- Haskell-Cafe - Haskell.org

readEitherChan
by S. Alexander Jacobson 19 Jun '04

19 Jun '04

I'd like a function that blocks until data is available on one of two channels and then returns that data wrapped in an Either. Here is my naive implementation: readEitherChan::Chan a -> Chan b -> Either a b readEitherChan a b = do var <- newEmptyMVar forkIO (readChan a >>= putMVar var . Left) forkIO (readChan b >>= putMVar var . Right) val <- readMVar return val eitherChan a b left right = readEitherChan a b >>= either left right But creating an MVar and starting two threads feels like a lot of overhead for a simple operation. Is there a better way or is forkIO so efficient that I shouldn't care? -Alex- _________________________________________________________________ S. Alexander Jacobson mailto:me@alexjacobson.com tel:917-770-6565 http://alexjacobson.com

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x11 libs
by Matthew Roberts 17 Jun '04

17 Jun '04

I have ghc installed on OSX 10.3 via darwin ports. I have no X11 libs and I can't find anywhere on the web to get them. Can anyone help me out? Cheers, Matt

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Modules and files
by Andrei de A. Formiga 16 Jun '04

16 Jun '04

Hi, Is it possible to have more than one module defined in a single file ? As far as I can tell, the Haskell Report doesn't prohibit this, but so far my tests with hugs and ghc indicate they don't accept multi-module files. Is this standard ? Thanks. --- []s, Andrei de A. Formiga __________________________________ Do you Yahoo!? Friends. Fun. Try the all-new Yahoo! Messenger. http://messenger.yahoo.com/

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EuroHaskell 2004 pictures
by Shae Matijs Erisson 15 Jun '04

15 Jun '04

The EuroHaskell 2004 Conference was great fun. Shae Erisson gave a Haskell tutorial, Jeremy Bobbio gave a talk about hOp, and John Hughes gave a talk/tutorial about Arrows. Code was written, pizza was purchased, most attendees claimed to enjoy themselves. A social event for programmers may be an oxymoron, but it was nice to get so many Haskellers in the same room. Until you've experienced the range of jokes and puns that can come from a CS Student Pub named "The Monad" you haven't plumbed the depths of FP humor. Anders Carlsson has pictures on his website: http://ridley.csbnet.se/gallery/eurohaskell Some of my favorites are the signs: http://ridley.csbnet.se/gallery/eurohaskell/p1010059 - (this is a bathroom / watercloset / toilet, insert your local term) http://ridley.csbnet.se/gallery/eurohaskell/p1010052 and of course the unicycle pictures: http://ridley.csbnet.se/gallery/eurohaskell/p1010026 http://ridley.csbnet.se/gallery/eurohaskell/p1010047 Tune in next year for EuroHaskell 2005! -- Shae Matijs Erisson - Programmer - http://www.ScannedInAvian.org/ "I will, as we say in rock 'n' roll, run until the wheels come off, because I love what I do." -- David Crosby

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RE: [Haskell-cafe] forkIO preemption
by Simon Marlow 15 Jun '04

15 Jun '04

On 07 June 2004 22:59, S. Alexander Jacobson wrote: > When you forkIO and one of the threads blocks > (e.g. on a socket or a readchan), do execution > automatically resume on other threads and does the > blocked thread consume timeslices while it is > blocked? In GHC: yes, no. > In other words, is there any performance > penalty in setting timeslices very large in a > server app (where all I care about is throughput)? No. But there probably isn't much to gain either. Cheers, Simon

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Type conversion problems
by Christian Hofer 13 Jun '04

13 Jun '04

Hi, after some playing around with monad transformers and starting to like those specialities of Haskell, I am currently really feeling thrown back by a simple problem: trying to write a sine-function... Here is the part of my code that is troubling me: fac :: Integer -> Integer fac n = product [1..n] term :: Double -> Integer -> Double term x n = (-1.0::Double)**(fromInteger n) * (x**(fromInteger (2*n + 1))) / (fromInteger (fac (2*n + 1))) The term function is supposed to be the direct translation of the formula of a term within the sum. But it isn't: it is actually cluttered with lots of type conversions, and I had a hard time figuring out how to make it work at all. I hope that this is not the easiest way to do that and I would appreciate any help on how to handle those type conversion issues... Thanks, Chris

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Calling Haskell from Java
by Andrei de A. Formiga 11 Jun '04

