- Beginners - Haskell.org

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 03 Jul '09

03 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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ghc profiler beginner
by José Prous 01 Jul '09

01 Jul '09

I never used the ghc profiler before, when I try to compile the program with: ghc -prof -auto -O2 -o Main --make Main.hs I get: Could not find module `Data.Binary.IEEE754': Perhaps you haven't installed the profiling libraries for package `data-binary-ieee754-0.3'? Use -v to see a list of the files searched for. Is there a way to tell the profiler to ignore the external libraries or I have to recompile all the libraries with profiling enabled? thanks

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High precision doubles
by Aaron MacDonald 29 Jun '09

29 Jun '09

Is there anything in Haskell that can represent doubles with higher precision than the regular kind? I'm working with formulas that generate sets of values, and values that I know should be zero are ending up as things like -7.105427357601002e-15 or -1.7763568394002505e-14, which is causing all kinds of butterfly effects. I've heard of Rational, but I need to also work with irrational numbers (namely, pi and anything that comes from cos and sin). What other options do I have? - Aaron

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Rigid type variables match error
by Darryn 29 Jun '09

29 Jun '09

Hi, I wonder if anyone can explain what is going on here and what to do about it. I'm fairly new to Haskell, so apologies in advance if my question seems naive. I've cut my code down to a minimum that reproduces the problem. I receive an error in GHCI in the following code, complaining that it cannot match the rigid type variables for the instance definition for Ainst for the function a3. Can anyone advise about what to do about it? ------------------------------------------------ class A a where a1 :: a a2 :: a -> a a3 :: (B b) => b -> a class B b where b1 :: Int -> b data (B b) => Ainst b = I | J (Ainst b) | K b instance (B b) => A (Ainst b) where a1 = I -- a2 :: (B b, A a) => a -> a a2 = J -- a3 :: (B b, A a) => b -> a a3 = K -- Error! -- a3 = K `asTypeOf` a3 -- Error even with this! data Binst = Val Int instance B Binst where b1 = Val ------------------------------------------------ Test5.hs:17:9: Couldn't match expected type `b1' against inferred type `b' `b1' is a rigid type variable bound by the instance declaration at Test5.hs:12:12 `b' is a rigid type variable bound by the type signature for `a3' at Test5.hs:5:13 Expected type: b -> Ainst b1 Inferred type: b -> Ainst b In the expression: K `asTypeOf` a3 In the definition of `a3': a3 = K `asTypeOf` a3 Failed, modules loaded: none.

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Re: [Haskell-cafe] What is an "expected type" ...
by Joe Fredette 29 Jun '09

29 Jun '09

When Haskell runs it's type checker, it tries to "guess" the type of each function. Thats why you can write: map (+1) and it knows that you're talking about a function of type: Num a => [a] -> [a] Another thing, called 'defaulting' resolves this, but you didn't ask about that, so I won't go into it. An expected type is one that you provide to the compiler in the form of a type signature, this can be used to specialize a general type (like the one I showed) or to resolve ambiguous types the compiler can't, or just for documentation/good practice. So when I write: foo :: Num a => [a] -> [a] foo ls = map (+1) ls The "expected type" for `foo` is `Num a => [a] -> [a]`. I imagine you're asking this because you got an error which said your expected type doesn't match your inferred type. That might, for instance, happen if I wrote: bar :: String bar = 'a' 'a' has type `Char`, since `String` is not `Char`, the type checker infers that 'a' has type char, but _expects_ it to be type String. Two solutions are as follows: --- Method 1 bar :: Char bar = 'a' --- Method 2 bar :: String bar = "a" Can you see why those two changes fix the problem? Also, just as a matter of process, I forwarded this to the haskell-beginners list, as I imagine type errors like these come up a lot, and someone probably has a better explanation over there. /Joe michael rice wrote: > as opposed to an "inferred type"? > > Michael > > > ------------------------------------------------------------------------ > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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Gtk2Hs and Cairo on Windows
by Philippe D.-P. 27 Jun '09

27 Jun '09

Hello, I would like to use Haskell with Gtk2Hs/Cairo for a project, but I have trouble making it work on Windows. I've installed the lastest GHC + Gtk2Hs and I tried to copy/paste the code from examples on this tutorial http://darcs.haskell.org/gtk2hs/docs/tutorial/Tutorial_Port/app1.xhtml. On linux; no problem. But for some reason I get a "not in scope: `eventSent`" error on Windows and I can't compile. How can I solve the problem and, more importantly, is it possible to write portable code with Gtk2Hs/Cairo ? Thank you

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if True than let...
by Bernhard Lehnert 26 Jun '09

26 Jun '09

Hi, I'm sorry because I am absolutely sure, this is bloody obvious to the knowing. Being a total beginner I'm stuck. In the main = do part I wrote: 1: if a == True then putStrLn "Yes!" else putStrLn "No." 2: if a == True then let b = "+" else let b = "-" Line #1 works perfectly well. Read line #2 as pseudocode and you'll see what I want to do. Read it in ghci and it produces " parse error on input `=' " I tried 'case of' but it doesn't work either. What am I doing wrong? Thank you for any help, Bernhard

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on
by Ivan Uemlianin 26 Jun '09

26 Jun '09

Dear All During some recent discussion about different ways of sorting a list by some comparison function, one of the methods suggested was like this: sortByFunc = sortBy (compare `on` func) Now, I can imagine how this works, and I don't want to discuss the efficiency of this implementation (I imagine it's not "Schwartzian", for example;). I know what the backticks do (allow infix use of a function). My question is: what is "on"? I'm afraid I haven't been able to find anything about this, no doubt because of all the false positives coming up in searches. Please could someone tell me its official title, or where I could find it in the docs? Thanks and best wishes Ivan -- ============================================================ Ivan A. Uemlianin Speech Technology Research and Development ivan(a)llaisdy.com www.llaisdy.com llaisdy.wordpress.com www.linkedin.com/in/ivanuemlianin "Froh, froh! Wie seine Sonnen, seine Sonnen fliegen" (Schiller, Beethoven) ============================================================

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