- Beginners - Haskell.org

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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concerning wxHaskell: wxcore fails to install
by Obscaenvs 29 Jun '11

29 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 29 Jun '11

29 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 29 Jun '11

29 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 28 Jun '11

28 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 28 Jun '11

28 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 28 Jun '11

28 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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Creating beautiful code: can you make this divide-and-conquer implementation of the "tails" function beautiful?
by Costello, Roger L. 28 Jun '11

28 Jun '11

Hi Folks, Below is a divide-and-conquer implementation of the "tails" function. Notice the two patterns (x:y:xs) and (x:[]). And notice that (x:y:xs) is used by the "length" function and again by the "splitAt" function. That doesn't seem elegant. Can the function be simplified and made beautiful? /Roger tails' :: [a] -> [[a]] tails' (x:y:xs) = map (++zs) (tails' ys) ++ tails' zs where m = length (x:y:xs) n = m `div` 2 (ys,zs) = splitAt n (x:y:xs) tails' (x:[]) = [[x]]

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DSL for Android development
by Sean Charles 27 Jun '11

27 Jun '11

As a yet to be published iPhone hacker, now somewhat saddened by the AppStore bouncers, I've turned to Android. I have a lot of Java and J2ME experience but only eight months with Haskell. I hate java. The JVM is awesome. Clojure is wish-fulfillment for Android right now. Having used Chicken (scheme) in the past for generating applications for people I know that compiling language A to language B can be very successful, especially when the end-user doesn't know or care how he got his app, only that it works. So... where would be a good place to start with a DSL that could produce Java code as its output? This is new to me! I've written simple lexers and parsers and I know the ropes on that front but I don't know haskell well enough yet to know how to go about it. I am thinking that I'd need to create data types for the major classes, or maybe not, if i can produce my own abstraction that generated multiple classes in the output etc and so i descend into confusion about where to start! I suspect that state carrying monadic types will be prevalent, good job I understand monads. Yes that was sarcasm. I've done the Scheme-in-48 hours thing, so I am ok with parsec. I did consider writing a "true" language that will code-generate java instead of a DSL, that's still in my mind too. As I understand DSL, it's up to me to invent my language to work within the "domain" of Android and then make it create Java code for subsequent feeding to the android tools. As a first guess, I am going to study the "Hello Android" example and then see how I could create a DSL around that little program. As I see it, programming boils down to just a couple of things: function calls and decisions. I appreciate that there is more to it: XML files and images etc to be bound in but that's nothing a good bash script won't fix for me! Thanks, Sean Charles. PS: I know this is a *big* thing to take on but I love big and I love learning so let's rip this one to bits.

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