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RE: [Haskell-cafe] Parsec question
by Bayley, Alistair 19 Nov '04

19 Nov '04

I've also used Parsec for separated lexer + parser and currently have something like this to invoke them: testParse inputString = do case (parse myLexer "" inputString) of Left err -> fail ("lexical error: " ++ err) Right tokens -> case (parse myParser "" tokens) of Left err -> fail ("parse error: " ++ err) Right result -> return result ... or was this the "manual startup" that you were referring to? The above seems clunky to me, so I'd also welcome suggestions for piping the lexer output into the parser. What do you mean by "makes position calculations very complex"? Are you talking about reporting the position of lexical or parse errors? (If so, Parsec supports this quite well.) Alistair. > -----Original Message----- > From: John Goerzen [mailto:jgoerzen@complete.org] > Sent: 19 November 2004 14:28 > To: haskell-cafe(a)haskell.org > Subject: [Haskell-cafe] Parsec question > > Hi, > > I'm porting a parser over from an OCamllex/Ocamlyacc version and I'm > using Parsec for both the tokenizer and the resulting token > stream parser. > > I have both of them working fine, but now my question is: how do I > combine them? I can't quite figure out how to say "take the > output from > this GenParser Char () Tok and use it as the input for this > GenParser Tok () ParserOutput". > > Well, I have figured out an ugly way involving manually > starting up both > parsers, but it makes position calculations very complex. I > suspect I'm > missing something. > > Ideas? ----------------------------------------- ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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Parsec question
by Tomasz Zielonka 19 Nov '04

19 Nov '04

On Fri, 19 Nov 2004 14:28:07 +0000 (UTC), John Goerzen <jgoerzen(a)complete.org> wrote: > Hi, > > I'm porting a parser over from an OCamllex/Ocamlyacc version and I'm > using Parsec for both the tokenizer and the resulting token stream parser. > > I have both of them working fine, but now my question is: how do I > combine them? I can't quite figure out how to say "take the output from > this GenParser Char () Tok and use it as the input for this > GenParser Tok () ParserOutput". > > Well, I have figured out an ugly way involving manually starting up both > parsers, but it makes position calculations very complex. Did you try to pair tokens with their SourcePos'es? What I did in my program was this: tokenize :: CharParser st tok -> CharParser st [(SourcePos, tok)] tokenize tp = do l <- many t eof return l where t = do pos <- getPosition tok <- tp return (pos, tok) token :: (Show tok) => (tok -> Maybe a) -> GenParser (SourcePos, tok) st a token f = Parsec.token (show . snd) fst (f . snd) This has a space leak, but I didn't care, because my inputs are at most 10kb long. You can do a lazy version with getInput. BTW, it would be useful if Parsec allowed to throw a ParseError from one type of parser in another parser. It would help me in a situation, where I parse the whole file with one parser, but I retokenize some parts of it and parse them with another parser type. Right now I am using this workaround, but it doesn't work well: cs <- chars pos <- getPosition x <- case p pos cs of Left err -> do setPosition (errorPos err) fail (messageString (head (errorMessages err))) Right x -> return x How about making an instance for instance Control.Monad.Error.MonadError ParseError (GenParser tok st) PS. I guess I could do this myself. Best regards, Tomasz

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Parsec question
by John Goerzen 19 Nov '04

19 Nov '04

Hi, I'm porting a parser over from an OCamllex/Ocamlyacc version and I'm using Parsec for both the tokenizer and the resulting token stream parser. I have both of them working fine, but now my question is: how do I combine them? I can't quite figure out how to say "take the output from this GenParser Char () Tok and use it as the input for this GenParser Tok () ParserOutput". Well, I have figured out an ugly way involving manually starting up both parsers, but it makes position calculations very complex. I suspect I'm missing something. Ideas?

