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Pattern matching articles/explanations
by Ian Duncan 17 Aug '07

17 Aug '07

Hello everyone, I've been working on improving my Haskell knowledge, and in doing so, I have read a little about as-patterns as well as some form of pattern that uses "~" that I don't really understand. I suspect there are even more lesser-known pattern matching expressions than just these, but I don't have the stamina to decode the Haskell 98 Report. Are there any articles anyone is aware of, or anything in the Haskell wiki that covers the built-in pattern matching facilities pretty comprehensively? I've looked in the Haskell wiki myself, but information on different pattern matching abilities seems rather scattered and I'm not entirely sure what I should be looking for. Thanks, Ian Duncan

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Re: [Haskell-cafe] monte carlo trouble
by Chad Scherrer 17 Aug '07

17 Aug '07

Yeah, I did have troubles with (StateT StdGen Maybe). If it hits a "Nothing", I'd like it to skip that one and try again with the next state. But instead, Nothing is treated as a failure condition that makes the whole thing fail. I just found MaybeT on the wiki, which looks like it could work. I'll take a look at that and the ListT thing. I'm starting to think the power of abstraction is a blessing and a curse. Haskell's abstraction mechanisms are so powerful that it's generally possible to come up with a way to solve a given problem elegantly and efficiently. On the other hand, if a problem isn't so well studied, the bulk of the work is in finding the right abstraction, which forces generalization beyond what would otherwise be needed (though it'll be easier the next time!). On 8/15/07, Paul Johnson <paul(a)cogito.org.uk> wrote: > Chad Scherrer wrote: > > Funny you should say that, I was just experimenting with generating > > one at a time using (StateT StdGen Maybe). If I get stuck (again) I'll > > check out ListT. Thanks! > > > > > You definitely want a list not a Maybe. List is for 0 or more results > whereas Maybe is for 0 or 1. Yes you can have a Maybe [a] with Nothing > instead of [], but its redundant. > > I'm fairly sure you need it the other way out too. All About Monads has > a demo of StateT s [a] where it is used to solve a logic problem. The > key point is that in the logic problem each possible result is > associated with a different state change. However here you want to have > a list of results and then a state change when you pick one (or a > subset, or whatever you want). So that would be ListT (StateT StdGen). > > Paul. > > -- Chad Scherrer "Time flies like an arrow; fruit flies like a banana" -- Groucho Marx

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Re: trouble building 6.7 on ubuntu
by Stefan O'Rear 17 Aug '07

17 Aug '07

On Thu, Aug 16, 2007 at 01:01:12PM -0400, Thomas Hartman wrote: > I repeated the install attempt described below using darcs head, including > the extra libs. > > I got the exact same error as before. > > Setup: Warning: Unknown fields: nhc98-options (line 173) .... and then a > cryptic error involving HsColour I'm running a full get from 20070712 without problems. (admittedly not Ubuntu) Stefan

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Re: [Haskell-cafe] Explaining monads
by Gregory Propf 17 Aug '07

17 Aug '07

I made this mistake myself at first too. It seems that the Monad = "side effect machine" error is common to Haskell newbies. Probably to do with the fact that the first thing every programmer wants to do is write a hello world program and for that you need the IO Monad which requires some explanation of how a Monad can allow for side effects (at least the IO Monad). - Greg ----- Original Message ---- From: peterv <bf3(a)telenet.be> To: Kim-Ee Yeoh <a.biurvOir4(a)asuhan.com>; haskell-cafe(a)haskell.org Sent: Monday, August 13, 2007 10:31:48 AM Subject: RE: [Haskell-cafe] Explaining monads Ronald Guida wrote: > > Given the question "What is a Monad", I would have to say "A Monad is > a device for sequencing side-effects." > There are side-effects and there are side-effects. If the only monad you use is Maybe, the only side-effect you get is a slight warming of the CPU. .... "Side-effects" is a piece of linguistic cruft played fast-and-loose by too many people in this game. "Sequencing" suffers the same disease. ____________________________________________________________________________________ Sick sense of humor? Visit Yahoo! TV's Comedy with an Edge to see what's on, when. http://tv.yahoo.com/collections/222

