- Haskell-Cafe - Haskell.org

Re: Haskell and scripting
by Pierre-Etienne Meunier 04 May '10

04 May '10

Hi, You do not need a DSL at all, in fact. The simplest way to do this, if you use GHC, is to use the GHC api. This can compile everything for you, and return a value with type "Dynamic", from Data.Dynamic and Data.Typeable. It is type-safe, you can write within very little time (even if the documentation for the GHC api is not -- yet -- perfect), and use a real language whose users are still alive. Cheers, PE ps : by the way, if you are interested by this kind of programming, I saw that someone was writing a shell in haskell, called hashell I think. If you want to help him... > Hello Café, > > I don't know if you know > conky<http://en.wikipedia.org/wiki/Conky_%28software%29>. > It's a well-known open-source system monitor (a software that displays > information on the desktop, like CPU frequency, disk usage, network rate, > etc.). > It is quite good, but it's very descriptive, and even if you can call shell > commands it's clearly not made for being scripted. > What I would do is to make a similar system monitor, which base would be > compiled Haskell code, but that would be scriptable with some DSL, or > already existing interpreted language. > I've thought about a Lisp/Scheme language, since those languages are > functional, dynamically typed and simple (so enable a quick scripting) and > I'm not very keen on making my own DSL > > What I would like to know is: > 1) If you have other solutions > 2) How do haskellers usually script their applications

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eclipse, haskell-platform and windows. Installation problem.
by Han Joosten 03 May '10

03 May '10

Hi all I have been using an old version of eclipsepf for quite some time now on my windows machine. Recently I saw that there is a newer version, as well as a newer version of haskell. However, I am not able to get it all to work. I wonder if you are able to help me out. First, I installed the newest version of ghc by means of the haskell-platform. Then I installed git, so I was able to download the special variant of scion, as specified at http://sourceforge.net/projects/eclipsefp/files/EclipseFP%202%20branch/1.11… Then I tried to build the downloaded scion: C:\data\gitRepository\scion>runhaskell Setup.hs configure Configuring scion-0.1.0.2... Setup.hs: At least the following dependencies are missing: binary ==0.5.*, ghc-paths ==0.1.*, ghc-syb ==0.1.*, hslogger ==1.0.*, json ==0.4.*, list-tries ==0.1.*, multiset >=0.1 && <0.3, uniplate ==1.2.* As you can see, this failed. However, since I have the new cabal, that comes with the haskell platform, I figured that the above dependancies should not be a problem. So I used cabal: Cabal install binary (and then I cabal installed all other missing packages. ) All cabal install commands completed succesfully. Then, again i tried 'runhaskell setup.hs configure', but this failed again with exactly the same message as before. It seems that all packages that I installed dissapeared! I probably do something stupid, but I cannot figure out what I should do. Any help is welcome! Thanks! Han Joosten -- View this message in context: http://old.nabble.com/eclipse%2C-haskell-platform-and-windows.-Installation… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Release: rangemin-2.0
by Louis Wasserman 03 May '10

03 May '10

The original rangemin project was my first Haskell package, and definitely my first attempt at super-advanced algorithms work in Haskell. (I here define "super-advanced" as "not in MIT OpenCourseWare undergraduate algorithms lecture notes from any year.") Here's the problem: Suppose we have an array of length n. Preprocess the array in O(n) time such that we can look up the minimum element of any slice in the array in O(1) time. Coming back to this project after two years, and especially rewriting the approach with the vector package, was *spectacularly* fun. rangemin-2.0<http://hackage.haskell.org/package/rangemin>is now on Hackage, with a new API as well. The constant factor on the O(n) preprocessing is, based on criterion benchmarks, 88 milliseconds per million elements, on my 2.5GHz Ubuntu laptop, with GHC HEAD and -O2 -fvia-c -optc-O3. I've spent the past three days unsuccessfully trying to get the LLVM backend to work, because this algorithm has a *lot* of the sort of tight loops that LLVM excels at. Unfortunately, I haven't been able to install the LLVM backend, which is very disappointing. That said, GHC with the C backend is about 10x slower than an optimized C++ implementation, which is all right considering that the algorithm plays so much to C++'s strengths. (If anybody has LLVM working and would be willing to help, I'd massively appreciate it if you could benchmark both the C++ implementation -- not mine, but included in a tarball inside the rangemin distribution package -- and my implementation, with LLVM -O2. Benchmarking code is here<http://hpaste.org/fastcgi/hpaste.fcgi/view?id=25309#a25309> .) Louis Wasserman wasserman.louis(a)gmail.com http://profiles.google.com/wasserman.louis

