August 2007 - Haskell-Cafe

Syntax for lambda case proposal could be "\of"
by Brian Hulley 16 Aug '07

16 Aug '07

Hi, On http://hackage.haskell.org/trac/haskell-prime/wiki/LambdaCase the proposed syntax for lambda case is: case of alts but this has a really bad downside for interactive editors: it doesn't allow one to distinguish between an incomplete construct and a completed construct thus removing one opportunity for an editor to alert the user (eg by highlighting) that there is something missing in the program text. Therefore I propose: \of alts which doesn't suffer this problem since the keyword "of" can never follow a '\' in the existing grammar. However I can't edit the H' wiki, so if anyone agrees that the above syntax is better please could they add the above to the page. Thanks, Brian.

9 12

Pattern matching articles/explanations
by Ian Duncan 16 Aug '07

16 Aug '07

So what I noticed that "A Gentle Introduction to Haskell" mentioned that wild-cards are useful in constructors. For example: head (x:_) = x So, does that offer any performance benefits over: head (x:xs) = x Or is it primarily to indicate to the coder that xs is useless? I get the impression it has a very similar meaning to the irrefutable pattern in regards to not evaluating it when the function is called. Or am I way off?

3 2

Re: hyperlinked haskell 98 grammar
by Peter Hercek 16 Aug '07

16 Aug '07

Hi Neil, For haskell-cafe members: This is what we are talking about: http://www.hck.sk/users/peter/ I'm glad somebody liked it. I do not understand what you mean by the extension request. Would you want to see the number of references of a production head even without opening the popup list box? If you meant that the pop-up list box should always display all backward links directly (without a scroll bar) then it would not work for a very big number of such links (in theory there can be hundreds there). If you do not like the default of 5 then change the nMaxRowCount "constant". Anyway, if it is complicated to describe, let it be. I do not expect to play with the java script code much. Only the most trivial things have a chance to be implemented by me. I have written the java script code few years ago and I do not really like java script ... Just wanted to give back something to the community - so I added the Haskell specific data. As for as putting it on the Haskell org I do not mind if somebody else does it. Although I'm not that sure if it would require license change. If I would believe giving it into public domain would make more grammars browsable like this one then I would do it. I do not really expect heavy loads. From my experience most language users never ever look at the grammar nor they care about it. And those who consult it a lot will take a local copy and tweak it anyway. Looking at the munin server loads, looks like you and Esa are the only ones who actually checked it out :-D Anyway, sending this to haskell cafe too (as recommended). I expect a discussion (if any) will continue there. Sorry about GHC user list abuse. Peter Hercek. Neil Mitchell wrote: > Hi > > In addition, perhaps this should be relocated to haskell.org, if your > server is not suitable for a large volume of users. I think it should > also be integrated somewhere (perhaps a link from the HTML report, and > certainly on the wiki) > > Thanks > > Neil > > On 8/16/07, Neil Mitchell <ndmitchell(a)gmail.com> wrote: >> Hi Peter, >> >> A nice application. One suggestion: I would have implemented it so >> that a maximum on one backward hyperlink menu could be visible at once >> - try clicking on two RHS's and you'll get too "menus" visible at the >> same time. >> >> You also might want to mail this out on haskell-cafe - which is more >> appropriate for "general haskell" things, ghc-users is more for direct >> issues with GHC (although haskell-cafe can also be used for those) >> >> Thanks >> >> Neil >> >> On 8/16/07, Peter Hercek <peter(a)syncad.com> wrote: >>> Hi, >>> >>> I was improving my Haskell knowledge lately and I created a small dhtml application which allows browsing of >>> Haskell 98 grammar. I contains both forward and backward hyperlinks. By backward hyperlink I mean that you can >>> click on an a production head and you get a popup list box where you can navigate to any production using the >>> nonterminal. If you like it please save and use your local copy since the server cannot handle much load. >>> >>> http://www.hck.sk/users/peter/ >>> >>> Peter. >>> >>> _______________________________________________ >>> Glasgow-haskell-users mailing list >>> Glasgow-haskell-users(a)haskell.org >>> http://www.haskell.org/mailman/listinfo/glasgow-haskell-users >>>

1 0

Pattern matching articles/explanations
by Ian Duncan 16 Aug '07

16 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

2 1

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

16 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

2 1

Re: trouble building 6.7 on ubuntu
by Stefan O'Rear 16 Aug '07

16 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

1 0

Re: [Haskell-cafe] Explaining monads
by Gregory Propf 16 Aug '07

16 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

21 40

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

4 4

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

3 3

monte carlo trouble
by Chad Scherrer 15 Aug '07

15 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

5 6