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Re: Yet Another Monad Tutorial
by Derek Elkins 13 Aug '03

13 Aug '03

On Wed, 13 Aug 2003 13:26:34 +0200 "blaat blaat" <l4t3r(a)hotmail.com> wrote: I have a long reply, perhaps I'll post it later. Here's a more pragmatic short one. > What did we lose? Well, introspection comes to mind. A monad is > strange in the sense that, actually, there does not seem to be any > reason to compute its value since we cannot possibly get anything out > of it. In case of IO, only the haskell interpreter can. And, > introspection is a nice thing to have, especially for compiler > writers. Although I think the benefits in Haskell would be limited, an > application might be that I could write my own promotion functions on > my own programs. Lot's of issues with this paragraph, but here are the questions. What do you mean by "promotion function"? How would you write it in your hypothetical Haskell? How is it related to IO? How do monads and/or the IO monad in particular cause problems? Why couldn't the same be done using monads or why would it be significantly harder if it can be done? > Think of the following though experiment. What if I would have a > function "unparse:IO a->String" which gives the textual representation > of the specification of the object of type IO a? Clearly, IO is not a > monad anymore. Moreover, I don't think it would break the purity of > Haskell. And surely, in case of the IO monad, we can think of better > manners to break a program down. Keith has already pointed out the flagrant error here; which I also point out in my long reply. But again, the questions... What is unparse supposed to return? What does it have to do with IO in particular, i.e. why do I want to unparse IO computations especially? Why can't it be done with (or over) the current IO monad? What other ways do you want to breakdown a program, still resulting in understandable behavior, but can't because of the IO monad? > Hmm, I am rambling. The point is, I think there is no reason to > discard all alternatives to monads; > I believe that central in the > IO-issue is not the fact that we are using monads for IO - but that we > can purely create impure programs which do the IO for us ;-), and that > those do not have to be of the monadic kind. I have comments for these too, but I'll keep this short. Basically, I don't really see how what you want has all that much to do with IO in general, and I don't see monads get in the way.

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[String]->[[Bool]]
by Tn X-10n 13 Aug '03

13 Aug '03

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[String]->[[Bool]] 2
by Tn X-10n 13 Aug '03

13 Aug '03

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Yet Another Monad Tutorial
by blaat blaat 13 Aug '03

13 Aug '03

Hmmm, I personally always thought of the IO monad as being a synonym for "Program" (or, for functions wich return an IO a, "a subroutine with side-effects which returns a value of type a"). We can build mini programs (like putStr), and compose them in a sequential manner (including something alike assignment). I personally use the following analogy. Someone, somewhere, sometime must have thought, "Great, if I cannot do imperative programming directly, I can damned well create imperative programs and have _them_ evaluated, grumbl!" And with that thought, he solved numerous problems ;-) Can we really make cheese out of it? (Dutch saying) For what is the interpretation of an object of type IO a? Well, it is a bit of a hack atop of Haskell, isn't it? If we return a complex type with an IO a object in it, the object cannot be evaluated, but if we return a IO object solely (program), the program get evaluated (passed to the (Haskell)-OS). However, given that observation, the fact that IO is a monad I find to be a _rather arbitrary design decision_; why not define a small term language which may be passed to the OS? Why not have (some limited) introspection on programs which we build? Purity will never be broken since the only way to get a "program" evaluated is to pass it to the Haskell-OS. Are we missing out on usefull stuff? Hehe... God, we might as well just generate impure graph rewrite rules,.... or plain C for that matter ;-). Cheers, l4t3r ps: Mind you, nothing wrong with that IO monad. It works, why break it. ;-) _________________________________________________________________ STOP MORE SPAM with the new MSN 8 and get 2 months FREE* http://join.msn.com/?page=features/junkmail

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[String] -> [[Bool]]
by Tn X-10n 13 Aug '03

13 Aug '03

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AW: Yet Another Monad Tutorial
by Markus.Schnell＠infineon.com 13 Aug '03

13 Aug '03

> The overall message I have taken from your post is that I need to > be more precise to convey the correct information and avoid confusion > (or worse, misinformation). As I understood it, the tutorial should be instructive. So I would not try to be more precise at the beginning, because too much detail would lead to confusion for a beginner. Start with something like "I'm now telling you some lies, which make it easier to grasp the idea of the monad, but further down the road, I'll reexplain some concepts and be more precise there." This was done by Don Knuth for the Typesetting-Books and it worked very well (for me). I hope this is a sensible idea. Markus -- Markus Schnell Infineon Technologies AG, CPR ET Tel +49 (89) 234-20875

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Database interface
by Thomas L. Bevan 13 Aug '03

13 Aug '03

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Does anyone know if there is work being done on a standard Haskell database interface. I was very impressed with HaskellDB but I understand that it uses Haskell extensions not present in GHC. Tom -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.2 (GNU/Linux) iD8DBQE/OhsFYha8TWXIQwoRAhkUAKDE/z2fepe7565J5+9JmMGZxpmQvwCgre/R JokIUilP8aF37d764FYFZbY= =Zlx6 -----END PGP SIGNATURE-----

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Help with Exceptions on I/O
by Alexandre Weffort Thenorio 13 Aug '03

