- Beginners - Haskell.org

Could not find file in module graph
by Øystein Kolsrud 01 Jul '11

01 Jul '11

Hi! I'm using version 2.0.4 of Eclipse FP and I very often get an error saying "Could not find file in module graph.". I did a quick search on the web for this message, but all I could find was some old threads discussing an issue with this symptom (for instance http://code.google.com/p/scion-lib/issues/detail?id=36#c1) Does anyone know what the error means and how I can avoid it? -- Mvh Øystein Kolsrud

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GOTCHA! OpenGL window behaviour is mis-spelled...
by Sean Charles 30 Jun '11

30 Jun '11

This caught me out for several frustrating minutes until I finally saw the problem... package: Graphics.UI.GLUT.Begin data ActionOnWindowClose has three constructors, and I wanted the third one which I glanced at and then spelled it how I knew it *should* be: ContinueExecution which gave me an error, and finally I realised that as an English word it is in fact spelled incorrectly in the code, like so: ContinueExectuoin HTML Link: http://hackage.haskell.org/packages/archive/GLUT/latest/doc/html/Graphics-U… Is this one of those "accepted things" as in, too difficult to change because it will break stuff or could it be changed ? All the best, Sean.

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Global variables
by ARJANEN Loïc Jean-David 30 Jun '11

30 Jun '11

Hello, For a pet project I need a global counter, giving how many times a given function was called. What would you use for that purpose ? Loic

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A first try
by Panagiotis Koutsourakis 30 Jun '11

30 Jun '11

Hello all, I've been spending some time recently learning Haskell. I am not a newbie in programming, and I've had some experience with lisp in the past, but I have not really studied functional programming. I am mostly seeking feedback regarding style, and glaring mistakes that I may make in some toy examples, but I am not sure if this is the proper place to ask. In case it is though, here is an attempt (not yet complete) at the word count program: http://pastebin.com/JZw22QGA Thanks in advance! -- Panagiotis Koutsourakis

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Re: [Haskell-beginners] Understanding category theory
by Arlen Cuss 30 Jun '11

30 Jun '11

On 30/06/2011 10:26 AM, Christopher Howard wrote: > Would anyone recommend a particular book or tutorial to help me > understand category theory? I've tried reading a few tutorials i've > googled, but even the "beginner" ones are very difficult and seem to > assume a lot. I've just started reading this, found on /r/haskell on Reddit: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.78.7317&rep=rep1&t… It's quite readable, may require a little effort though!

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Re: [Haskell-beginners] Understanding category theory
by Jake Penton 30 Jun '11

30 Jun '11

On 2011-06-29, at 8:26 PM, Christopher Howard wrote: > Would anyone recommend a particular book or tutorial to help me understand category theory? I've tried reading a few tutorials i've googled, but even the "beginner" ones are very difficult and seem to assume a lot. > I just picked up the book by Benjamin Pierce entitled Basic Category Theory for Computer Scientists. It is pretty good; at least I am making some headway in it. I have another more recent book by Pierce called Types and Programming Languages, which I found to be very readable, so I am a bit of a fan of Pierce. It is not an easy topic, IMHO. I have a B.Sc. in math, and am currently doing a master's degree in software engineering. I find functional programming literature that purports to be tutorial in nature (as opposed to research oriented) some of the most impenetrable stuff I have ever read. Reading it is like trying to eat a bowl of steam with a fork. Good luck.

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Understanding category theory
by Christopher Howard 30 Jun '11

30 Jun '11

Would anyone recommend a particular book or tutorial to help me understand category theory? I've tried reading a few tutorials i've googled, but even the "beginner" ones are very difficult and seem to assume a lot. Sent from my HTC

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opengl and application state
by Sean Charles 29 Jun '11

29 Jun '11

How!?!?! LOL I am getting back into OpenGL again with Haskell now and I have a program that listens to all the callbacks and prints stuff out, terminates on Ctrl+X and draws a grey sphere. Earth shattering. If I was using C/C++ I'd be using a (shudder) global pointer to a record containing state that the render callback would use to know what to do. I smell state monads or something but I cannot see how to pass them into "the loop". I want to organise my code so that all input events cause a record to be updated and then the render part just uses the current values to draw the right things, with the obvious caveat of "what if a key event happens half way through the render phase." MVars? Threads? What??? I'm a real newb on this right now! How do I,or what is a "best practice" for passing a continuously changing record across calls given that the "displayCallback" doesnt have any obvious place to plug one on? In Erlang or even Haskell, there is the idion of making a tail call back to yourself with the new modified record and so on but I cannot figure out where to actually pass it around! Thanks in advance. Sean.

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Why aren't David Harley's QT bindings more popular?
by Michael Serra 29 Jun '11

29 Jun '11

Hi all, Recently I have been doing the usual beginner's search for the "best GUI toolkit" available to Haskell, and today I installed David Harley's qtHaskell bindings. I was surprised to find they were easy to install (on Linux), and all the examples run flawlessly. So I have two related questions: (1) has anyone out there used qtHaskell, and what was your experience; (2) why do these bindings seem to be a less-popular option than wxwidgets and gtk2hs? I understand qtHaskell is much newer than the others, and so far its development appears to be a one-man operation; other than these obvious explanations, is there a bigger reason people don't seem to be using or discussing QT with Haskell? I would have thought working bindings to such a useful framework would be a bigger deal. - m. serra

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Various style at instance of a typeclass
by Haisheng Wu 29 Jun '11

29 Jun '11

The Control.Monad.Writer module exports the Writer w a type along with its Monad instance newtype Writer w a = Writer { runWriter :: (a, w) } instance (Monoid w) => Monad (Writer w) where return x = Writer (x, mempty) (Writer (x,v)) >>= f = let (Writer (y, v')) = f x in Writer (y, v `mappend` v') My question is Why the signature is "Monad (Write w)" but neither "Monad Write" nor "Monad (Write w a)"? Any difference among those three styles? Thanks a lot! -Haisheng

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