- Haskell-Cafe - Haskell.org

RE: [Haskell-cafe] RE: Haskell-Cafe Digest, Vol 21, Issue 31
by Bayley, Alistair 17 May '05

17 May '05

> From: Atwood, John Wesley [mailto:john.atwood@hp.com] > > Under my Linux install, with ghc6.4, it doesn't throw an > exception, but > also doesn't return a correct result; > it returns Just []. Under WindowsXP, ghc6.4, I get the exception you > get; under hugs, it can't find subRegex, but > matchRegex returns Just []. What does seem to work is > > import Text.Regex.Posix > > > > p1 = do > > r <- regcomp "\\." 0 > > b <- regexec r "foo.bar" > > putStr (show b) > > why it's in the IO monad, I don't know. > Where would I find the source? I browsed the ghc cvs at > haskell.org, but > can't find Text.Regex. > > > John http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/libraries/base/Text/Regex. hs?rev=1.15 ----------------------------------------- ***************************************************************** 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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RE: Haskell-Cafe Digest, Vol 21, Issue 31
by Atwood, John Wesley 17 May '05

17 May '05

> Date: Wed, 11 May 2005 17:22:25 -0400 (Eastern Standard Time) > From: "S. Alexander Jacobson" <alex(a)alexjacobson.com> > Subject: [Haskell-cafe] subRegex bug? > > Am I misunderstanding the regex docs? > > *MyMod> subRegex (mkRegex "\\." ) "foo.bar" "blah" > "foo*** Exception: Text.Regex.Posix.regcomp: error in pattern > Under my Linux install, with ghc6.4, it doesn't throw an exception, but also doesn't return a correct result; it returns Just []. Under WindowsXP, ghc6.4, I get the exception you get; under hugs, it can't find subRegex, but matchRegex returns Just []. What does seem to work is > import Text.Regex.Posix > > p1 = do > r <- regcomp "\\." 0 > b <- regexec r "foo.bar" > putStr (show b) why it's in the IO monad, I don't know. Where would I find the source? I browsed the ghc cvs at haskell.org, but can't find Text.Regex. John

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RE: Re[2]: [Haskell-cafe] Text search
by Bayley, Alistair 17 May '05

17 May '05

> From: Bulat Ziganshin [mailto:bulatz@HotPOP.com] > > if you really need KMP, you can find it at > http://haskell.org/hawiki/RunTimeCompilation > > > find (isSuffixOf "needle") (inits "haystack") > > find (isPrefixOf "needle") (tails "haystack") > > if you need an index - add it with zip: > > find (isPrefixOf "needle".snd) (zip [0..] (tails "haystack")) Or: findIndex (isPrefixOf "bla") (tails "dfvbdbblaesre") (I asked for something similar a few weeks ago: http://www.haskell.org//pipermail/haskell-cafe/2005-April/009690.html ) ----------------------------------------- ***************************************************************** 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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Re: [Haskell-cafe] NumberTheory library
by Yitzchak Gale 17 May '05

17 May '05

> As promised, here's the first attempt: > > darcs get http://andrew.bromage.org/darcs/numbertheory/ A few small comments about the function "factor" in Prime.hs: o I think you are testing w' * w' < n each time, even when you are repeating factors of the same prime p. You only need to do that when you move to the next p. o You can use divMod (or quotRem) instead of using div and then multiplying. (That's an ancient trick from C. It may not make a difference on modern CPUs.) o You are using the old functional programming trick of artificially creating state with extra function parameters. Nowadays we don't need that in Haskell - it is much simpler just to use the State monad. o I personally would much prefer a function that only returns prime numbers, not -1, 0, or 1. o I think that in most of the places where you fix the type as Int or Integer, you could leave it polymorphic and avoid a lot of coercing. Below is a version of "factor" that implements the above (except the last two, so that I preserve your interface). Regards, Yitz import Control.Monad.State -- ...etc. -- An infinite list of possible prime factors wheel :: Integral a => [a] wheel = iVerySmallPrimes ++ [f + d | d <- [0,iWheelModulus..], f <- wheelSettings] where iWheelModulus = fromIntegral wheelModulus iVerySmallPrimes = map fromIntegral verySmallPrimes wheelSettings = [f | f <- [z,z+2..iWheelModulus+1], null [p | p <- iVerySmallPrimes, f `mod` p == 0]] z = last iVerySmallPrimes + 2 -- |Factorize a number. factor :: Integral a => a -> [a] factor 0 = [0] factor 1 = [1] factor n | n < 0 = -1 : factor (-n) | otherwise = evalState (factorS n) wheel -- Factorize n with wheel as state factorS :: Integral a => a -> State [a] [a] factorS 1 = return [] factorS n = do p <- gets head if p * p > n then return [n] else factorPS n p -- Factorize n at a given prime p factorPS :: Integral a => a -> a -> State [a] [a] factorPS n p | rem == 0 = do ps <- factorPS quot p return (p:ps) | otherwise = do modify tail factorS n where (quot, rem) = n `quotRem` p

