- Beginners - Haskell.org

Why Functional Programming Matters <-- this will make you smarter (Part 2)
by Costello, Roger L. 31 May '13

31 May '13

Hi Folks, I completed the second half of my summary of John Hughes' outstanding paper titled, Why Functional Programming Matters. This second half describes the importance of lazy evaluation and function composition. I converted all of Hughes' examples into Haskell. It contains a lengthy example showing how to implement the game of tic-tac-toe, and how lazy evaluation enables the implementation of tic-tac-toe to have a level of modularity that would otherwise not be attainable. (Modularity is important. Hughes writes, "Modularity is the key to successful programming") Here is my summary: http://www.xfront.com/Haskell/Why-Functional-Programming-Matters-Part-2.pdf /Roger

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Re: [Haskell-beginners] Q 1 of 2: How stable is Haskell?
by Hans Georg Schaathun 31 May '13

31 May '13

> On 30/05/13 23:50, Gan Uesli Starling wrote: > > As compared against Python and Perl, how stable is Haskell? I must say, I find the comparison odd. The transition from imperative to declarative programming must be much more mind-bogging than figuring out how to deal with instability. Surely, there are both more and less stable options within each paradigm. On Fri, May 31, 2013 at 12:23:00AM +0100, Mateusz Kowalczyk wrote: > I think you're actually asking ``how much does Haskell change between > revisions'' and not how stable it is. If you're looking for a language > where your program will work with a compiler that's 5 years old or 5 > years from now, this probably isn't it. It's not like programs break > daily because of the changes but I believe there is no hesitation to > change stuff because some program from 5 years ago might no longer > compile. It may be useful to distinguish between stability of the language and stability of the libraries. Talking about libraries, you are probably right. In fact, I have tried to install a number of libraries from hackage recently, and only about half of them succeeded. The typical problem with the others seem to be changes in their dependencies. Typical age is about 3-6 years, I think. Python however, is not even stable in terms of core language. Although library evolution can also break your program, such breakage is generally easier to repair. The problem for haskell, of course, is that the community is too small to sustain a standard library of the extent seen for python or java. Hence one relies much more on beta versions and stale libraries. That does not make haskell any less stable; it is a very different kind of problem. -- :-- Hans Georg

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Re: [Haskell-beginners] Q 2 of 2: GUI and turnkey compiler?
by Hans Georg Schaathun 31 May '13

31 May '13

On Thu, May 30, 2013 at 06:54:54PM -0400, Gan Uesli Starling wrote: > Perl and Python both have GUI interfaces, which are handy for > publishing little give-away apps, as I used to do quite a lot in > Perl/Tk and also in ancient of days on JForth. Do you mean a GUI library? Or a GUI interface to the interpreter? There are plenty of GUI libraries. _Real World Haskell_ from O'Reilly presents one approach, and is otherwise a good read at http://book.realworldhaskell.org > Also to make > give-away apps, there needs to be a turn-key compiler, The glasgow haskell compiler (ghc) is a compiler. > something > into which I'd feed code and out would come an *.exe (albeit quite > bloated in size) well, you can always rename the file to *.exe or whatever else you want. I cannot see how that's relevant. -- :-- Hans Georg

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[Haskell-beginers] mutex monad
by Sergey Mironov 31 May '13

31 May '13

Hi, cafe. Imagine that we are writing an operating system FooOS which uses spinlocks (mutexes, critical sections) to guard shared resources and we want the typesystem to prevent application from re-locking the spinlock which have already been locked by the current thread. Below is the code which does it by explicitly defining MonadSpinlockLess class. Unfortunately, it requires tons of boilerplate instances because we need _any_ monad except SpinlockT to be an instance of MonadSpinlockLess. Is there a better way of doing it? Maybe GADTs? Basically, it would be sufficient to rewrite lock function as lock :: (! MonadSpinlock m) => Spinlock -> SpinlockT m a -> m a but afaik we can't use negations on a typelevel, can we? Thanks, Sergey --- {-# LANGUAGE GeneralizedNewtypeDeriving #-} import Data.IORef newtype Spinlock = SL (IORef Int) newtype SpinlockT m a = SpinlockT { unSpinlock :: m a } deriving(Monad) class (Monad m) => MonadSpinlockLess m instance MonadSpinlockLess IO instance MonadSpinlockLess m => MonadSpinlockLess (ReaderT r m) instance MonadSpinlockLess m => MonadSpinlockLess (WriterT r m) instance MonadSpinlockLess m => MonadSpinlockLess (StateT s m) -- .... lots of instances class (Monad m) => MonadSpinlock m instance Monad m => MonadSpinlock (SpinlockT m) lock :: (MonadSpinlockLess m) => Spinlock -> SpinlockT m a -> m a lock (SL r) h = {- atomicModifyIORef (+1) r (doesn't matter) >> -} unSpinlock h process :: Spinlock -> IO () process sl = handler_a where handler_a = do lock sl handler_b handler_b = do lock sl handler_c {- ^^^^ Second lock, should fail to typecheck -} handler_c = do {- do something -} return ()