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Erlang VM in Haskell
by Joel Reymont 16 Aug '07

16 Aug '07

Folks, I would like to write an Erlang VM in Haskell. I first thought of OCaml but Haskell has SMP and lazy evaluation may come in handy. Plus, I'll need help in this project like in no other and support from the Haskell community has always been outstanding. I'm doing this to learn more about the Erlang internals and to acquire some skills in the process. I'm also hoping to be at least as fast as the existing Erlang VM (written in C) and to be able to extend the VM in a functional language. With C you can implement a threaded VM using GCC labels or switch- style [1]. What is the most efficient approach in Haskell? I'm thinking using CPS and building up a chain of closures. Would this work well? Any other suggestions? The Haskell repo for the initial HEM (Haskell Erlang Machine) is at http://darcs.haskell.org/hem. A much improved HAW implementation will follow in the not so near future. Thanks, Joel [1] http://www.complang.tuwien.ac.at/forth/threaded-code.html -- http://wagerlabs.com

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Haskell for the Enterprise (topic for a MSc dissertation?)
by Lars Oppermann 16 Aug '07

16 Aug '07

Hi All, This is my first post here, so I'll start with a quick introduction... I live and work in Hamburg, Germany. My day job is as a software engineer at Sun Microsystems at the OpenOffice.org development team where I'm mostly doing XML (ODF) related things. Beside of that, I'm currently pursuing an M.Sc degree in software engineering in a part-time post graduate program where I am now nearing the phase where I have to develop a topic for my dissertation. Apart from a little dabbling with Miranda during my undergraduate CS days I haven't touched functional programming ever since and have been doing Java, C# and C++ most of the time. However, I have over the last few month "fallen in love" with the great expressiveness offered by Haskell and through that have gained some great new perspectives and discovered new ways to think when writing programs. Consequently, my idea to combine all of this into a topic for my dissertation is to write about the existing and potential benefits (and shortcomings) of a functional programming language for the development of typical enterprise applications (especially with regards to more and more concepts from functional programming being adopted in imperative languages). I would start off with an existing J2EE application, analyze the abstractions and patterns that it uses. I will then discuss similar and alternative abstractions and patterns available in functional programming which can be applied in order to solve the same problem in a functional setting, leveraging some existing infrastructure, such as HAppS. What would follow is a comparison of the two resulting applications from a software engineering standpoint: How maintainable are these implementations? How can they be tested? Can formal validation techniques be applied to parts of the functional implementation that can be used to establish its correctness. Maybe analysis of performance... One of my potential supervisors for the dissertation has already indicated some interest in the topic so it all looks quite well to me. The overview that I gave above obviously needs much more elaboration, but as I said, I have just started developing topics. I would love to hear your comments on this. Has this been done over and over already? Are there any source which you think of from the top of your head which I should check out? Books that you would want to recommend? As I wrote above, I'm quite new to Haskell. Thus if anyone wants to recommend some readings specifically on the aspect of modeling in a functional environment I'd be happy to hear about that too. All the best, Lars -- Lars Oppermann Hamburg, Germany

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monte carlo trouble
by Chad Scherrer 16 Aug '07

16 Aug '07

There's a problem I've been struggling with for a long time... I need to build a function buildSample :: [A] -> State StdGen [(A,B,C)] given lookup functions f :: A -> [B] g :: A -> [C] The idea is to first draw randomly form the [A], then apply each lookup function and draw randomly from the result of each. It's actually slightly more complicated than this, since for the real problem I start with type [[A]], and want to map buildSample over these, and sample from the results. There seem to be so many ways to deal with random numbers in Haskell. After some false starts, I ended up doing something like sample :: [a] -> State StdGen [a] sample [] = return [] sample xs = do g <- get let (g', g'') = split g bds = (1, length xs) xArr = listArray bds xs put g'' return . map (xArr !) $ randomRs bds g' buildSample xs = sample $ do x <- xs y <- f x z <- g x return (x,y,z) This is really bad, since it builds a huge array of all the possibilities and then draws from that. Memory is way leaky right now. I'd like to be able to just have it apply the lookup functions as needed. Also, I'm still using GHC 6.6, so I don't have Control.Monad.State.Strict. Not sure how much difference this makes, but I guess I could just copy the source for that module if I need to. Any help is greatly appreciated! -- Chad Scherrer "Time flies like an arrow; fruit flies like a banana" -- Groucho Marx

