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Re: [Haskell-cafe] Join and it's relation to >>= and return
by Ron de Bruijn 10 Jun '04

10 Jun '04

Hello again, I have thought a while about morphisms and although I had written down in my paper that a functor and also a natural transformation are also morphisms, but in a different category, I now am not sure anymore of this. If you see everything(objects and morphisms) as dots and arrows, and some arrows and some dots are just some more complex than others, then this holds, but is that legal? Intuitively seen, it is. According to some paper, morphisms are not like functions, in that you can apply some morphism to an object, but that you can only compose them. But I would say that if you have the morphism f:a->b, that is a arrow from dot a to dot b. That there clearly is a notion of following that arrow, in effect applying a function. And suppose there is the following path of morphisms: a---->b---->c---->d, with a..d are dots. Then I would say there are three functions(constructed by composition)(in fact more, because of identity mapping) from a that when followed give new objects. This following of arrows, looks a lot like general function application, as in f(x) = 2x for example. It's btw quite hard to write the essence of monads down in a clear and precise way. I hope you can give some feedback on the above. Ragards, Ron P.S. The question about multiplication still stands. Probably multiplication is a set of laws defined on a mathematical object that must hold. And for each mathematical object there is such definition. Is this correct? __________________________________ Do you Yahoo!? Friends. Fun. Try the all-new Yahoo! Messenger. http://messenger.yahoo.com/

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Re: how to write a list builder? fixpoint?
by Graham Klyne 09 Jun '04

09 Jun '04

[moved to Haskell-cafe] This makes my head hurt! Let me see if I get this right. When the type checker sees: build' as a a2 ... then it invokes the second instance of BuildList (i.e. BuildList a (a->r)) and returns the value: build'(a:as) a2 ... But when it sees just: build' as a (as a value, not used as a function application), then the type checker invokes the first instance and returns: reverse$ a:as So they key here appears to be that the type-checker can distinguish between function application and other uses of an expression? #g -- At 18:18 01/06/04 -0700, oleg(a)pobox.com wrote: > > Is it possible to write a function to build a list [a]? > > so that I can write [a,b,c,d] as "getBuilt $ build a b c d"? > >Yes, in the format very close to desired. > > > {-# OPTIONS -fglasgow-exts #-} > > {-# OPTIONS -fallow-undecidable-instances #-} > > > > module Foo where > > > > class BuildList a r | r-> a where > > build' :: [a] -> a -> r > > > > instance BuildList a [a] where > > build' l x = reverse$ x:l > > > > instance BuildList a r => BuildList a (a->r) where > > build' l x y = build'(x:l) y > >That's it. It works both on GHC and Hugs. > >*Foo> build' [] True :: [Bool] >[True] >*Foo> build' [] True False :: [Bool] >[True,False] >*Foo> build' [] True False False :: [Bool] >[True,False,False] >*Foo> build' [] 'a' 'b' 'c' 'd' 'e' :: [Char] >"abcde" >*Foo> build' [] (1::Int) :: [Int] >[1] >*Foo> build' [] (1::Int) (2::Int) :: [Int] >[1,2] >*Foo> build' [] (1::Int) (2::Int) (3::Int) :: [Int] >[1,2,3] > >Note that the type annotation [Bool] etc. at the end is required: it >is the delimiter of the list. Who would have thought that the type >annotation can play the role of Nil... > >_______________________________________________ >Haskell mailing list >Haskell(a)haskell.org >http://www.haskell.org/mailman/listinfo/haskell ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

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Modelling Java Interfaces with Existential data types
by Tom Pledger 09 Jun '04

09 Jun '04

Hi. Another way is to take advantage of Haskell's laziness: > class Foo a where > foo :: a -> Int > > data Oof1 = Oof1 Int > instance Foo Oof1 where foo (Oof1 i) = i > > data Oof2 = Oof2 Int > instance Foo Oof2 where foo (Oof2 i) = i > > list1 = [foo (Oof1 10), foo (Oof2 20)] That's all it takes. The applications of foo in list1 don't get evaluated until list1 is used, e.g. by sum. Regards, Tom