13 Aug '03

I have a program which creates textfiles out of other files. Since the program is runned from windows I output some text strings (Like "File created succefully") and I need to stop the program before it quits so that the user can read the line outputted to know what went on and then he can press ENTER to quit the program. I managed to do this fine if no error occurs but when a error occurs I am having problems. The code goes like that main :: IO() main =catch (do Do all the needed stuff here putStr "File created Successfully. Press RETURN to quit" dummy <- getLine --Halts the program so the user can read the above line) putStr "Exiting now..." --needed since I can't finish a do function with the "dummy<- getLine" line) (\_ -> do putStr "\nFile not found. Press RETURN (ENTER) to quit." dumb <- getLine putStr "\nExiting...") So when the program runs, if the input file is found the program writes successfull creation of file but if the file doesn't exist, after the user gives the input filename and press enter, the program creates a new line and Halts (Probably because of the getLine function) without writing out anything, then when the user press ENTER again it writes the line at the first putStr (File not...), then writes the next putStr line under it (Exiting...) and exits. I don't know why it doesn't wirte the first line, halts and then when user press enter it writes the second and quits. Can anybody help me as I am not very familiar with exception and catches. Another question I have is: Is there any other function rather than getLine that halts a program and continues when a user press any key (Instead of ENTER) and besides this is an ugly code since getLine wasn't made for that but I couldn't find anything else myself. Thank you in advance. Best Regards Alex

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RE: .. Drowning in stateful, side-effect riddled, spaghetti code? We have the answer. fdps[]c.
by Bayley, Alistair 13 Aug '03

13 Aug '03

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RE: Yet Another Monad Tutorial
by Bayley, Alistair 13 Aug '03

13 Aug '03

> From: Wolfgang Jeltsch [mailto:wolfgang@jeltsch.net] > > For example, the function readFile is pure. For a specific > string s the > expression readFile s always yields the same result: an I/O > action which > searches for a file named s, reads its content and takes this > content as the > result of the *action* (not the expression). What about getChar? This is a function which takes no arguments, yet returns a (potentially) different value each time. I know, I know: it returns an IO action which reads a single char from the terminal and returns it. Is the IO monad the only one (currently) in which you would say that it "returns an action", which is then executed by the runtime system? I would have thought that monads that are not involved in IO (e.g. State) would be pure in the sense that Van Roy was thinking. You wouldn't need to describe functions in them as "returning an action". ***************************************************************** The information in this email and in any attachments is confidential and intended solely for the attention and use of the named addressee(s). This information may be subject to legal professional or other privilege or may otherwise be protected by work product immunity or other legal rules. It must not be disclosed to any person without our authority. If you are not the intended recipient, or a person responsible for delivering it to the intended recipient, you are not authorised to and must not disclose, copy, distribute, or retain this message or any part of it. *****************************************************************

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RE: Yet Another Monad Tutorial
by Bayley, Alistair 12 Aug '03

12 Aug '03

> -----Original Message----- > From: Jeff Newbern [mailto:jnewbern@nomaware.com] > > Alistair, > > Hhhmmm. That is an interesting issue, and I am unsure how to treat it > in the tutorial. I did attempt to explain the ability of the monad to > isolate impure computations, but I think I need to make a better > explanation of what an action is and how it is used. > > As for the question of whether using a monad in this way casts Haskell > out of the ranks of the functionally pure, I don't know. > This feels to > me like a debate over personal interpretation, with decent > arguments on > both sides. I think the most accurate thing to say is that it enables > Haskell to incorporate non-pure features without destroying the purity > of the core of the language. The interesting point for me is: you can't escape from the IO monad, but you can from others. You can use non-IO functions from IO functions, but you can't use IO functions from non-IO functions. AFAICT, this is not true of all monads; as Wolfgang said, you can write a function that uses monadic values but returns a non-monadic ("pure") value. You (sort of) explain this in the first two paragraphs of "No way out", but things that might help are: - two example lists: of one-way monads (IO), and others (List, Maybe) - non-monadic function using monadic values ( http://www.nomaware.com/monads/html/listmonad.html#example ) > Any suggestions for how best to present this issue to a reader who may > be new to the world of FP? Should we mention it at all? I think you should mention it in a monad tutorial, hence the request. It will help (has helped) me understand a little more about monads. ***************************************************************** The information in this email and in any attachments is confidential and intended solely for the attention and use of the named addressee(s). This information may be subject to legal professional or other privilege or may otherwise be protected by work product immunity or other legal rules. It must not be disclosed to any person without our authority. If you are not the intended recipient, or a person responsible for delivering it to the intended recipient, you are not authorised to and must not disclose, copy, distribute, or retain this message or any part of it. *****************************************************************

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RE: Yet Another Monad Tutorial
by Jeff Newbern 12 Aug '03

12 Aug '03

Alistair, Hhhmmm. That is an interesting issue, and I am unsure how to treat it in the tutorial. I did attempt to explain the ability of the monad to isolate impure computations, but I think I need to make a better explanation of what an action is and how it is used. As for the question of whether using a monad in this way casts Haskell out of the ranks of the functionally pure, I don't know. This feels to me like a debate over personal interpretation, with decent arguments on both sides. I think the most accurate thing to say is that it enables Haskell to incorporate non-pure features without destroying the purity of the core of the language. Any suggestions for how best to present this issue to a reader who may be new to the world of FP? Should we mention it at all? Thanks, Jeff Alistair Bayley wrote: > Thanks. Quite comprehensive. > > Peter Van Roy (the Mozart/Oz guy) said this on the PragProg list, and > I didn't have the knowledge to respond. It's a point you might want to > address, perhaps in this section?: > http://www.nomaware.com/monads/html/laws.html#nowayout > > [Discussion of whether I/O monad makes Haskell impure] -- Jeff Newbern <jnewbern(a)nomaware.com> Nomaware, Inc.

