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Re: [Haskell-cafe] foldlWhile
by Serge D. Mechveliani 22 Nov '04

22 Nov '04

To my question of >> foldlWhile :: (a -> b -> a) -> (a -> Bool) -> a -> [b] -> a >> foldlWhile f p a bs = >> case >> (bs, p a) >> of >> ([], _ ) -> a >> (_, False) -> a >> (b:bs', _ ) -> foldlWhile f p (f a b) bs' >> >> foldl does not seem to cover this. Dylan Thurston wrote: On Nov 20, 2004 > An alternative is > > foldlWhile f p a bs = last $ takeWhile p $ scanl f a bs > Yes. Thank you. Inspecting Prelude, we find out that scanl expresses, mainly, the thing. ----------------- Serge Mechveliani mechvel(a)botik.ru

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Predicates in HaXml
by Tom Spencer 21 Nov '04

21 Nov '04

Hi there, In XSLT there is an XPath function that will let you select a particular node in the current context, for example; <xsl:value-of select="team[1] "/> in Michael Kay's article http://www-106.ibm.com/developerworks/xml/library/x-xslt/?article=xr This selects the first team element in the current context. Is there a work around to get similar functionality from the HaXml library? Many Thanks Tom Spencer

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Re: [Haskell-cafe] foldlWhile
by dpt＠bostoncoop.net 20 Nov '04

20 Nov '04

On Sat, Nov 20, 2004 at 03:48:23PM +0000, Jorge Adriano Aires wrote: > > On Sat, Nov 20, 2004 at 12:47:58PM +0300, Serge D. Mechveliani wrote: > > > foldlWhile :: (a -> b -> a) -> (a -> Bool) -> a -> [b] -> a > > > foldlWhile f p a bs = > > > case > > > (bs, p a) > > > of > > > ([], _ ) -> a > > > (_, False) -> a > > > (b:bs', _ ) -> foldlWhile f p (f a b) bs' > > > > Why not just > > foldlWhile f p a bs = takeWhile p $ foldl f a bs > > Quite different. The former stops a foldl when the accumulating parameter no > longer satisfies p, the later assumes the accumulating parameter of the foldl > is a list, and takes the portion of the list that does satisfy p. Yes, this was a mistake. > The following is closer to the original, but doesn't work when the whole list > is folded (i.e., p always satisfied): > foldlWhile f p a = head . dropWhile p . scanl f a Serge's version returns the last 'a' that satisfies 'p', while yours returns the first 'a' that does not satisfy 'p'. This should be an equivalent version: foldlWhile f p a = tail . takeWhile p . scanl f a But what about the version with Maybe? There ought to be a concise way to write that too. Peace, Dylan

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No need for integer literals
by Henning Thielemann 19 Nov '04

19 Nov '04

In addition to Paul Hudak's "The Haskell School of Expression", Appendix A "Built-in types are not special" I want to note that there is also hardly a need for integer literals. :-] module Digits where infixl 9 # (#) :: a -> (a -> b) -> b x # f = f x int :: Num a => a int = 0 d0, d1, d2, d3, d4, d5, d6, d7, d8, d9 :: (Enum a, Num a) => a -> a d0 n = n+n+n+n+n+n+n+n+n+n d1 = succ . d0 d2 = succ . d1 d3 = succ . d2 d4 = succ . d3 d5 = succ . d4 d6 = succ . d5 d7 = succ . d6 d8 = succ . d7 d9 = succ . d8 test :: (Enum a, Num a) => a test = int#d1#d4#d2#d8#d5#d7 * int#d2#d1

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Sample rate inference
by Henning Thielemann 19 Nov '04

19 Nov '04

I want to connect several functions for signal processing. The typical case is that in a network of signal processor there are parts that are already discretized such as sampled sounds, and there are processors with no particular sample rate such as amplifiers. But when it comes to computation each processor must choose a sample rate for processing. There are also processors for converting sample rates. They divide the network into components of equal sample rate. The computation sample rate should be propagated through the network as follows: If in a component of equal sample rate some processors have the same fixed sample rate, all uncertain processors must adapt that. If some processors have different fixed sample rates this is an error. If no processor has a fixed sample rate, the user must provide one manually. To me this looks very similar to type inference. Is there some mechanism in Haskell which supports this programming structure?

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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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