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How to write a lazy variant ?
by Dmitry Astapov 05 Dec '01

05 Dec '01

Consider the following functions: readRecFile fname = do ifh <- openFile fname ReadMode readRecords ifh readRecords ifh = do x <- decodeNextRecord ifh more <- isEof ifh if (more == False) then return [x] else do rest <- readRecords ifh return (x:rest) They allow me to read contest of file fname into a list of data structures. But "do rest <- readRecords ifh ; return (x:rest)" part makes them behave non-lazily. Maybe I'm just too tired right now, but it escapes me how can I write a lazy variant of those functions, so that they will behave like getContents. Does anyone have a clue? -- Dmitry Astapov //ADEpt E-mail: adept(a)umc.com.ua GPG KeyID/fprint: F5D7639D/CA36 E6C4 815D 434D 0498 2B08 7867 4860 F5D7 639D

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Re: Functional Metapost revival
by Ralf Hinze 04 Dec '01

04 Dec '01

Dear Feri, > You can find a version of Functional Metapost, which works > with current (2001 February) Hugs, on the following page: > > http://afavant.elte.hu/~wferi/funcmp/ > > This version is able to produce all the illustrations in the > enclosed Tutorial on my machine. > > Good luck, waiting for your comments: Feri. sorry for not being responsive. Currently, I don't have time to look into this. However, I put my current version of FMP on the web, which works both with Hugs (2001 February) and GHC. http://www.informatik.uni-bonn.de/~ralf/FuncMP-0.2.tar.gz Cheers, Ralf

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Integer to String Conversion?
by Chris 04 Dec '01

04 Dec '01

Hi everyone, is there a function that converts Integers to Strings and vice versa? Thanks in advance Chris

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Graphic User Interface Question
by Gabriel Alix 02 Dec '01

02 Dec '01

Hello All; My name is Gabriel Alix; I am currently an undergrad Computers Science major at Colgate University working with Proffessor Phillip Mulary(http://cs.colgate.edu/new/faculty/mulry.html). I am currently working with a group of other people to build a UI but am finding a lack of good examples to study off of. Would anyone have any references, links, or code that shows the inner workings of UI in Haskell? We are look for anything that will help us designs a good GUI. Any help would be appreciated Thank You ~Gabriel Alix

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Re: Strictness making it worst?
by Marcin 'Qrczak' Kowalczyk 01 Dec '01

01 Dec '01

Sat, 1 Dec 2001 15:58:47 +0000, Jorge Adriano <jadrian(a)mat.uc.pt> pisze: > So my question is, is there anyway to force an argument to be reduced to > *normal form*? The meaning of "normal form" (applied to an arbitrary Haskell type) is not precisely defined. It depends on the type and must be implemented for each type separately, you can at most group it in a class. It can't be implemented for functions at all if you want to evaluate their free variables. -- __("< Marcin Kowalczyk * qrczak(a)knm.org.pl http://qrczak.ids.net.pl/ \__/ ^^ QRCZAK

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Re: Strictness making it worst?
by Jorge Adriano 01 Dec '01

01 Dec '01

> How high can you go with fibac before getting a stack overflow? I'm > guessing that it's no more than 200000, and that the suspensions have > the following structure. 100 000 works fine 110 000 stack overflow > > fibac (as you described): > > c = (+) a b > d = (+) b c > e = (+) c d > > sfibac: > > c = ($!) ((+) a) b > d = ($!) ((+) b) c > e = ($!) ((+) c) d > > i.e. the ($!) is in the wrong place, and is only making the suspension > twice as deep. Addition is already strict in both its parameters. correct, dumb mistake, thanks :) > > sfibac' n (a,b) | n == 0 = (a,b) > > | otherwise = sfibac' (n-1) ((,) b $! a+b) Yeap, got it. Well, in fact my first aproach was not using pairs: > sfibac2 :: IntPos -> IntPos -> IntPos -> IntPos > sfibac2 n a b | n == 0 = a > | otherwise = sfibac2 (n-1) b $! (a+b) With pairs I thought I'd write > dumbfibac :: IntPos -> IntPos -> IntPos -> IntPos > dumbfibac n a b | n == 0 = a > | otherwise = dumbfibac (n-1) $! (b,(a+b)) But this would obviously not work because ($!) reduces its 2nd argument to 'head normal form', not 'normal form'. Anyway IMO it looks way much better then yours sfibac' function :-) So my question is, is there anyway to force an argument to be reduced to *normal form*? a strict fold function defined like, > sfoldl f a [] = a > sfoldl f a (x:xs) = (sfoldl f $! (f a x)) xs would have the same problem as my dumbfibac function. How do you correctly define a strict fold function? J.A.