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Re: Yet Another Monad Tutorial
by Jeff Newbern 12 Aug '03

12 Aug '03

Hello, I have put up all-in-one archives of the tutorial for people who prefer to read it offline. http://www.nomaware.com/monads/monad_tutorial.tgz and http://www.nomaware.com/monads/monad_tutorial.zip Thanks, Jeff Newbern jnewbern(a)yahoo.com __________________________________ Do you Yahoo!? Yahoo! SiteBuilder - Free, easy-to-use web site design software http://sitebuilder.yahoo.com

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RE: Yet Another Monad Tutorial
by Bayley, Alistair 12 Aug '03

12 Aug '03

Thanks. Quite comprehensive. Peter Van Roy (the Mozart/Oz guy) said this on the PragProg list, and I didn't have the knowledge to respond. It's a point you might want to address, perhaps in this section?: http://www.nomaware.com/monads/html/laws.html#nowayout I can't decide for myself if he's right or not. It seems to me that monadic IO functions aren't pure, but that doesn't mean that *all* monadic functions are impure, does it? Or could you argue that monadic IO functions are pure: it's just that their input includes the outside world, which means that you can never invoke an IO function more than once with the same arguments? ----------------------------------------------------------------------- > Care to explain why you think Haskell is not pure? Purely functional = you are always defining and calling pure functions. Calling a pure function with the same arguments always gives the same results. In Haskell, if monadic state is used, then this is not true (there is support from the syntax + type system to hide the state argument). So Haskell's monadic state is an implementation of state *on top of* a purely functional language. Saying that Haskell is 'purely functional' is reasonable if you want to present it to non-functional programmers. But the reality is not that simple. It's perhaps better to say that Haskell allows a clear separation of the purely functional core from the rest, which is a Good Thing. It's like saying that Erlang is a functional language. It's true that Erlang has a functional core. But the concurrent message passing built on top is very important and definitely not functional! Peter > -----Original Message----- > From: Jeff Newbern [mailto:jnewbern@yahoo.com] > Sent: 12 August 2003 10:41 > To: haskell-cafe(a)haskell.org > Subject: Yet Another Monad Tutorial > > > Hello everyone, > > Due to the scarcity of monad tutorials available (:^), I have > written one of my own. I hope that this one is both more gentle > and more comprehensive than many of the other tutorials out on > the 'net, and I am looking for feedback from experienced > Haskellers and novices alike. > > You can find the tutorial at > > http://www.nomaware.com/monads/html/ > > The tutorial begins from first principles and introduces the monad > concept, details the implementation and usage of all of the standard > monads in Haskell and the monad template library and explains the > use of monad transformers. The tutorial contains 23 individual > sections and 18 files of example code illustrating different monad > concepts. > > I hope that this tutorial is useful to many people, and I would > love to hear your experiences and suggestions for improving it. > > Thanks, > Jeff Newbern > jnewbern(a)yahoo.com ***************************************************************** The information in this email and in any attachments is confidential and intended solely for the attention and use of the named addressee(s). This information may be subject to legal professional or other privilege or may otherwise be protected by work product immunity or other legal rules. It must not be disclosed to any person without our authority. If you are not the intended recipient, or a person responsible for delivering it to the intended recipient, you are not authorised to and must not disclose, copy, distribute, or retain this message or any part of it. *****************************************************************

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Yet Another Monad Tutorial
by Jeff Newbern 12 Aug '03

12 Aug '03

Hello everyone, Due to the scarcity of monad tutorials available (:^), I have written one of my own. I hope that this one is both more gentle and more comprehensive than many of the other tutorials out on the 'net, and I am looking for feedback from experienced Haskellers and novices alike. You can find the tutorial at http://www.nomaware.com/monads/html/ The tutorial begins from first principles and introduces the monad concept, details the implementation and usage of all of the standard monads in Haskell and the monad template library and explains the use of monad transformers. The tutorial contains 23 individual sections and 18 files of example code illustrating different monad concepts. I hope that this tutorial is useful to many people, and I would love to hear your experiences and suggestions for improving it. Thanks, Jeff Newbern jnewbern(a)yahoo.com __________________________________ Do you Yahoo!? Yahoo! SiteBuilder - Free, easy-to-use web site design software http://sitebuilder.yahoo.com

3 2

.. Drowning in debt? We have the answer. fdps[]c.
by koxwhdsinsad＠msn.com 12 Aug '03

12 Aug '03

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Some humor
by Josef Svenningsson 11 Aug '03

11 Aug '03

Hi! All you people nourishing a dream of being part of the Haskell committee. Forget it. You might be taken for a serial killer. Check out: http://www.malevole.com/mv/misc/killerquiz/ :) /Josef

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Editor in Linux for Hint/Helium
by Tim Stitt 11 Aug '03

11 Aug '03

Hi all, I'm using the Helium interpreter/compiler with the Hint interface within Linux. Unfortunately, I want an editor that allows me to jump to the row/column of the source when I receive an error. Does anyone know of a good editor in Linux to do this or the commands for Emacs etc. that can be used from within Hint to achieve it? Context works fine in Windows but I want something similar in Linux. There isn't much help from the Helium website. Thanks in advance, Tim Stitt

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[newbie] UTF-8
by danon＠epita.fr 11 Aug '03

11 Aug '03

Hi, We try to make a program which write on stdout the UTF-8 character corresponding to an input unicode value. And this under MacOS X, Debian linux, and Windows. It seems to be easy, but we can't have found anything valuable, on the net. Would you mind to help us? Niko. Ps: excuse my poor english.

