December 2002 - Haskell

Enum on Float/Double
by George Russell 23 Oct '03

23 Oct '03

The situation with Enum on Ratio is pretty bad but at least it's not hopeless, since rational numbers are at least exact. But for Float/Double it seems to be a total disaster area. Since it seems to be impossible to abolish Enum Float/Double altogether, would it at least be possible to make it rather more useful, by making succ x return the next representable float after x (or x if x is non-finite) and pred x the previous representable float. This would (a) be useful; (b) make mathematical sense; (c) is tricky to implement with the existing primitives. Before someone else points it out, this is of course unimplementable should someone in the future implement infinite-precision reals in Haskell, but as (barring a solution to the halting problem) Eq would be as well, that does not bother me. My preference would be for succ (+-0) to return the smallest positive real, since then you could define succ x to be the unique y with x < y and forall z . z < y => not (x < z), where such a y exists, and I'm not sure if the Haskell standard knows about signed zeros. Feri wrote > Everybody working with floats should know their quirks While in an ideal world, everyone who represented a mouse position or anything else as a float should have a PhD in numerical analysis, in practice they do not. Indeed I think the Haskell Library Report contains quite a few examples of floating point code which a numerical analyst would have written rather better. So I don't think it's good enough to treat every Float/Double operation as if it had an implicit "UNSAFE" flag indicating that the compiler was entitled to wierd behaviour only comprehensible to experts, as seems to be the case with floating enumerations now.

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tuple component functions
by trb＠eastpac.com.au 03 Jan '03

03 Jan '03

S.D.Mechveliani writes: > As Haskell has the standard functions fst, snd to decompose (a,b), > maybe, it worths to provide also > tuple31, tuple31, tuple31, > ... > tuple51, tuple52, tuple53, tuple54, tuple55 > > for the tuples of n = 3,4,5 ? I've found some of these useful, except I named them differently: > fst3 :: (a,b,c) -> a > fst3 (x,_,_) = x > snd3 :: (a,b,c) -> b > snd3 (_,x,_) = x > thd3 :: (a,b,c) -> c > thd3 (_,_,x) = x .... never got around to quadruples etc. I also defined (some of these may be untested): > tripl :: a -> (a,a,a) > tripl x = (x,x,x) > curry3 :: ((a, b, c) -> d) -> a -> b -> c -> d > curry3 f x y z = f (x, y, z) > uncurry3 :: (a -> b -> c -> d) -> ((a, b, c) -> d) > uncurry3 f t = f (fst3 t) (snd3 t) (thd3 t) > applyTriple :: (a -> b, a -> c, a -> d) -> a -> (b,c,d) > applyTriple (f,g,h) x = (f x, g x, h x) > cross3 :: (a -> b, c -> d, e -> f) -> (a,c,e) -> (b,d,f) > cross3 (f,g,h) (x,y,z) = (f x, g y, h z) > mapTriple :: (a -> b) -> (a, a, a) -> (b, b, b) > mapTriple = cross3 . tripl > applyFst3 :: (a -> b) -> (a, c, d) -> (b, c, d) > applyFst3 f = applyTriple (f . fst3, snd3, thd3) > applySnd3 :: (a -> b) -> (c, a, d) -> (c, b, d) > applySnd3 f = applyTriple (fst3, f . snd3, thd3) > applyThd3 :: (a -> b) -> (c, d, a) -> (c, d, b) > applyThd3 f = applyTriple (fst3, snd3, f . thd3) > applyArgs3 :: (a -> b) -> (c -> d) -> (e -> f) -> (b -> d -> f -> g) -> (a -> c -> e -> g) > applyArgs3 af1 af2 af3 f = \x y z -> f (af1 x) (af2 y) (af3 z) > rotl :: (a -> b -> c -> d) -> (b -> c -> a -> d) > rotl f y z x = f x y z > rotr :: (a -> b -> c -> d) -> (c -> a -> b -> d) > rotr f z x y = f x y z > rotl4 :: (a -> b -> c -> d -> e) -> (b -> c -> d -> a -> e) > rotl4 f y z t x = f x y z t > rotr4 :: (a -> b -> c -> d -> e) -> (d -> a -> b -> c -> e) > rotr4 f t x y z = f x y z t Some more combinators (only for pairs so far): > twin :: a -> (a,a) > twin x = (x,x) > swap :: (a,b) -> (b,a) > swap (x,y) = (y,x) -- apply a pair of functions to one argument > applyPair :: (a -> b, a -> c) -> a -> (b, c) > applyPair (f, g) x = (f x, g x) -- apply a pair of functions to a pair > cross :: (a -> b, c -> d) -> (a, c) -> (b, d) > cross (f, g) = applyPair (f . fst, g . snd) -- compose a pair of functions onto one function > dotPair :: (a -> b, a -> c) -> (d -> a) -> (d -> b, d -> c) > dotPair (f, g) h = (f . h, g . h) -- compose a pair of functions onto a pair of functions > dotCross :: (a -> b, c -> d) -> (e -> a, f -> c) -> (e -> b, f -> d) > dotCross (f, g) (h, i) = (f . h, g . i) > mapPair :: (a -> b) -> (a, a) -> (b, b) > mapPair = cross . twin > applyFst :: (a -> b) -> (a, c) -> (b, c) > applyFst f = applyPair (f . fst, snd) > applySnd :: (a -> b) -> (c, a) -> (c, b) > applySnd f = applyPair (fst, f . snd) > applyArgs :: (a -> b) -> (c -> d) -> (b -> d -> e) -> (a -> c -> e) > applyArgs af1 af2 f = \x y -> f (af1 x) (af2 y) > betweenFst :: (a -> b -> c) -> (a, d) -> (b, e) -> c > betweenFst = applyArgs fst fst > betweenSnd :: (a -> b -> c) -> (d, a) -> (e, b) -> c > betweenSnd = applyArgs snd snd I find these combinators useful, and probably other people have their own versions - it would be nice to get them standardised, so we all speak the same language. By standardised, I mean a prominent version that will be adopted by users, not necessarily a committee process. Tim

