- Haskell-Cafe - Haskell.org

Getting my feet wet - small browser game
by Joachim Durchholz 24 Dec '06

24 Dec '06

OK, after years of looking and discussing and doing HSoE exercises, I have finally decided that Haskell is far enough into practical usefulness that I should give it a try in a "real" project. The basic idea is a browser game - this touches all the potentially hard issues without miring me too deeply in target platform details. I'd like to lay out my plans here and ask how they are going to work out, and take advice. THE PLAN I'll start with http://haskell.org/haskellwiki/How_to_write_a_Haskell_program and get a toolchain together. I haven't decides which compiler (interpreter?) to choose; I'll probably go for the one that give the least amount of trouble. Next would be library selection. I'm willing to undergo some modest amount of hassle here, since I don't expect all libraries that I need to be mature yet. My preliminary plan is to split the application into a central world simulation process, and satellite processes that accept HTTP requests, feed them into the simulation, read back the results, and generate the response HTML. The interface between simulation and satellite is: * Satellites can read a snapshot of the game data. * Satellites cannot directly write game data. What they can do is to post commands to a blackboard, which are marked as "no more updatable" as soon as the simulation starts executing them. I expect the simulation and the satellites to be separate OS processes, so I'll need a way to marshall command and game data between processes. The simulation will have to store its entire state to disk, too - in theory, it could run forever and never write to disk (and I don't need a database, too), but in practice, I have to plan for the occasional reboot. Since the server will be running Apache for other purposes anyway, and I don't want to force the players to use a URL with a port number, I think I'll set up Apache so that it proxies game-related URLs to the Haskell software. I just hope that Apache doesn't incur much overhead in that mode. I have no idea how to organize software upgrades. The satellites are easy, but how do I make sure that revision N+1 of the simulation can read the marshalled data from revision N? The final software should be efficient. Ideally, the satellites are able to saturate the network card of today's typical cheap rootserver if the simulation is a no-op. I have two data points for a "typical cheap rootserver": * 100 MBit/s Ethernet, 256 MB RAM, 1.2 GHz Celeron (~3 years old) * 1 GBit/s Ethernet, 1 GB RAM, 2.2 GHz Athlon (current) Of course, not needing an RDBMS will give the system a head start efficiency-wise. Comments and suggestions welcome :-) Regards, Jo

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Writing "Haskell For Dummies Or At Least For People Who Feel Like Dummies When They See The Word 'Monad'"
by Kirsten Chevalier 23 Dec '06

23 Dec '06

It's not as if this is the first time that this has been suggested, but some people have suggested that a practical book about Haskell would be a good idea. I agree. Some people have also suggested that the right moment for this hasn't arrived yet, and I see that as a challenge. I'm willing to take the lead in at least thinking about what such a book might look like. I'm potentially about to have some free time for such things, and am still young and foolish enough to think that writing a book would be a good idea. Of course, there are many good Haskell books out there already, but many of them are intended as class textbooks or are aimed at more theoretical-minded people. There's nothing wrong with that, but I think that it would be nice if a friendly, conversational, informal book about Haskell existed, since after all this is such a friendly and informal community. (If there already is a book like this, point it out, but I get the impression there's not.) There's also excellent Haskell documentation available on the web already, but people like to buy books and they like to have an artifact that they can hold in their hands without getting laser printer toner all over themselves. But if I were going to do this, I'd need all the help I could get, so if you're interested in working with me on this, email me off-list and we'll talk. Don't feel like you need to be named "Simon" for this; I don't think you need to be a Haskell guru to contribute to a book like this (I know I'm not one), though it wouldn't hurt. Being interested in good writing and explaining things to a wider audience is more important. And, the more people who are interested in working on this, the more we can all pool our various talents to create something awesome. Cheers, Kirsten -- Kirsten Chevalier* chevalier(a)alum.wellesley.edu *Often in error, never in doubt "Everyone's too stupid." -- _Ghost World_

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Followup on writing "Haskell for Dummies..."
by Kirsten Chevalier 23 Dec '06

23 Dec '06

I think I've decided that this isn't the right time for me to take the lead on this book-writing project, and in addition, I've come to think that maybe the things *I'd* like to say about Haskell hacking can't be transmitted so easily in book form. However, if any others would like to organize the writing of one (or several) books based on any of the ideas brought up in the mailing list discussion, they should feel free to step forward and do so, and know that they won't be duplicating work (or at least not my work). Cheers, Kirsten -- Kirsten Chevalier* chevalier(a)alum.wellesley.edu *Often in error, never in doubt "You can't learn everything you need to know legally."--John Irving

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"Secret Santas" in Haskell / better data structure?
by Paul Brown 23 Dec '06

23 Dec '06

Cafe Crew -- I posted my simple-but-slow solution to Ruby Quiz problem #2: http://mult.ifario.us/articles/2006/12/22/secret-santas-in-haskell-iii-coll… (Or http://tinyurl.com/y3l2re if your mail client messes up the URL...) It's more than practically fast enough unless you have, say 10^8 friends to do secret Santa with, but the problem just cries out for a well-chosen data structure. (One obvious improvement over the approach I posted is to sort the list first, but maybe there's something even better...) As I'm learning, critiques of syntax and usage welcome. I'll collect the source and post to the Haskell Quiz wiki as well. -- Paul

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partial functions / failure, Maybe and MonadError and good style
by Steve Downey 23 Dec '06

23 Dec '06

Although terse, the subject really says it all. If i've a partial function, like a parser, what is considered good style for a library. The tradeoffs that I can see are that Maybe is a binary operation, while Error can communicate more information in the type of the error case. Is there some way to defer the error handling Monad to the calling context?