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[Newbie] Streams and IO
by Christian Hofer 10 Aug '03

10 Aug '03

Hello, I am completely new to functional programming, am just reading through SOE (great book so far!) and try to understand streams in client-server-interactions. Therefore I wrote the following program (which has nothing to do with the book exercises), which is intended to read integers from stdin and to write the sum of the inputs so far to stdout. When I start it (in Hugs), I can enter numbers, but don't get back the sum, but every second time a zero, every forth time the value I just entered and every forth time the sum of the two values I entered the latest. Could s.o. explain me, what is going wrong? I can't figure out, what is happening here. (I know that there are easier solutions to the problem than using streams, but the latter was my intention for learning.) Thanks, Christian module Main where main = sequence reqs type Request = IO Int type Response = IO Int reqs :: [Request] reqs = client resps resps :: [Response] resps = server reqs client :: [Response] -> [Request] client (y:ys) = (do toPrint <- y putStr ("So far: " ++ show toPrint ++ "\nEnter number: ") line <- getLine return (parseToInt line)) : client ys server :: [Request] -> [Response] server = scanl helper (return 0) where helper :: IO Int -> IO Int -> IO Int helper sofar x = do a <- x sf <- sofar return (a + sf) parseToInt :: String -> Int parseToInt str = foldl nextDigit 0 (intList str) where intList :: String -> [Int] -- only for positive numbers so far intList = map (\x -> fromEnum x - 48) nextDigit :: Int -> Int -> Int nextDigit a b = a * 10 + b

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award-winning
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10 Aug '03

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Outlook
by birene＠erols.com 09 Aug '03

09 Aug '03

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idiom for producing comma-seperated lists?
by Antony Courtney 08 Aug '03

08 Aug '03

I often need to format a list of strings using some character as a *seperator* rather than a terminator for the items. Is there some simple combinator or idiom from the Prelude or standard libraries that could be used for this purpose? I ended up defining my own variation on foldr1 to solve this (code below), but I'd be much happier using some more standard solution: -- A variation on foldr1 that takes an extra argument to be returned if -- the list is empty. foldrs :: (a -> a -> a) -> a -> [a] -> a foldrs f z [] = z foldrs f _ xs = foldr1 f xs -- Example: format a list of strings, using a comma as a seperator: mkSepStr :: [String] -> String mkSepStr xs = foldrs (\x s -> x ++ ", " ++ s) "" xs t0 = mkSepStr [] -- ==> "" t1 = mkSepStr ["hello"] -- ==> "hello" t2 = mkSepStr ["10","20","30"] -- ==> "10, 20, 30" What do the rest of you do to solve this particular problem? If there isn't a standard solution, perhaps something like foldrs would be a useful addition to the Prelude. Thanks, -antony -- Antony Courtney Grad. Student, Dept. of Computer Science, Yale University antony(a)apocalypse.org http://www.apocalypse.org/pub/u/antony

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Re: Another typing question
by oleg＠pobox.com 08 Aug '03

08 Aug '03

> Is there any way to parametrize a type by a value, rather than another > type? I believe the following web page answers your question: http://pobox.com/~oleg/ftp/Haskell/number-parameterized-types.html It uses parameterization by decimal numbers -- which seem more natural to read. > It would be interesting to know if/how you could, e.g., define a > concatenation function with a type like Yes, you can -- and the page referenced above does that.

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Re: HI && TypeCast
by Iavor Diatchki 08 Aug '03

08 Aug '03

hello, this is not supposed to be serious :-) Sebastian Sylvan wrote: > ... > List comprehensions are overrated =) > > How about this: > > concatMap ( (':':) . (:[]) . fst ) [('d',3),('f',3)] > or even better: [('d',3),('f',3)] >>= (':':) . return . fst bye iavor -- ================================================== | Iavor S. Diatchki, Ph.D. student | | Department of Computer Science and Engineering | | School of OGI at OHSU | | http://www.cse.ogi.edu/~diatchki | ==================================================

2 1

why are you waiting
by a_tatarsky2＠saksinc.com 07 Aug '03

07 Aug '03

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Re: Another typing question
by Jon Cast 06 Aug '03

06 Aug '03

I wrote: > Wolfgang Jeltsch <wolfgang(a)jeltsch.net> wrote: > > On Tuesday, 2003-08-05, 15:22, CEST, Nick Name wrote: > > <snip> > > > > This is called "dependent types" and is not a feature of haskell > > > (nor of any language that I know); there was "cayenne" (try a google > > > search) but I don't believe it is still mantained. > > <snip> > > > > BTW, why is there no general interest for such a thing as dependent > > > types? > > > What negative consequences does their implementation have? > > It depends on whether you just want to write functions to take in > values of such types, or whether you want to write functions to return > values of such types. Doing the former uses dependant products, which > would require a significant extension to GHC's guts, but not (AIUI) > inherently unpleasant. Doing the latter (AIUI) essentially requires > dependant sums. Basically, a family of types A_i parameterized by a > value i :: I is written, for long-honored traditional reasons too > complex to get into here, Sigma i::I.A_i. Now, if you have such > dependant sums, there's a bad interaction with polymorphism: either > you must have a type alpha which cannot appear in a family of types > A_i in a dependant sum, or, for any function f :: alpha -> alpha, > there exists a type beta such that f cannot be aplied at either beta > or Sigma i::Int.beta. Clear? Actually, on second thought, I think this could be simplified to: if we have dependant types, then either there is a type which cannot be used in a dependant type, or for every polymorphic function there is a type it cannot be applied at. Jon Cast