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ICFP programming contest
by trb＠eastpac.com.au 03 Jan '03

03 Jan '03

Michael Hobbs writes: > Just took a look at the results for the ICFP contest. Am I correct in > assuming that "LA" stands for Lennart Augustsson and that "Si^3" stands > for Simon Marlow, Simon Peyton-Jones, Sigbjorn Finne? Is there a URL for the results ? Tim

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ANNOUNCE: Haskell Wiki resurrected
by Keith Wansbrough 03 Jan '03

03 Jan '03

Hi all... last night I resurrected the Haskell Wiki, http://haskell.org/wiki/wiki This is a set of web pages on Haskell which can be edited and updated by anyone. The intention is to accumulate the combined wisdom of posters to the Haskell lists - if you ever reply to a FAQ with what you think is a particularly good answer, please add it to the Wiki. If you see a FAQ that's been Frequently Asked, just point the poster at the appropriate page on the Wiki. Sadly, the Wiki isn't very stable at the moment. I'm hoping that one day it will be made more stable, but in the meantime, I've taken a snapshot of the state as it was last night, and placed a link to it on the haskell.org front page: http://haskell.org/wikisnapshot/FrontPage.html Note that no updates to the Wiki are ever lost; they're kept in a version control system. So if you see a blank page on the main Wiki where you once spent several hours writing the perfect page, don't worry - it's not lost! Hope this is useful to people! --KW 8-) PS: I'm not officially in charge of the Wiki, John Heron <jheron(a)enteka.com> is; I'm just helping him out at the moment.

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Question About lists
by Cesar Augusto Acosta Minoli 02 Jan '03

02 Jan '03

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RE: Question About lists
by Jong Keun Na 31 Dec '02

31 Dec '02

> long = sum . map (const 1) How's this? /JongKeun -----Original Message----- From: William Lee Irwin III [mailto:wli@holomorphy.com] Sent: Tuesday, December 31, 2002 5:18 AM To: Artie Gold Cc: Cesar Augusto Acosta Minoli; haskell(a)haskell.org Subject: Re: Question About lists On Mon, Dec 30, 2002 at 01:47:37PM -0600, Artie Gold wrote: > One suggestion, though is that you're working too hard; there's really > no reason to define a locally defined function. The much simpler: > long [] = 0 > long (x:xs) = 1 + long xs > will do quite nicely. > HTH, > --ag There is already a length function in List and/or the Prelude. Bill _______________________________________________ Haskell mailing list Haskell(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell

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Re: real-time heap profiling?
by Jan-Willem Maessen 29 Dec '02