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Re: [Haskell-cafe] Why monad tutorials don't work
by Dan Piponi 16 Aug '07

16 Aug '07

On 8/14/07, Dan Weston <westondan(a)imageworks.com> wrote: > Conor McBride and Ross Paterson said it best in the introduction to > their paper "Applicative programming with effects" [1]: As von Neumann said: "Young man, in mathematics you don't understand things, you just get used to them." Getting used to something is, practically by definition, something that you can't do just by reading the ultimate tutorial. You just have to write the code, see the pattern happen again and again, and abstract it. There's no short cut. (Well...sometimes...) On 8/14/07, Michael Vanier <mvanier(a)cs.caltech.edu> wrote: > I'm reminded of > a physics teacher who was having a similar problem explaining the concept of tensors, until he said > that "a tensor is something that transforms like a tensor does!". Grrr...must...hold...my...tongue... -- Dan

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Re: [Haskell-cafe] monte carlo trouble
by Chad Scherrer 16 Aug '07

16 Aug '07

Funny you should say that, I was just experimenting with generating one at a time using (StateT StdGen Maybe). If I get stuck (again) I'll check out ListT. Thanks! Chad On 8/15/07, Paul Johnson <paul(a)cogito.org.uk> wrote: > Chad Scherrer wrote: > > Thanks for your replies. > > > > I actually starting out returning a single element instead. But a > > given lookup might return [], and the only way I could think of to > > handle it in (State StdGen a) would be to fail in the monad. But > > that's not really the effect I want - I'd rather have it ignore that > > element. Another option was to wrap with Maybe, but then since I > > really want a sequence of them anyway, I decided to just wrap in a > > List instead. Is there a way Maybe would work out better? > Ahh. How about using ListT Gen then? That (if I've got it the right > way round) works like the list monad (giving you non-determinism), but > has your random number generator embedded as well. Take a look at "All > About Monads" at http://www.haskell.org/all_about_monads/html/. Each > action in the monad may use a random number and produce zero or more > results. > > Paul. -- Chad Scherrer "Time flies like an arrow; fruit flies like a banana" -- Groucho Marx

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Re:Explaining monads
by Gregory Propf 15 Aug '07

15 Aug '07

Sorry to spam you Jeff, again I sent my email to the poster rather than the list. I'm using Yahoo beta webmail and don't see a way to set it to reply to the list rather than the originator. Anyway, this was my post: Hence the need to perform a "run" operation like runIdentity, evalState or runParser (for Parsec) to get something useful to happen. Except for lists we don't seem to do this. I suppose lists are so simple that the operators :, ++ and the [] constructor do all we ever need with them. Finally there is no runIO because "main" is essentially that function in every real program? - Greg ----- Original Message ---- From: Jeff Polakow <jeff.polakow(a)db.com> To: lanny(a)cisco.com Cc: haskell-cafe-bounces(a)haskell.org; Haskell-Cafe <haskell-cafe(a)haskell.org> Sent: Tuesday, August 14, 2007 8:45:06 AM Subject: Re: [Haskell-cafe] Explaining monads One general intuition about monads is that they represent computations rather than simple (already computed) values: Boardwalk for $500? In 2007? Ha! Play Monopoly Here and Now (it's updated for today's economy) at Yahoo! Games. ____________________________________________________________________________________ Sick sense of humor? Visit Yahoo! TV's Comedy with an Edge to see what's on, when. http://tv.yahoo.com/collections/222

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