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Re: Another typing question
by Jon Cast 06 Aug '03

06 Aug '03

> On Tuesday, 2003-08-05, 15:22, CEST, Nick Name wrote: <snip> > > This is called "dependent types" and is not a feature of haskell > > (nor of any language that I know); there was "cayenne" (try a google > > search) but I don't believe it is still mantained. <snip> > > BTW, why is there no general interest for such a thing as dependent > > types? > What negative consequences does their implementation have? It depends on whether you just want to write functions to take in values of such types, or whether you want to write functions to return values of such types. Doing the former uses dependant products, which would require a significant extension to GHC's guts, but not (AIUI) inherently unpleasant. Doing the latter (AIUI) essentially requires dependant sums. Basically, a family of types A_i parameterized by a value i :: I is written, for long-honored traditional reasons too complex to get into here, Sigma i::I.A_i. Now, if you have such dependant sums, there's a bad interaction with polymorphism: either you must have a type alpha which cannot appear in a family of types A_i in a dependant sum, or, for any function f :: alpha -> alpha, there exists a type beta such that f cannot be aplied at either beta or Sigma i::Int.beta. Clear? The alternative, btw., is to have both dependant sums and an undecidable type-checking problem. > I think, sometimes they could be quite handy. Sure. <Sarcastic comment censored>. Jon Cast

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Another typing question
by Konrad Hinsen 06 Aug '03

06 Aug '03

Is there any way to parametrize a type by a value, rather than another type? What I would like to do is to define "list of length 3" and "list of length 4" as separate parametrization of the same type, such that I could write functions that accept lists (under my new typename) of any length as long as parameters have the same length. The goal is to remove the need for runtime checks all over the code. Konrad.

4 7

Show with Strings
by Thomas L. Bevan 05 Aug '03

05 Aug '03

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Is there any reason why show :: String -> String is not equivalent to 'id' ? At the moment, show "one" = "\"one\"" which leads to problems because it often requires String to be treated as a special case, rather than just a member of Show. Tom -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.2 (GNU/Linux) iD8DBQE/Li8QYha8TWXIQwoRApCgAJ49iJcOuf88WTIKVJWBkHF/QyugOwCfS2Gu jcJFn48T4uc+ooeSAaCkAhs= =pIel -----END PGP SIGNATURE-----

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Newbie Design Question
by Tanton Gibbs 05 Aug '03

05 Aug '03

Haskellers, I'm currently working on my first Haskell program. I really like the language so far, though it has been hard to break the OO frame of mind. Normally, I'm a C++/Perl programmer, but I have really enjoyed the type-safety of Haskell over Perl as well as the Hugs interpreter over a make cycle. Since execution time is not important, I haven't found any issues with using Hugs. Now, on to my problem. Our company uses XML to describe our processes. I'm writing a DSL in Haskell that will print that XML out. I've done most of it, but now I'm describing the I/O in the XML. Basically, we have files come in in various formats from various file contributors and we use a mapping program to map the fields to the right spots in the XML. So, for example, one contributor could have the following layout: Field Name Start Pos Length First Name, 1, 10 Last Name, 11, 10 and another contributor could have them in exactly the opposite order. Furthermore, some contributors may have fields that others don't (for example: DOB). I'm now trying to come up with a way to map this information into Haskell dynamically. It would be trivial to create a module for each Contributor and then import all the modules. However, we have hundreds of contributors and it would be a pain to have to change and re-release the program every time we added a new contributor. Therefore, I need someway to take this information and map it into my Haskell structures. I have come up with two options. One would be to let Haskell dynamically import a contributor module (but I don't think that is currently possible); the other would be to use a configuration file and have the user specify the information through it. Is there a standard Config file module? Is there any other way to do this that I missed? Thanks in advance, Tanton Gibbs, Ph.D. Technical Architect Abilitec Knowledge Base Acxiom Corporation Conway, AR 72034

3 2

Persistent storage
by Nick Name 05 Aug '03

05 Aug '03

Is there someone who has implemented some limited form of persistency in haskell? I don't mean the longly-debated persistence of functional values, but something rough, like a persistent MVar with a thread saving modified values every n seconds or so. Vincenzo

2 2

Parsec allocating a lot of memory
by Nick Name 04 Aug '03

04 Aug '03

Hi all, I am using "parsec" to parse the output from "xmame -listinfo" wich is a list of records of the form game ( attr1 value1 ... attrN valueN ) and for approx. 3500 records I got ~250 mb of RSS memory during parsing, wich takes 20 seconds on my athlon 1400. I think that I must have done something wrong (this is the first time I use parsec), here is my parser: --- Begin games = many game game = do openGame x <- manyTill attribute closeGame return (mkGameInfo x) attribute = do whitespaces x <- identifier whitespaces y <- tillEOL return (x,y) openGame = do string "game (" newline closeGame = do string ")" newline newline whitespace = "\v\f\t\r " whitespaces = skipMany (oneOf whitespace) tillEOL = manyTill anyChar newline identifier = many alphaNum --- End Thanks for any advice Vincenzo