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Building regex-posix for windows
by Станислав Черничкин 03 May '10

03 May '10

I'm having trouble building regex-posix for Windows under MinGW because MinGW does not include regex.h. As far as I know Haskell Platform uses MinGW and it includes regex-posix. I'd like to build regex-posix by myself, like Haskell Platform does. I would appreciate it if someone suggests some resources about building regex-posix and other packages for windows. I’m also looking for Haskell Platform developer’s blog or something similar, where I can read about porting Haskell on Windows. Thanks.

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Learning about Programming Languages (specifically Haskell)
by Samuel Williams 03 May '10

03 May '10

Dear Friends, I'm looking for some help from the Haskell community. I hope this is the right place to ask for information. I'm putting together a website aimed at high school students and teachers, and would like to add a few paragraphs to the following page: http://programming.dojo.net.nz/languages/haskell/index ... explaining why one would want to learn Haskell. I don't know the answer to this since I don't use Haskell extensively. What are the main application areas and how is it useful? Any other suggestions or ideas for the Haskell page would be fantastic, and any suggestions to other pages in general is also very helpful. http://programming.dojo.net.nz/ Kind regards, Samuel

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Re: FRP for game programming / artifical life simulation
by Ben 03 May '10

03 May '10

slightly off topic, but how does one handle pausing / saving / restarting in the FRP framework, especially the arrowized version? i've only been able to do this via explicit (or monadic) state-passing, e.g. imperative / piecemeal versus declarative / wholemeal, which seems against the spirit of FRP. b

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MonadPlus or Alternative or ...?
by Sean Leather 03 May '10

03 May '10

I want to generalize a set of functions from lists to some functor type. I require the following three operation types. f a a -> f a f a -> f a -> f a Should I choose MonadPlus and use these? mzero return mplus Or should I choose Alternative and use these? empty pure (<|>) Or should I make my own class? Or is there another option? Thanks, Sean

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FRP for game programming / artifical life simulation
by Ben Christy 03 May '10

03 May '10

I have an interest in both game programming and artificial life. I have recently stumbled on Haskell and would like to take a stab at programming a simple game using FRP such as YAMPA or Reactive but I am stuck. I am not certain which one I should choose. It seems that Reactive is more active but is it suitable for game programming. Also has anyone attempted to implement neural networks using FRP if so again which of these two approaches to FRP would you suggest?

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are forkIO threads event-driven?
by Aran Donohue 03 May '10

03 May '10

Hi Cafe, In GHC, if a thread spawned by forkIO blocks on some network or disk IO, is the threading system smart enough not to wake the thread until an IO event occurs on its input/output? The Control.Concurrent documentation doesn't specify, and the previous discussions I could find on this topic are out-of-date. There is a years-old GHC ticket, too, recently revived[2]. Put another way, is it possible yet to use forkIO for making a server to handle tens of thousands of concurrent network connections? If not, what is the best current Haskell/GHC way? Thanks, Aran [1] http://www.monkey.org/~provos/libevent/ [2] http://hackage.haskell.org/trac/ghc/ticket/635

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Leibniz equality can be made injective
by oleg＠okmij.org 03 May '10