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Cabal licence files
by Dougal Stanton 23 Dec '06

23 Dec '06

The cabal setup recognises a small set of licences which I don't think are well explained. I'm trying to put together a canonical list for setting up new projects. GPL: <http://www.gnu.org/licenses/gpl.txt> LGPL: <http://www.gnu.org/licenses/lgpl.txt> Thankfully the FSF are particularly anal^W well-organised about this kind of thing and they make it very easy to determine what each licence is and how to apply it BSD3/BSD4: There appears to be one explicitly-named BSD licence on the OSI site. There is also a closely related MIT licence. I don't know which would be which, if these are the two referred to in the licence data type. <http://www.opensource.org/licenses/bsd-license.php> <http://www.opensource.org/licenses/mit-license.html> PublicDomain: The Creative Commons people have a public domain licence, though I am uncertain whether it is intended for code. Maybe this doesn't matter when what you're saying is "I give up all rights"? <http://creativecommons.org/licenses/publicdomain/> AllRightsReserved: This is the very opposite of a PD licence and, I believe, the default licence when there is no other explicitly stated. However it would be nice to say as much rather than relying on people's legal knowledge. ;) OtherLicense: This one's easy enough... If anyone can fill in gaps and correct the errors in the above I think it would be useful. Obviously if this has been discussed elsewhere just point me off in the right direction! :) Cheers, D.

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Showing the 1 element tuple
by Neil Mitchell 23 Dec '06

23 Dec '06

Hi, A weird question, what does the 1 element tuple look like? () -- 0 element tuple (,) a b -- 2 element tuple (,,) a b c -- 3 element tuple () a - meaningless (a) - a in brackets GHC has the 1 element unboxed tuple, (# a #), and all the other sizes unboxed as well, but how would you visually represent the 1 element boxed tuple? As it happens, Yhc _does_ have the 1 element tuple, you just can't use it from normal programs, its only created by desugarings of class instances. I'd quite like to change this, and I also need to render the 1 element tuple in some way, so wondered if anyone had any good ideas (or if there is even some sort of convention) I currently use ()1, but don't like that as it doesn't follow Haskell rules - the () and 1 would be separate lexemes. My other thought is (?), where ? is something appropriate - but all the appropriate things I can think of would either not be a lexeme (i.e. 1), having an existing meaning (i.e. . | etc) or seem wrong. Thoughts? Neil

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ST and Transformers
by J. Garrett Morris 22 Dec '06

22 Dec '06

Hello everyone, I recently found myself attempting to use ST at the base of a stack of transformers, ala: type Test t r = ErrorT String (ST t) r I imagined, given the type of ST, that I would need a run function along the lines of: runTest :: (forall t. Test t r) -> Either String r (which was, in fact, exactly what I wanted). Unfortunately, even with that type signature, I can't find a way to convince the compiler that the state thread is properly universal in the call to runST. Is it possible to write this function, or is ST (and other monads that use its style of threading) not transformable? Thanks, /g -- It is myself I have never met, whose face is pasted on the underside of my mind.

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"automonadization" of code?
by Adam Megacz 22 Dec '06

22 Dec '06

Is there any work on automatic translation of code in some tiny imperative language into Haskell code that uses the ST and/or IO monads (or perhaps even pure functional code)? For example, the user writes something vaguely like array = newArray (1,100) 1 for x=2 to 99 array[x] := array[x-1]+array[x+1] And it is transformed into something like foldl (>>=) (newArray (1,100) 1) $ map (\n arr -> do a <- readArray arr (n-1) b <- readArray arr (n+1) writeArray arr n (a+b) return arr) [2..99] Obviously the "small imperative language" would have to be highly restricted and carefully chosen in order for the translation to always work and be predictable. I'm interested in any existing work on choosing such a sublanguage. Thanks! - a -- PGP/GPG: 5C9F F366 C9CF 2145 E770 B1B8 EFB1 462D A146 C380

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Re: Shrinking the Prelude: The "categorical" approach
by Imam Tashdid ul Alam 22 Dec '06

22 Dec '06

so the main issues are these: Bulat Ziganshin raised: Num is intrinsically bound to the compiler. sad. so let's leave Num out. this basically means we will avoid "abstract algebra" in general. that is, forget "groups", "rings", for now, the principle focus will be "functor", "monad"... Henning Thielemann provided links to CAL: hi Henning, would you mind if we "steal" some of the good looking names in your Prelude? Brian Hulley raised one serious issue: even if we were to define a "newbie" friendly Prelude, which should be preferred, alphanumeric names or operators? I have a thing for operators, they look good! but Brian's issue is solid, the precedence isn't documented by Haddock. any Haddock developer reading this? so mplus stays. I would still suggest plus/zero over mplus/zero but Haskell's scoping is rather liberal. I suggest the convention that Control.Monad is always imported as M. this would mean we use M.plus, M.zero, M.lift and so on. Diego Navarro said: this will confuse beginners. I disagree. I think type classes and monads are the basics of Haskell (before that, it's just commonsense ;)... ) so if the error message successfully conveys the message that there's a problem, that should be enough to inspire the beginner to start on the basics. besides, people will ultimately use the standard Prelude. what I am trying to do is to prove that with a good choice of names programs written in Haskell can look more "comprehensible" to beginners. like Gosling said about Java, the language syntax has one very good property: it's easy to read it. Garrett Morris seems to agree with me on this. Greg Fitzgerald raised the further issue of monads being overlooked by the beginners. I think "do notation considered harmful" blog was at least partially right: it's better to expose the concept rather than hiding it. so with enough people interested (I assume) it's time to move this discussion to the wiki. the wiki code is not to be taken seriously, it's a "proof-of-concept", a "guideline". cheers! Imam __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com

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