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Strictness making it worst?
by Jorge Adriano 29 Nov '01

29 Nov '01

Hi, I've just started messing around with strictness. My goal now is understanding when and how to use it. I began with simple examples like making a strict foldl. When trying to sum a list of 60000 elements with lazy foldl I'd get a stack space overflow, with a strict foldl I was able to sum a list of 1E8 elements :) Then I thought I'd mess with the fibonnaci function. I began with: > fibac :: IntPos -> (IntPos,IntPos) -> (IntPos,IntPos) > fibac n (a,b) > | n == 0 = (a,b) > | otherwise = fibac (n-1) (b,a+b) Then I tried: > sfibac :: IntPos -> (IntPos,IntPos) -> (IntPos,IntPos) > sfibac n (a,b) > | n == 0 = (a,b) > | otherwise = sfibac (n-1) (b, (+b) $! a) I tried both functions with ghc 5.00.1 The strict version was not only slower, but I'd get stack overflows for numbers that worked with the 1st version!! (like 100000) I expect sfibac and fibac work this way: assuming that a+b = c b+c = d c+d = e sfibac: sfibac 4 (a, b) -> sfibac 3 (b, a+b) -> sfibac 2 (a+b, b+(a+b)) -> sfibac 1 (b+(a+b), (a+b)+(b+(a+b))) -> sfibac 0 ((a+b)+(b+(a+b)), (b+(a+b)) + (a+b)+(b+(a+b))) now if I had to evaluate the first element I'd get (a+b)+(b+(a+b)) -> c+(b+c) -> c+d -> e fibac: sfibac 4 (a, b) -> sfibac 3 (b, a+b) -> sfibac 2 (c, b+c) -> sfibac 1 (d, c+d) -> sfibac 0 (e, d+e) I'd expect sfibac to be faster, but what I find even more strange is the stack space overflow in sfibac! why? Greetings, J.A.

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Re: Strictness making it worst?
by Jorge Adriano 29 Nov '01

29 Nov '01

> Then I tried: > > sfibac :: IntPos -> (IntPos,IntPos) -> > > (IntPos,IntPos) sfibac n (a,b) > > > > | n == 0 = (a,b) > > | otherwise = sfibac (n-1) (b, (+b) $! a) I'm sorry I meant: > sfibac :: IntPos -> (IntPos,IntPos) -> > (IntPos,IntPos) sfibac n (a,b) > > | n == 0 = (a,b) > | otherwise = sfibac (n-1) (b, (a+) $! b) otherwise the reductions would be wrong > fibac: > sfibac 4 (a, b) -> > sfibac 3 (b, a+b) -> > sfibac 2 (c, b+c) -> > sfibac 1 (d, c+d) -> > sfibac 0 (e, d+e) In fact I would expect that using ((a+) $! b) would be better than (b, (+b) $! a), but the 1st seemed to be the worst in terms of both speed and stack space problems. J.A.

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Re: categorical prelude
by Christopher Milton 27 Nov '01

27 Nov '01

Oops, that should have been "Constructor Classes". --- Christopher Milton <cmiltonperl(a)yahoo.com> wrote: > I have a copy of the paper by Erik Meijer and Luc Duponcheel, > "On the expressive power of Combinator Classes", but I can't > seem to find a copy of the prelude file anywhere. > > Has anyone still got a copy anywhere? > > I look to hearing from you. __________________________________________________ Do You Yahoo!? Yahoo! GeoCities - quick and easy web site hosting, just $8.95/month. http://geocities.yahoo.com/ps/info1

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categorical prelude
by Christopher Milton 27 Nov '01

27 Nov '01

I have a copy of the paper by Erik Meijer and Luc Duponcheel, "On the expressive power of Combinator Classes", but I can't seem to find a copy of the prelude file anywhere. Has anyone still got a copy anywhere? I look to hearing from you. ===== Christopher M. Milton cmiltonperl(a)yahoo.com Phone 719/380-7665 Colorado Springs, CO __________________________________________________ Do You Yahoo!? Yahoo! GeoCities - quick and easy web site hosting, just $8.95/month. http://geocities.yahoo.com/ps/info1

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