May 2013 - Beginners - Haskell.org

[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 ()

2 2

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

5 5

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.

3 5

cabal -j
by harry 30 May '13

30 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?

6 7

(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.

2 2

Re: [Haskell-beginners] Heterogeneous Lists
by Benjamin Edwards 29 May '13

29 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

1 0

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

1 0

Which IDE use a professional Haskeller?
by Giacomo Tesio 28 May '13

28 May '13

I know the page on haskell.org about IDEs, but I'd like to know which IDE professional Haskell programmers actually use at work. Giacomo

8 12

Re: [Haskell-beginners] Looking for a project to join
by Ivan Jovanovic 27 May '13

27 May '13

Hi Emanuel, I have recently found here (http://www.seas.upenn.edu/~cis194/final.html) some projects where authors were keen to help students to join an open source project. Might help. Cheers, Ivan

2 1

Looking for a project to join
by Emanuel Koczwara 27 May '13

27 May '13

Hi, I'm looking for a project to join. Who need a volonteer? Best regards, Emanuel

5 4