '09

03 Jul '09

Oops...wrong list address...sorry ---------- Forwarded message ---------- From: Alexander Dunlap <alexander.dunlap(a)gmail.com> Date: Thu, Jul 2, 2009 at 8:54 PM Subject: Re: [Haskell-beginners] help with types and composition To: Britt Anderson <britt(a)uwaterloo.ca>, haskell-beginners(a)haskell.org On Thu, Jul 2, 2009 at 10:54 AM, Britt Anderson<britt(a)uwaterloo.ca> wrote: > I would like to write a function that takes two lists of numbers and returns > a number. For simplicity, let's assume an integer, i.e. > myfunc:: [Integer]->[Integer]->Integer > But I can't quite get things right when trying to > define myfunc = ... > and making the arguments implicit. > > My difficulty might be seen with the following two functions, > let, > > f1 = (zipWith ($)) . (map (*)) > > f2 = sum > > > if I set the types such that f1 returns [Integer] and sum accepts [Integer] > it seems that somehow I should be able to compose them in to a single > function that takes two lists and returns the number, but f1 . f2 and > numerous variations don't work. I can curry some list l and f1 . (f2 l) > works, but is there a way to get f1 and f2 combined into a function that > accepts two lists? > > > My problem is not really about writing the function but about composition. I > can write the function with arguments or a local lambda that suits my > practical purpose. But the fact that I think I should be able to compose f1 > . f2 somehow reveals a conceptual misunderstanding that I hope someone will > help me clear up. > > Thank you, > Britt Anderson > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners > > Try (f2 .) . f1 . The type of (f2 .) is (a -> [Integer]) -> a -> Integer and the type of (.) is (b -> c) -> (a -> b) -> (a -> c) so for the second (.), b :: ([Integer] -> [Integer]), c :: ([Integer] -> Integer) and a :: [Integer] so the final type is [Integer] -> [Integer] -> Integer. (It might be easier to work out the types yourself if you can - my description is doubtless rather difficult to follow.) A word of caution, though - most Haskellers in my experience prefer to not do this type of composition; they find it clearer to specify one or both of the arguments explicitly. Hope that helps you, Alex

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Problems with IO actions
by Adam Bergmark 02 Jul '09

02 Jul '09

I have an issue with IO actions. At the moment I have the following snippet of code, which works as I expect it to. mapM_ (writeChan ch) . lines =<< hGetContents h That is, it gets each line in the file and writes it to the channel. Previously I had just: writeChan ch =<< hGetContents h but when i readChan this I get the empty string every time. I have two questions: * I'm still a little unsure of why mapM_, sequence and the like are necessary, could someone please explain this, or point me somewhere where I can read up on it? * Why does this problem occur when there is only one action to perform? Cheers / Adam Bergmark

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Is there a replacement for network-minihttp that allows lazy downloads?
by Adam Bergmark 02 Jul '09

02 Jul '09

I'm trying to write a program that will download possibly large files using HTTP. In Real World Haskell I read: Warning The HTTP library used here does not read the HTTP result lazily. As a result, it can result in the consumption of a large amount of RAM when downloading large files such as podcasts. Other libraries are available that do not have this limitation. We used this one because it is stable, easy to install, and reasonably easy to use. We suggest mini-http, available from Hackage, for serious HTTP needs. Looking at http://hackage.haskell.org/cgi-bin/hackage-scripts/package/network-minihttp I see that minihttp is deprecated. Is there a replacement I can use? / Adam Bergmark

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