April 2014 - Beginners

Clocks, scheduling, jack and supercollider
by martin 27 Apr '14

27 Apr '14

Hello all, I just installed the haskell-supercollider bindings ("hsc3") and I am able to send events to supercollider and hear sound. But I have no idea how to schedule events to be sent in the future - a prerequisite to play a song. Ideally I would sync to an external clock source, like a time code (SMPTE/MIDI). Also it would be nice to listen to jack-transport, so I can fast-forward and rewind from another application. But the jack bindings for Haskell do not mention jack-transport. Do you have any advice where I should look next.

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easy question: transform Float -> Int how?
by John M. Dlugosz 25 Apr '14

25 Apr '14

If I have a value of type Float, and I want to pass it (approximately rounded) to a function wanting an Int, how do I do that?

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How to build a package?
by John M. Dlugosz 24 Apr '14

24 Apr '14

I used cabal unpack to get the source of a package to read. Now I want to make a change. How do I tell Haskell Platform to make that (the same way cabal knows how to) and do so with the provided files rather than downloading into temp. I got a package to work before using Leksah, but I'd like to do it directly since the cabal file is present and works just fine. A related issue: what should I do about the local copy of a module name that is also installed? Can I make my project use that instead, without having to globally change the name in all the files?

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why do classes require the type variable in type signatures?
by Dimitri DeFigueiredo 23 Apr '14

23 Apr '14

Hello All, Why does this compile class Special a where isSpecial :: a -> Bool whereas, GHC 7.6 complains about this class AlsoSpecial a where isAlsoSpecial :: b -> Bool This is the error message I get: The class method `isAlsoSpecial' mentions none of the type variables of the class AlsoSpecial a When checking the class method: isAlsoSpecial :: forall b. b -> Bool In the class declaration for `AlsoSpecial' My question is: Why must I use the type variable of the class declaration (i.e. *a*) in the type signature for the associated method? Is there a fundamental reason for this or is it just a GHC specific limitation? I didn't see a need for it when watching this video http://channel9.msdn.com/posts/MDCC-TechTalk-Classes-Jim-but-not-as-we-know… that explains the translation that GHC does to turn type classes into core. Thanks! Dimitri

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How would you write...
by John M. Dlugosz 22 Apr '14

22 Apr '14

chain1 = [ translate x y $ color red $ Circle r | (x,y,r) <- pappus 100 [1..10] ] The above is not right, as the comprehension syntax doesn't see the input range buried in an argument. What's the right way to express pap1 = translate x y $ color red $ Circle r where (x,y,r) = pappus 100 1 where 1 is [1..10] and I get a list of results? (pap1 does work as expected) (1) how can the list comprehension syntax manage it, (2) what's the recommended way to express that (not necessarily via list comprehension syntax) ? —John

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graphical output?
by John M. Dlugosz 22 Apr '14

22 Apr '14

How might I (easily) use Haskell to draw simple 2D graphics? Outputting a SVG file is OK for simple static results, but I want to animate. (Windows platform is strongly preferred.)

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Haddock and XML
by Emanuel Koczwara 21 Apr '14

21 Apr '14

Hi, I want to include some XML in documentation produced by haddock. Here is my simple test case: http://lpaste.net/103047 I generate the docs like this: haddock -h Doctest.hs Can I set some command line options or maybe there are some formatting options (like '@' in my example) to get this XML rendered correctly? Emanuel

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How do I return Nothing from a function?
by KC 21 Apr '14

21 Apr '14

How do I return Nothing from a function? When there is no edge in the graph this code fails. *** Exception: Graph.hs:65:9-26: Irrefutable pattern failed for pattern (Data.Maybe.Just w) *Main> :t weight weight :: (Ix t1, Ix t2) => t1 -> t2 -> Array (t1, t2) (Maybe t) -> t weight x y g = w where (Just w) = g!(x,y) Thank you for your time. -- -- Sent from an expensive device which will be obsolete in a few months! :D Casey

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How do I return Nothing from a function?
by KC 21 Apr '14

21 Apr '14

-- -- Sent from an expensive device which will be obsolete in a few months! :D Casey

1 0

Re: [Haskell-beginners] type problems with my little code
by raffa f 21 Apr '14

21 Apr '14

thanks for everyone that replied! you all sound really nice. removing the type info from pack' worked, i hadn't thought of that because i believed that when pattern matching you always had to write the type info first. the ScopedTypeVariables thing sounds really interesting too, and i'm gonna read more about it and maybe i'll understand more how types work. and gesh, thanks for the comments on the rest of my code, it's always nice learning new and useful functions. and i did realize that pack repeats the same list for every element in the list, but i had written just before that a function that takes care of repeated elements in a list, so now my final code just applies that function to pack. i don't know how efficient that is, but it works!

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