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Categories in Haskell
by Robert Goss 30 May '13

30 May '13

I am interested in trying to use category to perform some mathematical computations. It seems natural to want to talk about categories of mathematical objects e.g. Groups,Rings,Algebras ... etc. But I have found some difficulty doing this with Haskell's standard Category libraries. class Category m where id :: m a a comp :: m a b -> m b c -> m a c The main issue is that the objects of the category (represented by the id morphism) are in bijection to some set of haskell types. The problem (I think) this leads too is that 1. All the objects that may appear in a program must be known to the compiler. Which leads to problems (for me at least) as the application I want to study I don't know all the (Groups,Rings,... etc) that I will need. 2. This requires representing (for example) all groups as types. In general I have found while possible it leads to pretty horrendous types. Now this all makes sense Haskell's categories are for reasoning about programs while I want to use it more for pure maths. Has anyone else had a similar problem with categories in haskell? Or am I missing a way of implementing such structures within the standard Category framework for haskell. For a (toy) example take trying to model the category of CyclicGroups --Cyclic n is the object representing the cyclic group Z_n newtype Cyclic = Cyclic Int --CyclicMor n m p is represents a group morphism from Z_n to Z_m taking 1 -> p newtype CyclicMor = CyclicMor Int Int Int This now gives the definition of id :: Cyclic -> CyclicMor id (Cyclic n) = CyclicMor n n 1 comp :: CyclicMor -> CyclicMor -> Maybe CyclicMor comp (CyclicMor n m p) (CyclicMor n' m' p') | m==n' = Just (CyclicMor n m' (p+p')) | otherwise = Nothing The main loss in this approach is compiler can no longer determine if morphisms are compatible. In my project I use a slightly more generic variant : class OrdinaryCategory m where id :: a -> m a a comp :: m a b -> m b c -> Maybe (m a c) I haven't seen a similar construction in haskell libraries does anyone know if this is because 1. It is already there and I just haven't come across it. 2. It solves a problems that no-one else has had. 3. My idea is fundamentally broken / useless in some key way. Thanks for any comments anyone has. Robert

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Cabal Dependency Hell
by harry 30 May '13

30 May '13

According to a few articles I've read, cabal dependency hell is caused by installing packages with -o, which inlines code from dependant packages. Why isn't this avoided by installing packages without inlining? Packages could still be recompiled with -o in a cabal-dev sandbox, but the package repository would be free of dependency hell.

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cabal -j
by harry 29 May '13

29 May '13

I've noticed that cabal install now has a -j option for parallel builds, and has to be told how many builds to run in parallel. This seems strange - most parallel capable systems run in parallel by default, with one process/thread per core. Why is cabal different?

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(no subject)
by emacstheviking 29 May '13

29 May '13

On this page: http://donsbot.wordpress.com/2008/06/04/haskell-as-fast-as-c-working-at-a-h… there is a link to a paper: http://www.cse.unsw.edu.au/~dons/papers/CLS07.html Alas, being only human it seems I am not allowed to read it lest I should reveal the inner secret of the magic Circle. Does anybody know if that paper is downloadable from another location? Thanks.

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Re: [Haskell-beginners] Heterogeneous Lists
by Benjamin Edwards 28 May '13

28 May '13

Might be worth reading http://lukepalmer.wordpress.com/2010/01/24/haskell-antipattern-existential-… Especially in the context of guis and widgets. Ben

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haskell solution for assignment
by Arthur Vard 28 May '13

28 May '13

HI, i am new in Haskell my thinking mostly fit in OOP languages I attached and post link to assignement which i done using C# (can post source if anyone interested in), now i am want to implement it using Haskell. I need feel true Haskell style, for this I think will help i I have the haskell solution for this assignment as well and compare. Does anybody have similar solution in haskell? Assignment 3: Managing a<http://www.cs.cornell.edu/Info/Courses/Spring-98/CS211/assgts/assgt3/assgt3…> Component Dependency Database<http://www.cs.cornell.edu/Info/Courses/Spring-98/CS211/assgts/assgt3/assgt3…> thank you in advance --------------------------------- Arthur Vardanyan Yerevan, Armenia Tel: +374(10)647301 Mobile : +374(93) 184646

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