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IO and State (was Re: [Haskell] Re: Global Variables and IO initializers)
by Iavor S. Diatchki 17 Nov '04

17 Nov '04

Hello, Just wanted to point out that the suggested idea is not quite correct. (well that has to be quantiifed a bit, see bellow) Krasimir Angelov wrote: >--- Ben Rudiak-Gould ><Benjamin.Rudiak-Gould(a)cl.cam.ac.uk> wrote: > > >>This is solved by merging the IO and ST monads, >>something that ought to >>be done anyway: >> >> type IO = ST RealWorld >> type IORef a = Ref RealWorld a >> type STRef s a = Ref s a >> >> It is not (should not be?) the case that IO = ST RealWord, as IO is not a state monad as we understand it. In a state monad, the changes to the state are all in the program, i.e. one can always point to the part of the program that modified the state. On the other hand, the "state" of the RealWorld can change on its own, without anything in the program affecting it. I guess this is similar to "volatile" state in C. For example, one might expect the following rule in a state monad: do x <- readSTRef r y <- readSTRef r f x y = do x <- readSTRef r f x x But this is not true for the IO monad, as for example reading a file twice does not guarantee that you will get the same result, even if no part of the program ever wrote to the file. Now the above law already doesn't hold when all GHC extensions are used, as when concurrency is present we cannot assume that nobody modified the state concurrently. As a result all pointers in GHC probably behave as if they were "volatile", which is not very nice. I think that inherently there are two concepts here, and I see no point in mixing them (even though they are already a bit mixed up, because of stToIO). The one is the usual sequential state that we know and love (and I think we use that a lot of the time). In the sequential state the above optimization is one of the laws specifying the monad. The other concept is that of "volatile" or concurrent state, and then the law doesn't hold. The two monads have a very similar inetrafce (reading and wrinting pointers, and creating new pointers) but the laws that the operations satisfy are different. -Iavor

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Pure Haskell Printf
by John Goerzen 17 Nov '04

17 Nov '04

(Sorry for the crosspost; I'm not sure which list this should go to.) I've just completed a pure-Haskell printf. Docs at [1], download at [2]. Here are some examples: vsprintf "Hello" > "Hello" vsprintf "Hello, %s\n" "John" > "Hello, John\n" vsprintf "%s, your age is %d\n" "John" (10::Integer) > "John, your age is 10\n" sprintfAL "%(name)s, your age is %(age)d\n" [("name", v "John"), ("age", v (10::Integer))] > "John, your age is 10\n" I have more examples available at the doc page[1]. I used Ian Lynagh's Printf.Printer module to do the actual formatting (I converted it away from TH first). I got the idea for simulating handling variable numbers of function arguments from the haskell-xml-rpc code. I would appreciate comments/critiques. -- John [1] http://gopher.quux.org:70/devel/missingh/html/MissingH.Printf.html [2] http://gopher.quux.org:70/devel/missingh

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Re: [Haskell-cafe] Re: Pure Haskell Printf
by MR K P SCHUPKE 16 Nov '04

16 Nov '04

Of course you can do intertationalisation with show... There is a paper on using type classes to define implicit confugurations, perhasps someone can provide the reference? This can be used to nicely redefine show... Ill see if I cant dig out an example. Keean.

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Re: [Haskell] Problems installing HTTP and Browser modules
by Bjorn Bringert 13 Nov '04

13 Nov '04

Ranasaria, Kamal Kishor wrote: > Dear Sir, > > I was trying to install the HTTP module and came across the following > error. Would really appreciate if could help me find the cause of the error. > [...] I am guessing that you are using the version from http://homepages.paradise.net.nz/warrickg/haskell/http/ There is an updated version of the package at http://www.bringert.net/~d00bring/haskell-xml-rpc/http.html The code is in a darcs repo at http://cvs.haskell.org/darcs/http/ Graham Klyne has a slightly different version which uses the new Network.URI module. His code can be found at http://www.ninebynine.org/Software/HaskellUtils/Network/ /Bjorn

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RE: [Haskell-cafe] One-shot? (was: Global variables and stuff)
by Simon Marlow 13 Nov '04