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forkIO preemption
by S. Alexander Jacobson 08 Jun '04

08 Jun '04

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 other words, is there any performance penalty in setting timeslices very large in a server app (where all I care about is throughput)? -Alex- _________________________________________________________________ S. Alexander Jacobson mailto:me@alexjacobson.com tel:917-770-6565 http://alexjacobson.com

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Join and it's relation to >>= and return
by Ron de Bruijn 08 Jun '04

08 Jun '04

Hello, The last 3 or 4 days I have been studying a lot of Category Theory so that I would be able to explain the concept of a monad to some people at the university in a "learn to present something"-course. newtype S a = State -> (a,State) -- functor T to map objects mapS::(a-> b) -> (S a -> S b) -- functor T to map morphisms unitS :: a -> S a --\eta joinS::S(S a)-> S a -- \mu This is a complete monad using a direct mapping from Category Theory. I really like it, because it's mathematically grounded. But I don't know how to map this to Haskell monads using the standard "bind" and "return", as I explain below. Although, I do believe I understand most parts of it(as in Functor, Natural Transformation and so on), I don't really understand how the join function (conventional monadic multiplication as it's called in the documentation) relates to the >>= and return functions that are mostly used. I read somewhere at Google that join can be expresed in terms of >>= and return, but I couldn't find it anymore. I also read that the Kleisli triple has more resemblance to how monads are used in practice, but it has some additional constraints. I don't understand why there are these additional constraints and I don't understand it's relevance, because according to some book there's a one-one mapping from monads to Kleisli triples and reverse. I also don't understand why join is a "multiplication", because a multiplication takes two operands (as in 5*4), at least all multiplication operators I know do so. I think what's happening is that one structure is taken out of the other. And then, the two are "merged" in some way. So this way, there are two operands. Is this correct? (I really could use a function definition in Haskell of join in terms of bind and return). Can somebody explain me (with a lot of detail) how this works? Regards, Ron __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com

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emacs haskell-mode indentation
by Nathan Weston 02 Jun '04

02 Jun '04

I am learning haskell from "The Haskell School of Expression", and finding the indentation in the emacs mode rather unhelpful. When I try to type in the following example from the book: main = runGraphics ( do w <- openWindow "Graphics" (300,300) drawInWindow w (text(100,200) "Hello, world!") k <- getKey w closeWindow w ) emacs wants to indent it like so: main = runGraphics ( do w <- openWindow "Graphics" (300,300) drawInWindow w (text(100,200) "Hello, world!") k <- getKey w closeWindow w ) (For those not using a fixed-width font, in the first example, everything lines up under the 'w <-', in the second, everything lines up under the 'do') If I let emacs have its way, I get a syntax error. Am I doing something wrong here? Is the emacs mode broken? Thanks, Nathan

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new Library Infrastructure spec.
by Isaac Jones 02 Jun '04

02 Jun '04

(followups to libraries(a)haskell.org) Hail Haskellers! A commonly-identified obstacle to the spread of Haskell libraries is that there is no standard way a) for authors to package a library or other tool for distribution to others b) for customers to install such a package Platforms vary. Haskell compilers vary. Packages have dependencies. Etc. Isaac Jones has coordinated an effort to address this problem. We've had a lot of discussion between him and (at least some of) the folk involved in the GHC, Hugs, and nhc implementations. This discussion has led to a new concrete proposed specification[1]. Here it is: http://www.haskell.org/libraryInfrastructure/proposal See also the project home page: http://www.haskell.org/libraryInfrastructure/ We would welcome your feedback, either as a potential library or tool author, or as a potential consumer, or both. The specification isn't complete in every detail, but it seems better to post it before investing in details that may be rendered void by subsequent discussion. We hope this will be an important spec, and will be with us for a long time. So now's the time to look at it. Send feedback to libraries(a)haskell.org. (You can subscribe to this list at: http://www.haskell.org/mailman/listinfo/libraries .) Sincerely, Isaac Jones, Simon Marlow, Ross Paterson, Malcolm Wallace, Simon Peyton Jones [1] This specification differs from the previous proposal both in technical details, and in that it is the combined efforts of the undersigned. Those familiar with the previous proposal will not find large high-level differences, but some details have been made more concrete and some details have changed.