2 1

PROMOTION
by SANDER SMITH 04 Aug '03

04 Aug '03

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by webmaster 02 Aug '03

02 Aug '03

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by webmaster 02 Aug '03

02 Aug '03

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(no subject)
by Marcus D. Gabriel 02 Aug '03

02 Aug '03

-- Marcus D. Gabriel, Ph.D. Email: mdg(a)wanadoo.fr 213 ter, rue de Mulhouse F68300 Saint Louis FRANCE

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Re: a breaking monad (to haskell cafe)
by Iavor Diatchki 01 Aug '03

01 Aug '03

hi, Derek Elkins wrote: > Anyways, most of my uses of the Cont monad have been for my own personal > entertainment, and usually I break the abstraction (using > Control.Monad.Cont) to do the things I want. I don't know if I've ever > written something that's used callCC that I kept. except for entertainment purposes i feel pretty much the same way :-) i think that in probably 95% of the cases that arise there is a simpler monad that does the job (i.e. exceptions, backtracking, etc). of course it could be that we are simply not used to program using the callcc. bye iavor -- ================================================== | Iavor S. Diatchki, Ph.D. student | | Department of Computer Science and Engineering | | School of OGI at OHSU | | http://www.cse.ogi.edu/~diatchki | ==================================================

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FTP, XMLRPC libraries?
by Bryn Keller 01 Aug '03

01 Aug '03

Hi folks, I'm looking for FTP (client) and XMLRPC (client) libraries for Haskell. Does anyone know of any, or do I need to roll my own? Thanks, Bryn

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Exporting GHC functions.
by Thomas L. Bevan 31 Jul '03

31 Jul '03

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Here is a copy of all the files I am using to try to export a Haskell function. This compiles but produces a executable that simply hangs. Tom -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.2 (GNU/Linux) iD8DBQE/KUHbYha8TWXIQwoRAs8RAKCIFcnH9uuqfMpDYXA0gfFjBtMfEwCgjZeD 5dkfLbz/lz4btmsR9G4kL2A= =pCBN -----END PGP SIGNATURE-----

1 1

Linking to exported GHC functions.
by Thomas L. Bevan 31 Jul '03

31 Jul '03

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Could someone give me a command line example of how to link a C programme using functions exported via GHC FFI? I've compile C object files using an FFI function but can't for the life of me work out what arguments to pass to the compiler to make it link correctly. Thanks, Tom -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.2 (GNU/Linux) iD8DBQE/KTyZYha8TWXIQwoRAlAwAKDu/gh3/TVvrVo8Damrr5K8YvbGgACgsz17 2MZqp/GG0K51O+6Gn829I0A= =mIS3 -----END PGP SIGNATURE-----

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RE: Ann: HAllInOne bug fix release
by Hal Daume 31 Jul '03

31 Jul '03

(cafe) This seems like a reasonable hypothesis. Is there a way to get GHC to keep trying, despite the enormity of the input file? Specifically, I expect it to do a better job of (a) inlining and (b) specialization. Is there a way to hint to it to try a bit harder? :) - Hal -- Hal Daume III | hdaume(a)isi.edu "Arrest this man, he talks in maths." | www.isi.edu/~hdaume > -----Original Message----- > From: Koen Claessen [mailto:koen@cs.chalmers.se] > Sent: Thursday, July 31, 2003 1:49 AM > To: Simon Peyton-Jones > Cc: Hal Daume; Wolfgang Jeltsch; The Haskell Mailing List > Subject: RE: Ann: HAllInOne bug fix release > > > Simon Peyton-Jones wrote: > > | I don't know why the all in one version might go > | slower though. > > How about some (artificial) cut-offs during optimization > phases? Optimizing a hugs module could lead to some kind of > combinatorial explosion (which gets cut off by the > optimizer) which does not happen when you have separate > modules? > > Just my 2 öre! > > /Koen > >

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RE: Ann: HAllInOne bug fix release
by Hal Daume 31 Jul '03