11 Jun '04

Greetings, I need to call Haskell functions from a Java program. In the Java side, I can use JNI, which expects C-call functions exported from dynamically linked libraries. So, a primitive way to get what I want is to generate a dynamic library from Haskell code with exported ccall functions. This brings me to my first question: 1) is ghc able to create dynamic libraries in all platforms ? I searched the documentation and found only mention of a currently unsupported way to create DLLs in Win32, and nothing about other platforms. In case this is not possible, one solution might be to statically link ghc-generated code with a C dynamic library which calls the exported Haskell functions. Going further from the primitive, low-level JNI x FFI way, it would be great to automate the wrapping and marshalling necessary, so: 2) is there any tool to help with Haskell function exports and data marshalling to other languages ? I looked at HDirect but it predates the standard FFI definition and it doesn't seem to be supported. I am aware of tools to help calling Java from Haskell, but none to go the other way around. Thanks in advance for any help. --- []s, Andrei Formiga __________________________________ Do you Yahoo!? Friends. Fun. Try the all-new Yahoo! Messenger. http://messenger.yahoo.com/

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Re: Modelling Java Interfaces with Existential data types
by Ralf Laemmel 11 Jun '04

11 Jun '04

Hi Mike, Let's redirect to Haskell cafe. http://www.mail-archive.com/glasgow-haskell-users%40haskell.org/msg06288.ht… http://www.mail-archive.com/glasgow-haskell-users%40haskell.org/msg06289.ht… thanks for your time to look into the HList paper. >1. It looks like your HList is basically a "sugarized" version of > >data AnyMyInterface = Impl1 MyImplementation1 | Impl2 MyImplementation2 > >with the same drawback: you require me to explicitly list all the possible >implementations of my interface I don't think that anything was required to be explicit: If you mean this here: type MyList = MyImplementation1 :*: MyImplementation2 :*: HNil ... then note that this type is inferred! So with HLists, there is nothing like your Impl1 and Impl2, and no new datatype like your AnyMyInterface. Just for clarity: (i) I like comparing Haskell datatypes with Java classes. (ii) In Haskell, when you want to say that you have different implementations for the same interface, or when you want to say that different datatypes provide implementations of a certain interface, then you use Haskell's type class system. (iii) When you define an interface in Java, you rather define a type class in Haskell. Type class are of course more powerful, but we don't mind. (iv) When you say "implements" in Java, you rather say "instance" in Haskell. (v) When you define the methods of the interface to be implemented in Java, you define the methods of the instance. I would claim the Haskell code ends up being more modular :-) You immediately get a simple form of Java-like interface polymorphism in Haskell, say when you got a constraint, then you are polymorphic over implementations (instances) of that constraint. The crux of your concern seems to be that you want to package values of different types (Java: classes). Indeed you want to have heterogeneous containers modulo subtyping. And our lovely HLists are just good for that, but they come with the added value that they don't make the elements opaque as it is the case with the sometimes troublesome existentials. > It >might be possible to load them via hs-plugins or to obtain using something >similar to Clean Dynamics (btw, is there anything similar available for >Haskell?). Data.Dynamics Not needed here. >2. There will also be problems in gluing the utility/legacy code. E. g. if I >want to call any list function on HLists (e.g. reverse). The idea of having >heterogeneous FiniteMap can be implemented by storing the single-element >lists, but it will complicate the code, which will work with the map. I concur. Using HList for everything is perhaps too invasive. The HList paper makes clear that there are certain cases where HLists are really needed, and they are complementary what generics provide you with in Java. I was just able to use them for something were a Generic Java programmer uses Generics and subtyping for. So existentials are perhaps Ok. However, directly comparing the opaque values is untypeable, >I'm completely confident with applying (==) to different type. It's quite a >common operation in OO-world. Yes, what you need is type-safe cast. See paper 1 here: http://www.cs.vu.nl/boilerplate/ >I would like to have the following behaviour: > >instance Eq AnyMyInterface where > (==) (AnyMyInterface a1) (AnyMyInterface a2) = > if (typeOf a1) == (typeOf a2) then false else a1 == a2 > Java is just a weakly typed subset of Haskell :-) Here we go ... -- Yet another heterogeneous equality yaHEq :: (Typeable a, Typeable b, Eq a) => a -> b -> Bool yaHEq a b = case cast b of Just a' -> a == a' Nothing -> False *Main> yaHEq True True True *Main> yaHEq True "True" False *Main> As you said yesterday that the equality issue is your main problem, let's wipe this out for existentials as well. I revise your existential wrapper type to include all goodies that are needed, say Eq and Typeable: data AnyMyInterface = forall a. ( Eq a , Typeable a , MyInterface a ) => AnyMyInterface a This is just your list type: type MyList' = [AnyMyInterface] Here are two lists: list4 = [ AnyMyInterface $ MyImplementation1 10 , AnyMyInterface $ MyImplementation1 10 ] list5 = [ AnyMyInterface $ MyImplementation1 10 , AnyMyInterface $ MyImplementation2 10 ] Here is a demo: *Main> list4!!0 == list4!!1 True *Main> list5!!0 == list5!!1 False Cheers, Ralf PS1: All code is here: http://homepages.cwi.nl/~ralf/HList/code.html PS2: I guess Sheard's work on parameterised modules might also interest you. http://portal.acm.org/citation.cfm?id=507648&dl=ACM&coll=portal And John Huges Restricted Data types as it shows how to reify dictionaries:

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Re: Haskell-Cafe Digest, Vol 10, Issue 3
by Ron de Bruijn 11 Jun '04

11 Jun '04

>G'day all. Good day to you too. >I don't know what you mean by "more complex". A dot >is just a dot, and >it has no internal structure that we can get at using >category theory >alone. Some dots may play specific roles in relation >to other dots and >arrows, but no dot is any more complex than any other, >really. Well, intuitvively seen, I meant. In some categories, the dot might stand for a functor, and in some other as a simple object. >For a counter-example, think of the dual category >Set^{op}. A morphism >f : a -> b in that category means that there is a >function f^{op} : b -> a > >where a and b are sets, however f probably isn't a >function at all. Well, what is it then? I see a function as something where you put something in and you get a result. In this case, you already say, there is a morphism b->a, wel than the following of b to a and return a then is a function? A couple of days ago, I thought of the distinction between a function and a morphism, as in that a function operates on a hole set of objects, a.k.a. domain. And a morphism only on one. Is that the distinction you mean? If all of the above is false, then probably I don't know what a function is. (It looks like the more you are busy with things, the less you seem to know of it). >In a category which is a partial order, there is a >morphism f : a -> b >if and only if a <= b. (Or is it a => b? Can never >remember.) Here, >the morphisms really have no internal structure at >all. If the >category >has a finite number of objects, you can represent the >whole thing using >a bit matrix, and each morphism can be identified with >a bit set to >"true". Preserving internal structure is not much more than preservation of composition, right? >I think the problem here is that you have the idea >that a morphism is >a process that turns one object into another. In many >(probably most) >interesting, practically significant cases, that's >true, but it need >not be. Well, I think I don't know what a function is, because the following of an arrow, represents at least for me a clear mapping of turning one object in to another. Or do you mean things like in logic, that you can see the morphism, as relations (instances of axioms) between logic objects. So the morphism a->b would mean that a implicates b. This way, the arrow is not a process of turning an a into a b. Hmm, that seems logical. >I think it really helps to try to understand category >theory mostly as >a language for talking about things, and not >necessarily "things" in >and of themselves. Using this understanding, a >morphism is a noun, not >a verb. It's a concrete thing describing a >relationship between >objects, >not necessarily an action that you perform on objects. I think you have already answered my question this way. But a confirmation would be nice. It seems I had to read your explanation >1 time. >I don't know if any of this helps or not. Well, it certainly helps. Then the multiplication issue: Is the following a good summary? A multiplication is just a name for an operation that is defined or not defined for each mathematical construction in terms of to which laws the operation should comply. The laws are then things like communativity and so on. Regards, Ron __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com

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Re: [Haskell-cafe] Join and it's relation to >>= and return
by Ron de Bruijn 11 Jun '04

11 Jun '04

I am pretty sure, that >>= is to monads what * is to for example natural numbers, but I don't know what the inverse of >>= is. And I can't really find it anywhere on the web(papers, websites, not a single sole does mention it. It should have type, at least that's what I think: inv::M a->M b I say this, because I find this definition of a multiplication operation: 1. There exists a unique special element called neutral such that the operation on any element and the neutral does not change the element. 2. For every element there exists an inverse such that the operation on an element and its inverse is always neutral. 3. The operation is associative: it does not matter how you apply the operation to three elements. You may apply it to the first two and then to the result and the third element. Or you may first apply the operation to the last two and then to the first and the result of the previous operation. An operation may also be commutative 4. The order of two elements in operation is not important. According to 2 there should be an inverse. For join such an inverse is simple: to apply the type constructor on the monad. But I tried to somehow link it with bind, but than the types don't seem to match. So to be concrete: what's the inverse of bind? If I did make some errors, please tell me so. __________________________________ Do you Yahoo!? Friends. Fun. Try the all-new Yahoo! Messenger. http://messenger.yahoo.com/

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