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[Haskell-cafe] ANNOUNCE: Haskell Communities & Activities Report (6th ed., May 2004)
by Arthur van Leeuwen 25 May '04

25 May '04

Just a bit more than half a year after the previous edition, I am pleased to announce on behalf of the many contributers that the Haskell Communities and Activities Report (6th edition, May 2004) http://www.haskell.org/communities/ is now available from the Haskell Communities home page in several formats: PDF, for screenreading as well as printing, HTML, for those of you who prefer not to deal with plugins or external viewers, and PostScript, for those of you who have nice quick printers that do not grok PDF. Many thanks go to all the people that contributed to this report, both directly, by sending in descriptions, and indirectly, by doing all the interesting things that are reported. I hope you will find it as interesting a read as we did. If you haven't encountered the Haskell Communities and Activities Reports before, you may like to know that the first of these reports was published in November 2001. Their goal is to improve the communication between the increasingly diverse groups, projects and individuals working on, with, or inspired by Haskell. The idea behind these reports is simple: Every six months, a call goes out to all of you enjoying Haskell to contribute brief summaries of your own area of work. Many of you respond (eagerly, unprompted, and well in time for the actual deadline ;) ) to the call. The editor collects all the contributions into a single report and feeds that back to the community When we try for the next update, six months from now, you might want to report on your own work, project, research area or group as well. So, please put the following into your diaries now: ---------------------------------------- End of October 2004: target deadline for contributions to the October 2004 edition of the HC&A Report ---------------------------------------- Unfortunately, many Haskellers working on interesting projects are so busy with their work that they seem to have lost the time to follow the Haskell related mailing lists and newsgroups, and have trouble even finding time to report on their work. If you are a member, user or friend of a project so burdened, please find someone willing to make time to report and ask them to `register' with the editor for a simple e-mail reminder in the middle of October (you could point us to them as well, and we can then politely ask if they want to contribute, but it might work better if you do the initial asking). Ofcourse, they will still have to find the ten to fifteen minutes to draw up their report, but maybe we can increase our coverage of all that is going on in the community. Enjoy, communicate, and program! Arthur van Leeuwen -- /\ / | arthurvl+hcar(a)cs.uu.nl | Work like you don't need the money /__\ / | A friend is someone with whom | Love like you have never been hurt / \/__ | you can dare to be yourself | Dance like there's nobody watching

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[Haskell-cafe] Solving differential equations in terms of power series
by Henning Thielemann 24 May '04

24 May '04

I've attached a Haskell module demonstrating the beauty of solving ordinary differential equations in terms of power series. A power series is expressed by the (infinite) list of its coefficients. What do you think about that?

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Re: [Haskell-cafe] Re: Lazy XML handling
by MR K P SCHUPKE 24 May '04

24 May '04

>The standard representation should be a tree datatype Why not separate the parser and tree builder, have the parser stream the tags out, and define the tag stream as the interface to a DOM builder... parse :: String -> [(XmlTreeDepth,XmlElement)] buildDOM :: [(XmlTreeDepth,XmlElement)] -> DOM Thats how I would do it anyway. I appreciate the comments about efficency, I was really talking about simplicity. The algorithms on the steam can be quite simple, and hence avoid bugs. I would still want to dispense with the DOM, which if you think about the system thus: Parser -> [stream] -> DomBuilder -> DOM - operations on DOM You can push the operations on the dom via a structural transformation back through the DomBuilder, such that they operate directly on the stream... In other words the user composes the functions as if they are operating on a real DOM... only the DOM is never constructed and the operations are applied direct to the stream output of the parser. make sense? Keean.

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