31 Jul '03

(haskell-cafe now) Interesting. I've stripped them and they are exactly the same size. diff says they differ though. pre-stripping, the normal nhc (non ai1) is marginally larger (6.8m versus 7.0m). I'll try to measure memory usage if someone tells me how :) - Hal -- Hal Daume III | hdaume(a)isi.edu "Arrest this man, he talks in maths." | www.isi.edu/~hdaume > -----Original Message----- > From: haskell-admin(a)haskell.org > [mailto:haskell-admin@haskell.org] On Behalf Of John Meacham > Sent: Wednesday, July 30, 2003 5:22 PM > To: The Haskell Mailing List > Subject: Re: Ann: HAllInOne bug fix release > > > Another useful thing would be the striped binary sizes of the two > versions. I am curious how much AllInOneing affects the resultant > binary. Memory usage would also be interesting. > John > > On Wed, Jul 30, 2003 at 04:22:58PM -0700, Hal Daume wrote: > > Ah, apparently it is not (at least not the Cygwin version). > > > > I recompiled NHC with GHC -O2, both the separate > compilation version and > > the all-in-over version. Averaged over five runs, we see that the > > separate compilation version is actually *faster* than the > ai1 version: > > > > standard nhc compiled with ghc: > > > > real 0m27.167s > > user 0m9.991s > > sys 0m1.304s > > > > nhc all-in-one: > > > > real 0m31.411s > > user 0m10.007s > > sys 0m1.299s > > > > > > > > i am completely unable to explain this. someone want to > hazard a guess? > > > > - hal, who is a bit disappointed now :( > > > > -- > > Hal Daume III | hdaume(a)isi.edu > > "Arrest this man, he talks in maths." | > www.isi.edu/~hdaume > > > > > > > -----Original Message----- > > > From: haskell-admin(a)haskell.org > > > [mailto:haskell-admin@haskell.org] On Behalf Of Wolfgang Jeltsch > > > Sent: Wednesday, July 30, 2003 2:59 PM > > > To: The Haskell Mailing List > > > Subject: Re: Ann: HAllInOne bug fix release > > > > > > > > > On Wednesday, 2003-07-30, 23:36, CEST, Hal Daume III wrote: > > > > [...] > > > > > > > A few people have asked me for speed-up results from > > > All-In-One-ifying code, > > > > so here's a good one. We take two versions of NHC. One is > > > the original > > > > binary distribution and the other is the All-In-One-ified > > > version, compiled > > > > by GHC. > > > > > > Hi, > > > > > > is the original binary distribution compiled with GHC? If > > > not, the speed-up > > > may also be the result of using a different compiler (i.e., GHC). > > > > > > > [...] > > > > > > Wolfgang > > > > > > _______________________________________________ > > > Haskell mailing list > > > Haskell(a)haskell.org > > > http://www.haskell.org/mailman/listinfo/haskell > > > > > -- > > Haskell mailing list > > Haskell(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell > > > > -- > -------------------------------------------------------------- > ------------- > John Meacham - California Institute of Technology, Alum. - > john(a)foo.net > -------------------------------------------------------------- > ------------- > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell >

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RE: Ann: HAllInOne bug fix release
by Hal Daume 31 Jul '03

31 Jul '03

(moved to haskell cafe -- hopefully the other threads will follow) > >I don't know why the all in one version might go slower though > > Virtual memory thrashing? (Hal did say something about > needing lots of RAM.) only to compile. i haven't measured, but the resulting executable shouldn't require any more memory. it only hits about 400mbs max, though, which won't swap on my machine. - hal

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Re: Ann: HAllInOne bug fix release
by Malcolm Wallace 31 Jul '03

31 Jul '03

> Normal: > real 3m14.295s > user 3m9.879s > sys 0m2.143s > > All-In-One: > real 1m11.380s > user 0m10.102s > sys 0m1.711s > Something very odd is going on. GHC does not generate programs than run > 3x faster between GHC versions. This comparison is actually between an nhc98-built program (the first one) and a ghc-built program (the second one), showing that (in this specific case) nhc98 is generating code that is three times slower than that generated by ghc. In my experience, a speed difference of between 2x and 5x of the (unoptimised) generated code between the two compilers is fairly typical. The really odd thing is in the second set of figures. Unless there has been a cut-n-paste mistake, the user+system times do not add up even close to the wall-clock time, suggesting a much greater difference of 18x, which would start to worry me a little if it were true. However, Hal posted an updated set of figures comparing a program separately compiled with ghc to all-in-one compiled with ghc, and these figures seem to have a similar same user+system /= real discrepancy: > separately compiled with ghc: > real 0m27.167s > user 0m9.991s > sys 0m1.304s > all-in-one compiled with ghc: > real 0m31.411s > user 0m10.007s > sys 0m1.299s Is this some kind of known Cygwin timing problem? Do ghc executables fiddle with the internal timers at runtime? :-) Regards, Malcolm

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CSV parser, quotes?
by Shawn P. Garbett 31 Jul '03

31 Jul '03

In attempting to improve the CSV parser based on comments, I have the following code that's attached. I'm having a heck of a time getting the double quotes = an escaped quote thing to work. There is some commented out code which was my last attempt. As it stands the code works, minus the escaped quotes. Remove the comments and it just hangs. module CSV (contents, csv) where import Parsec ---------------------------------------------------------------------- -- CSV Module ---------------------------------------------------------------------- -- Useful common parsers comma :: Parser Char comma = char ',' quote :: Parser Char quote = char '\"' -- How to handle these buggers.... --esc_quote :: Parser Char --esc_quote = do {char '\"'; char '\"';} --text :: Parser String --text = do {esc_quote; t<-text; return ('\"':t)} -- <|> do {c <- noneOf "\""; t<-text; return (c:t) } -- <|> return "" -- A cell can be a quoted value, a number or empty -- Quotes can be embedded by using double quotes "" cell :: Parser String cell = between quote quote (many (noneOf "\"")) -- quoted values --cell = between quote quote text -- quoted values <|> many1 (noneOf "\",\n") -- unquoted values <|> return "" -- give up, Empty cell -- Group of cells with a newline cells :: Parser [String] cells = do c <- sepBy cell comma newline return c -- Comma Separated Values, set of rows followed by eof csv :: Parser [[String]] csv = manyTill cells eof -- Useful For extracting comma delimited values of a cell contents :: Parser [String] contents = sepBy1 (many (noneOf ",")) comma

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RE: Type design question
by Hal Daume 31 Jul '03