13 Nov '04

On 09 November 2004 11:54, Graham Klyne wrote: > I've not been following the Global variables debate too closely, it > seeming to have something of a religious wars flavour, but I noticed > that an example being proposed was how to achieve a "one shot" > execution. Here's something I did when working on modifications to > the HaXML parser: > > [[ > -- Memoization of withSocketsDo to prevent multiple calls. > -- (cf. http://tangentsoft.net/wskfaq/articles/lame-list.html) > socketsInitialized :: Bool > socketsInitialized = unsafePerformIO ( withSocketsDo ( return True ) ) > ]] > > Does it work as I think it does? ARe there any problems I'm > overlooking? You should add {-# NOINLINE socketsInitialized #-} to be on the safe side, but in practice GHC won't inline it anyway. The pragma would be good documentation though. Cheers, Simon

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Currying confusion
by John Goerzen 12 Nov '04

12 Nov '04

I'm trying to do something -- I think it's called uncurrying. I've boiled up a little test case to show what I mean: test1 :: Int -> String test1 = show test2 :: Int -> Int -> String test2 x y = (show x) ++ (show y) OK, that works OK. Now let's say I want to provide a function that generates the string to use in place of (show y) -- perhaps, test1. I like the partial application feature (and used it in test1). So, I thought I could use that to my advantage in test3 and test4: test3 :: Int -> Int -> String test3 x f = (show x) ++ f test4 :: Int -> (Int -> String) test4 x f = (show x) ++ f Of course, I could say test3 x f y = (show x) ++ (f y) but for various reasons that's not the most useful in my program. Thoughts?

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Re: [Haskell] Re: About Random Integer without IO
by Georg Martius 12 Nov '04

12 Nov '04

Hi, Sorry, but I don't really get the point of the discussion so far. I think any computation should be done in pure functional style, that gets a random generator (seed) and passes it around. Your program starts in the IO monad anyway, so you have the choice of using newStdGen to get a new seed or initialise it by hand from a command line argument or whatever. I think automatic random initialisation is very important and handy in programs that run non-deterministic simulations. Cheers, Georg On Thu, 11 Nov 2004 22:43:28 +0000, Thomas Davie <tatd2(a)kent.ac.uk> wrote: > On 11 Nov 2004, at 22:02, karczma wrote: > >> Thomas Davie writes: >>> This method unfortunately depends on having a seed first though. >> >> Which "this method"? Please, quote the text you are referring to >> *before* >> your answer. >>> One must use a different value every time the program is started, >>> commonly time or the first few bytes from /dev/random. Any one of >>> these is going to require a monadic function to generate (i.e. it >>> must come from the environment in some way - it must change every >>> time you run the program) So while this eliminates the IO monad from >>> most of the program, it must still be initiated using it. >> >> I understand that you mean the seed-propagation method. >> You are *partly* right, that the same program executed several times >> will >> generate the same sequence, if initialized identically. But there is no >> need for monads, it suffices to pass a parameter, or to read a >> different >> initialization file each time. >> Of course, anybody, especially somebody contributing to this list has >> the right to be a nit-picker. But, if one is a *practical* user of >> random stuff, then he/she usually knows such details. Moreover, it is >> sometimes very useful to control absolutely the initialization, to be >> able to repeat the experiment, so, professionals in Monte Carlo, etc. >> rarely if ever, use time, or system-dependent (and hidden) random >> values. >> For me the initialization is a *context* problem, which I separate from >> the properties of the random generator. It is its quality, lack of >> correlations, etc. which is important. Do you know of many problems >> requiring an automated, truly random initialization? In this case one >> may well delegate this issue to the *launching* of the functional >> program, >> instead of struggling with it from within. >> This is my personal philosophy, everybody is entitled to his/hers. >> > While I agree that it is often useful to start your program with > different parameters each time to seed the random number generator, I > would argue that this is often inappropriate - supposing we are writing > a very simple program to roll a virtual dice. If the user finds one > input that rolls a six, they can then use this repeatedly. Reading an > initialisation file is back to the situation of using IO to generate > our seed. > > Thanks > > Tom Davie > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell > -- ---- Georg Martius, Tel: (+49 34297) 89434 ---- ------- http://www.flexman.homeip.net ---------

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