29 Dec '02

John Meacham <john(a)repetae.net> writes: > I have been playing with the heap profiling graphs from ghc and find > them quite useful and was wondering if there was a tool to display them > in real-time? Oddly, I found myself looking at heap profiles in real time in the past week. It turns out to be pretty easy to generate a complete heap profile for a running program using a slightly tricky unix command line: head -`fgrep -n END_SAMPLE FOO.hp | tail -1 | cut -d : -f 1` FOO.hp \ | hp2ps > FOO.ps This takes the FOO.hp file of your running program and turns it into a heap profile FOO.ps which includes all the data from the running program. I create a little script which runs this program every few seconds or so. By running gv in "watch file" mode I can see the heap profile of my entire program to date. It's not hard to cook up similar scripts (for example, to display a sliding window of the last n profiling samples) if you load FOO.hp into your favorite editor and take a look at the format. For example, here's parts of the file phc.hp, generated by running the pH/EH compiler on a problematic program: JOB "phc -hC" DATE "Thu Dec 26 18:17 2002" SAMPLE_UNIT "seconds" VALUE_UNIT "bytes" BEGIN_SAMPLE 0.00 END_SAMPLE 0.00 BEGIN_SAMPLE 15.07 ... sample data ... END_SAMPLE 15.07 BEGIN_SAMPLE 30.23 ... sample data ... END_SAMPLE 30.23 ... etc. BEGIN_SAMPLE 11695.47 END_SAMPLE 11695.47 By gluing the header (JOB, DATE, SAMPLE_UNIT, VALUE_UNIT) onto a bunch of samples extracted from the .hp file, you can run hp2ps on arbitrary sample data. Indeed, I use an editor to select and extract interesting chunks of profile data and look at those chunks in greater detail. By using fgrep -n we find the line numbers of the end of each complete sample: % fgrep -n END_SAMPLE phc.hp ... lots of output ... 1004960:END_SAMPLE 11621.34 1006610:END_SAMPLE 11636.61 1008257:END_SAMPLE 11651.85 1009926:END_SAMPLE 11667.08 1011524:END_SAMPLE 11682.33 1011526:END_SAMPLE 11695.47 We select the line number of the last complete sample (the end of a .hp file will contain partially-written sample data while the program is running, causing hp2ps to choke): % fgrep -n END_SAMPLE phc.hp | tail -1 | cut -d : -f 1 1011526 This tells us that the first 1011526 lines of phc.hp will form a properly-formatted heap profile, which we can feed to hp2ps: head -1011526 phc.hp | hp2ps > phc.ps The "watch file" mode of gv is a godsend for this sort of application. It just wouldn't have been worth the effort to me to cook up a gui for the sole purpose of displaying an up-to-date profile. I suspect, by the way, that if .hp files were xml-formatted I would have had a far less pleasant time of things. There's such a thing as TOO much structure. A lesson for us tool-builders, perhaps? -Jan-Willem Maessen

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real-time heap profiling?
by John Meacham 28 Dec '02

28 Dec '02

I have been playing with the heap profiling graphs from ghc and find them quite useful and was wondering if there was a tool to display them in real-time? For interactive long-running applications written in haskell it would be nice to see how the heap profile reacts to user actions. I imagine a scrolling graph of the last minute or so of time. Does such a thing exist? would writing one be possible without modifying ghc? it seems you can poll the .hp file and update the graph, but i have not looked into the format of .hp files to see if this is plausable. any thoughts? John -- --------------------------------------------------------------------------- John Meacham - California Institute of Technology, Alum. - john(a)foo.net ---------------------------------------------------------------------------

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Question About Random Module
by Cesar Augusto Acosta Minoli 25 Dec '02

25 Dec '02

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Re: Question About Random Module
by Zdenek Dvorak 24 Dec '02

24 Dec '02

Hello, >Hello! I just Wanna Know, What should I do, to build a List of random >Numbers, >without IO type, just Float or Int the answer for this is either "you can't, read something about IO and monads (good starting places are http://haskell.org/wiki/wiki?UsingIo and http://haskell.org/wiki/wiki?UsingMonads, and reading other FAQs there is a good idea, too)" or "you should not, but if you really have to, browse this mailing list archives, where you'll find the answer 1000x", depending on what you are asking for. Zdenek _________________________________________________________________ MSN 8 helps eliminate e-mail viruses. Get 3 months FREE*. http://join.msn.com/?page=features/virus&xAPID=42&PS=47575&PI=7324&DI=7474&… http://www.hotmail.msn.com/cgi-bin/getmsg&HL=1216hotmailtaglines_virusprote…

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