31 Jul '03

Hi Konrad, > I am a Haskell newbie working on my first serious test case, > and I would like > some feedback from the experts to make sure I am not doing > anything stupid > ;-) Well, I may not exactly qualify, but I can give you a few suggestions, nonetheless... > data Floating a => Vector a = Vector !a !a !a > deriving (Eq, Show) Here. Do not use 'data C x =>'. It's essentially a useless part of the language that really should have gone away. What you want is just: | data Vector a = Vector !a !a !a deriving (Eq, Show) The Floating constraint can come in on the functions. (I say this feature is useless because it only does one thing: disallows you to construct Vectors whose elements aren't instances of Floating. However, the useful thing that it might do -- namely, allow you to *use* the fact that the elements are instances of Floating -- it doesn't.) > I would like to keep the numerical representation as general > as possible, in > particular to postpone decisions about precision (Float, > Double, eventually > arbitrary precision arithmetic) as much as possible. > Therefore the "Floating > a" constraint. Which leads to my first question: doing it the > way I did, I > find myself having to add a "Floating a" constraint to almost > every function > specification with a vector argument. I would prefer to > specify once and for > all that vector elements need to be "Floating", everywhere. > Is there a way of > doing that? Not really. > -- Periodic universe > data Floating a => OrthorhombicUniverse a = OrthorhombicUniverse a a a Again, get rid of the 'Floating a =>' part. > instance Floating a => Universe (OrthorhombicUniverse a) where > distanceVector (OrthorhombicUniverse a b c) > (Vector x1 y1 z1) (Vector x2 y2 z2) > = Vector (fmod (x2-x1) a) (fmod (y2-y1) b) (fmod (z2-z1) c) > where fmod x y = x - y*truncate (x/y) There are actually two problems. First, you want to write fmod as: | fmod x y = x - y * fromInteger (truncate (x/y)) otherwise the type is wrong. Secondly, since you're using truncate, you need to be in RealFrac, not in Floating. Changing the signature to: | instance RealFrac a => ... fixes this problem. However, once we fix this, we can see the real problem. Your Universe class has a method, distanceVector, of type: | distanceVector :: Universe u, Floating a => u -> Vector a -> Vector a -> Vector a And here's the problem. When 'u' is 'OrthorhombicUniverse x', it doesn't know that this 'x' is supposed to be the same as the 'a'. One way to fix this is to parameterize the Universe data type on the element, something like: class Universe u where distanceVector :: (RealFrac a, Floating a) => u a -> (Vector a) -> (Vector a) -> (Vector a) distance :: (RealFrac a, Floating a) => u a -> (Vector a) -> (Vector a) -> a distance u v1 v2 = norm (distanceVector u v1 v2) -- Infinite universe data InfiniteUniverse a = InfiniteUniverse instance Universe InfiniteUniverse where distanceVector u v1 v2 = v2 <-> v1 -- Periodic universe data OrthorhombicUniverse a = OrthorhombicUniverse a a a instance Universe OrthorhombicUniverse where distanceVector (OrthorhombicUniverse a b c) (Vector x1 y1 z1) (Vector x2 y2 z2) = Vector (fmod (x2-x1) a) (fmod (y2-y1) b) (fmod (z2-z1) c) where fmod x y = x - y*fromInteger (truncate (x/y)) This is beginning to get a little ugly, but I think that's largely because you're using type classes in too much of an OO style (this is what a lot of my first programs looked like and I think for reasons similar to this, I moved away from that). However, not knowing more about the domain of discourse, I don't know what to suggest. Perhaps using the module system would serve you better. Instead of having a universe class, simply put each universe in a separate module and provide the same functions in each. Then you just import them selectively, or qualified. HTH. - Hal

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RE: Useful list output function
by Kevin S. Millikin 31 Jul '03

31 Jul '03

On Wednesday, July 30, 2003 1:52 PM, Shawn P. Garbett [SMTP:Shawn@Garbett.org] wrote: > > output :: (a -> IO b) -> [a] -> IO () > output f = (foldr (>>) (return ())).(map f) Study the Prelude :) > output :: (a -> IO b) -> [a] -> IO () > output = mapM_

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Useful list output function
by Shawn P. Garbett 30 Jul '03

30 Jul '03

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 I've got a little function which I derived from Bird's book "Intro to Functional Programming using Haskell". Basically I had several instances of lists that needed outputing with all kinds of text decoration. This function was used repeatedly throughout the code, by removing all the cut and paste instances, this was the common function: output :: (a -> IO b) -> [a] -> IO () output f = (foldr (>>) (return ())).(map f) So given a function that transforms a value of type 'a' into an output of 'IO' and a list of 'a', all IO is performed across the list. Example usage: output f [1,2,3] where f a = putStr ("List contains "++(show a)++"\n")) - -- List contains 1 List contains 2 List contains 3 Probably not very exciting for the old hats at Haskell, but pretty handy to have around at my level of expertise. Shawn -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.1 (GNU/Linux) iEYEARECAAYFAj8oIdAACgkQDtpPjAQxZ6AC0ACaA8boOZc8Hjmp9rTAyZ00lPLX QwkAniPsBPHUvBfUcsZlU6TvF1pzkTox =3aEG -----END PGP SIGNATURE-----

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RE: Parsing CSV files
by Simon Peyton-Jones 30 Jul '03

30 Jul '03

| > Hey, I wrote a CSV parser too. seeing as how this seems like a common | > thing (having 3 independent implementations at least), perhaps it | > belongs in libraries somewhere? some area dedicated to useful little | > grammers would be handy. csv, c header files, .x (rpcgen), various | > preference file formats, etc... | | What haskell needs is something along the lines of CPAN. Contributed modules | of various utility. Not part of the base libraries, but never-the-less useful | and maybe needed for your project. There's a lot of activity on this front on the libraries mailing list (libraries(a)haskell.org) http://haskell.org/hawiki/LibraryInfrastructure Simon

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