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Re: [Haskell-cafe] Re: OCaml list sees abysmal Language Shootout results
by MR K P SCHUPKE 12 Oct '04

12 Oct '04

One to add to your list, string edit distance - as its hard, and useful. Keean.

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RE: [Haskell-cafe] strictness and the simple continued fraction
by Simon Peyton-Jones 12 Oct '04

12 Oct '04

If you are interested in arbitrary precision arithmetic using continued fractions, you may want to check out the work of David Lester. And Peter Potts et al. Just type "exact real arithmetic" into Google. Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of | William Lee Irwin III | Sent: 12 October 2004 04:53 | To: Scott Turner | Cc: haskell-cafe(a)haskell.org | Subject: Re: [Haskell-cafe] strictness and the simple continued fraction | | On Mon, Oct 11, 2004 at 09:53:16PM -0400, Scott Turner wrote: | > I tried using continued fractions in a "spiffy lazy list" implementation a | > while ago. Never got them working as well as expected. | > Evenutally I realized that calculating with lazy lists is not as | > smooth as you might expect. | > For example, the square root of 2 has a simple representation | > as a lazy continued fraction, but if you multiply the square root of 2 by | > itself, your result lazy list will never get anywhere. The calculation will | > keep trying to determine whether or not the result is less than 2, this being | > necessary to find the first number in the representation. But every finite | > prefix of the square root of 2 leaves uncertainty both below and above, so | > the determination will never be made. | > Your problems may have some other basis, but I hope this helps. | | I hit that one, too. That's nasty enough it may be best to give up on | the infinite case, at least. I can't think of a way to salvage all this. | | | -- wli | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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Re: [Haskell-cafe] Re: OCaml list sees abysmal Language Shootout results
by MR K P SCHUPKE 12 Oct '04

12 Oct '04

I though clean was always strict, and that was the major difference between clean and haskell (that and the fact clean is a proprietry language) Keean.

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Being Compatible
by John Goerzen 11 Oct '04

11 Oct '04

Hi, I have appreciated some of the comments here about what exactly is standardized. My next question is how to work at the greatest number of places. I am working on a library of Haskell-related utilities. There is some code in there that works with binary I/O, and some code that works with rawSystem(), etc. Some of these are not supported on Hugs, nhc98, or ghc5 to varying degrees. My initial thought was to use the cpp-style ifdefs I've seen elsewhere to mask those unsupported features on those particular systems. But Hugs at least doesn't support that, and I've found it extremely difficult to find a list of predefined macros for the other systems anyway. What is the suggested way to work around these problems?

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Re: [Haskell-cafe] Re: OCaml list sees abysmalLanguage Shootoutresults
by MR K P SCHUPKE 11 Oct '04

11 Oct '04

>No, you still have to copy "aaaa" so you can change the tail pointer Do you? If "aaaa" is treated as a single buffer contents (IE implemented as a UArray Int Char for example) then as our 'new' list implementation# can have cells which are single elements of buffers of elements, we simply 'cons' the buffer onto the beginning of the list (IE the next pointer is not in the constants string but the cell containing it) Keean.

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empty Array?
by Abraham Egnor 11 Oct '04

11 Oct '04

As a project mostly motivated by personal curiosity, I've been implementing a Sequence class: class Sequence s where front :: s a -> (a, s a) back :: s a -> (s a, a) null :: s a -> Bool empty :: s a prepend :: a -> s a -> s a append :: s a -> a -> s a concat :: s a -> s a -> s a The intent is to make it almost as easy to use, via pattern guards, as pattern matching on lists: map :: (a -> b) -> [a] -> [b] map _ [] = [] map f (x:xs) = f x:map f xs smap :: Sequence s => (a -> b) -> s a -> s b smap f s | null s = empty | (a, s') <- front s = prepend (f a) $ smap f s' It is, of course, trivial to implement this for lists. I've run into a snag, however, when trying to implement this for Arrays (as in Data.Array) - I can't seem to find a way to represent an empty array, which makes implementing 'empty' and 'null' impossible. Suggestions? Abe

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RE: [Haskell-cafe] Re: OCaml list sees abysmalLanguage Shootoutresults
by MR K P SCHUPKE 11 Oct '04

11 Oct '04

> - take this further and have list cells with 2 (or more) unboxed > characters, or even a full buffer. This sounds like the best idea to me... with each list cell being a full buffer you could effectively write nieve [Char] code and have it implemented in about as fast a way as possible... A little improvement on this would be to do a bit of read ahead, so that the filesystem latency can be hidden. (ie one thread reads the next buffer, whilst the application thread processes the previous buffer)... This would also benefit string processing... Imagine: test = "aaaa" ++ "bbbb" This could be implented as two list cells, one for each string, anf the cost of the "++" becomes the same as the cost of ":" test = 'a' : "bbbb" could be implemented in exactly the same way (a cell of size one and a cell of size 4) In this way with the cell size dependant on the source, lists generated by prepending characters would remain exactly as they are. The only slight complexity would be allowing 'next' pointers to point halfway through a cell, so that if you start with "dddd" remove the first character and prepend 'c' the original list "dddd" could still be in scope. Keean.

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NewbieQ: colon prefix for operators, e.g., Ratio ?
by Brian Beckman 11 Oct '04

11 Oct '04

Apologies if this is the wrong mailing list in which to pester folks with Newbie Questions, but I couldn't find my answer after half an hour of scouring the Haskell Report, the Haskell Wiki, the School of Expression book and a couple of tutorials. The frustrating thing is that I know I saw the answer in ONE of those sources and it didn't stick at the time: what does it mean when an operator is prefixed by a colon? For instance, in the Ratio module, the meaning of x % y is clear, but x :% y appears in multiple places and I'm confused.

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Re: Language extension idea (was Re: [Haskell-cafe] Re: OCaml list sees...)
by J.Garrett.Morris＠Dartmouth.EDU 11 Oct '04

11 Oct '04

--- Malcolm Wallace wrote: As an example, instead of the following list-only code, f :: List a -> ... f [] = ... f (h:t) = ... you could write this more general version, which assumes only some class Sequence with operations null, head, tail, etc. f :: Sequence s => s a -> ... f list | null list = ... | h <- head list, t <- tail list = ... --- end of quote --- I guess that's about half way there. I (and, I think, the original poster) was thinking more along the lines of generalizing the existing interface and considering lazy lists to be one implementation. It seems like the "list" syntax is general enough to apply to any ordered collection. Further, that would only require minimal changes in large amounts of existing code that relies on, for example, Strings being [Char]s were Strings changed to a more efficient representation. /gXm

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Language extension idea (was Re: [Haskell-cafe] Re: OCaml list sees...)
by oleg＠pobox.com 09 Oct '04

09 Oct '04

Tom Pledger wrote: > Something along these lines: > > class List l a | l -> a where > nil :: l > cons :: a -> l -> l > > But that's not of much use, because there isn't a class method to > recover the elements of a List. We could add more methods (corresponding > to null, head, and tail) Actually we merely need to add a deconstructor. Also, we can leave the type of elements fully polymorphic. Something like this: > class List l where > nil :: l a > cons :: a -> l a -> l a > decon :: l a > -> w -- on empty > -> (a -> l a -> w) > -> w > > instance List [] where > nil = [] > cons = (:) > decon [] on_nil on_cons = on_nil > decon (h:t) on_nil on_cons = on_cons h t we can then write truly generic list-processing functions: > rev:: List l => l a -> l a > rev l = rev' l nil > where > rev' l acc = decon l acc (\h t -> rev' t (cons h acc)) not in a terribly convenient way though. *Foo> rev "abc" "cba"

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