June 2011 - Beginners

opengl and application state
by Sean Charles 30 Jun '11

30 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 30 Jun '11

30 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 30 Jun '11

30 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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concerning wxHaskell: wxcore fails to install
by Obscaenvs 30 Jun '11

30 Jun '11

Attempting to install the wxcore package yields the following result on my Arch Linux box, got the same result on an Ubuntu box: <code> cabal: Error: some packages failed to install: wxcore-0.12.1.7 failed during the configure step. The exception was: ExitFailure 1 </code> As a result, the package wx cannot be installed either. I am trying to get started with wxHaskell. Is there any way to fix this retaining one's sanity? /Fredrik

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type question
by Roelof Wobben 30 Jun '11

30 Jun '11

Hello, I'm now following the book Programming in Haskel by Hutton. IM now reading the chapter about types. But I don't understand this : False :: Bool True :: Bool ¬ :: Bool ---> Bool So False and True has the type Bool. That's clear. I find out that ¬ makes the opposite of True and False. But why is it also the type Bool --> Bool Roelof

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Re: [Haskell-beginners] type question
by ARJANEN Loïc Jean-David 30 Jun '11

30 Jun '11

If I understand correctly, ¬ is the logical not. It's a function from booleans to booleans (denoted by it's type: Bool -> Bool) because it takes as its argument a boolean to negate and returns the negation of its parameter. Hoping to have helped you, Loic > On Wed, 29 Jun 2011 14:55:50 +0000, Roelof Wobben wrote: > Hello, > > I'm now following the book Programming in Haskel by Hutton. > IM now reading the chapter about types. > > But I don't understand this : > > False :: Bool > True :: Bool > ¬ :: Bool ---> Bool > > So False and True has the type Bool. > That's clear. > > I find out that ¬ makes the opposite of True and False. > But why is it also the type Bool --> Bool

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Re: [Haskell-beginners] book question
by Sean Charles 29 Jun '11

29 Jun '11

RWH is the first haskell book I bought after deciding that reading pages on a browser wasn't doing it for me... I bought it eight months ago and to be honest, it looks like I've owned it for years... it takes you from beginner level, and I have to say that it does a very good job at guiding you through the type system and onto IO, monads, parsec, the works. Some of the projects in the book are just awesome, not only in their execution but in the step-by-step explanation, for example the bar code reading and unix pipes projects are really well done. Kudos to RWH for that. Having learned Erlang and LISP did give me some advantages in that the concepts of higher order functions and pattern matching were already under my belt, along with guard clauses etc. If I could say one thing so far to beginners to haskell it would be two things: * learn to "read" type signatures * learn to "decipher" the type related error messages Everything else just comes naturally after practice. Practice. And more practice. A famous author once said "You get good at writing by reading, and writing lots." :)

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book question
by Roelof Wobben 29 Jun '11

29 Jun '11

Hello, What's a good book for a absolute beginner in Haskell programming with a lot of exercises so I can see if I understand it. Roelof

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book review?
by Ramy Abdel-Azim 29 Jun '11

29 Jun '11

has anyone read this book? beginners(a)haskell.org There are some mixed reviews on the product page. I'm a mid-level software engineer. I've only started learning about Functional programming (didn't even really learn about lisp in college) in the past 12 months. I'm looking to learn more about functional programming to help me improve as a programmer generally. Thanks, _Ramy

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Re: [Haskell-beginners] Do I understand this well
by Roelof Wobben 29 Jun '11

29 Jun '11

________________________________ > Date: Mon, 27 Jun 2011 16:14:28 +0100 > Subject: Re: [Haskell-beginners] Do I understand this well > From: colinpauladams(a)gmail.com > To: rwobben(a)hotmail.com > > No. > > You are onfusing defining a function with using it. > > > addVectors (x1, y1) (x2, y2) = (x1 + x2, y1 + y2) > > defines the function addVectors > > (x, y) = addVectors (3.0, 4.5, -3.4, -5.6) > > calls the function, and binds x and y to the components of the result > (pattern matching). > > Alternatively: > > fst . addVectors (3.0, 4.5, -3.4, -5.6) > > will return the x-compnent of the result (by composing functions) > Oke, I understand that I think But when I ghci I enter : (x, y) = addVectors (3.0, 4.5, -3.4, -5.6) after I made a file with the file defenition. I get this error : :1:7 parse error on input = So something is not right here. Roelof

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