03 May '10

We show that type families let us write Leibniz equality witnesses of the injectivity of type constructors. We may use Leibniz equality not only for introduction but also for elimination. The paper on ``Typing Dynamic Typing'' (Baars and Swierstra, ICFP 2002) demonstrated the first implementation and application of Leibniz equality witnesses in Haskell: > newtype EQU a b = EQU{subst:: forall c. c a -> c b} A term (eq :: EQU a b) witnesses the equality of the types a and b: in any context c, the type a can be substituted with the type b. The context is represented by the type constructor c. The witness gives the constructive prove of substitutability: given a term of the type c a, we can always obtain a term of the type c b. Since we don't know anything about the context c, such a substitution can only happen if the two types are indeed equal. Hence the sole non-bottom witness is > refl :: EQU a a > refl = EQU id testifying that equality is reflexive. Leibniz equality became quite popular since it lets us represent GADTs to a large extent. The power of Leibniz equality comes from the fact that the context c may indeed be anything we want. The following example illustrates that power, by concisely proving that the equality is transitive. The proof of transitivity is very short > tran :: EQU a u -> EQU u b -> EQU a b > tran au ub = subst ub au We treat the type EQU a u as (EQU a) u, that is, as an application of the `type constructor' (EQU a) to the type 'u'. We then use the witness EQU u b to replace the type u with the type b in the context (EQU a), giving us the desired EQU a b. When writing type checkers and type inferencers in Haskell along the lines of typing dynamic typing, we also need to know that two arrow types are equal if and only if their components are equal. In one direction, > eq_arr :: EQU a1 a2 -> EQU b1 b2 -> EQU (a1->b1) (a2->b2) if the argument and result types of two arrow types are equal, the arrow types themselves are equal. Fortunately, for this, commonly used direction, the proof is easy, after we define suitable type-level combinators F1 and F2 to place the types being equated in the right context: > newtype F1 t b a = F1{unF1:: EQU t (a->b)} > newtype F2 t a b = F2{unF2:: EQU t (a->b)} > eq_arr :: EQU a1 a2 -> EQU b1 b2 -> EQU (a1->b1) (a2->b2) > eq_arr a1a2 b1b2 = > unF2 . subst b1b2 . F2 . unF1 . subst a1a2 . F1 $ refl In the type EQU (a1->b1) (a2->b2), the type `constructor' (F1 (a1->b1) b2) represents the context of the type a2, and the type constructor (F2 (a1->b1) a2) is the context of the type b2. Leibniz equality only applies if the context of a type is represented as a type constructor; therefore, we sometimes have to define auxiliary newtypes to put the contexts in the required form. In the reverse direction, we have to prove that if two arrow types are equal, their components (e.g., the argument types) are equal as well. The proof will witness the injectivity of the arrow type constructor. It is thought that injectivity cannot be proven with Leibniz equality at all. For example, the paper ``Implementing Cut Elimination: A Case Study of Simulating Dependent Types in Haskell'' by Chen, Zhu and Xi, PADL'04, needed the reverse, elimination direction and found it impossible to obtain with Leibniz equality witnesses. We demonstrate that type families help. Type families let us define ``subtractive contexts'', so that we can view a type 'a' as the type (a->b) placed into the context that removes (->b). Again we use an auxiliary newtype to make the subtractive context appear as a type constructor. > type family Fst a :: * > type instance Fst (x->y) = x > type instance Fst (x,y) = x > -- etc. > newtype FstA a b = FstA{unFstA :: EQU (Fst a) (Fst b)} > eq_arr1 :: EQU (a1->b1) (a2->b2) -> EQU a1 a2 > eq_arr1 eq = unFstA . subst eq $ ra > where > ra :: FstA (a1->b1) (a1->b1) > ra = FstA refl The complete code is available at http://okmij.org/ftp/Haskell/LeibnizInjective.hs The code includes a simple test of the injectivity.

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