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concatMap generalizes to msumMap?
by S. Alexander Jacobson 16 May '05

16 May '05

What is the name of the monadic generalization of concatMap? In which lib is it located? -Alex- ______________________________________________________________ S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com

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Type system extension
by Thomas Davie 16 May '05

16 May '05

Hi, I'd just been writing some code and an interesting idea for an extension to Haskell's type system sprang into my head. I have no idea if people have played with it, but it looked vaguely useful to me, so I thought I'd see what everyone else thought. Supposing you have these types: type Export = String data SCode = SModule String [Export] | SUnknown It would be useful to specify a function as so: doSomethingToAModule :: SCode(SModule) -> SomeRandomOtherType which would specify, not only that the first argument was of type SCode, but more specifically, that it used the SModule constructor. This would then allow you to safely only write a case for the SModule constructor, and not worry about a runtime error when pattern matching failed on an SUnknown (as this would be checked at compile time). I hope this makes sense What does anyone think of the idea, and is there an obvious flaw in the plan? Thanks Tom Davie

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fptools in darcs now available
by John Goerzen 16 May '05

16 May '05

Hello, I am pleased to announce that I have used tailor.py to successfully convert the entire history of fptools HEAD branch, dating back to 1996, from CVS to darcs. For those of you that don't know, fptools represents the development area for the GHC Haskell compiler as well as many other related projects. It contains 21,949 changesets. You may check out the darcs repository by running: darcs get --partial http://darcs.complete.org/fptools *IMPORTANT REQUEST* PLEASE use --partial when you download this. If you really want a full copy of the repo to play with, contact me offline and I can give you a tar.bz2 somewhere. This server is not really set up to handle requests for 21,000 individual files. Currently, there is one checkpoint, made near the present time. I am going to try to add some more at various points in history if that would be useful. I'm not quite sure how to checkpoint old versions in a repo yet, though, so if anyone has tips... send 'em my way. Anyway, comments welcome. I intend to sync this up with the CVS sources every few days until either 1) the fptools people adopt darcs, or 2) it becomes apparent that it's not being useful for people. -- John

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NumberTheory library
by Bo Herlin 15 May '05

15 May '05

Hi I've been running Hugs for a couple of weeks now and I'm slowly starting to get the Haskell idea, and I certanly like it! But there are functions that I cant find and that I assume others before me must have missed and then perhaps also implemented them. Is there any standard library with functions like: binomial isCatalan nthCatalan nextCatalan isPrime nthPrime nextPrime factorize isFibonacci nthFibonacci nextFibonacci and possibly/hopefully other essential functions as well? (and of course better naming...) /Bo

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Re: Should inet_ntoa Be Pure?
by Dominic Steinitz 14 May '05

14 May '05

On Monday 09 May 2005 11:45 am, you wrote: > On 07 May 2005 11:59, Dominic Steinitz wrote: > > Does anyone know why these are in the IO monad? Aren't they pure > > functions converting between dotted-decimal strings and a 32-bit > > network byte ordered binary value? > > > > Dominic. > > http://www.haskell.org/ghc/docs/latest/html/libraries/network/Network.So > cket.html#v%3Ainet_addr > > http://www.haskell.org/ghc/docs/latest/html/libraries/network/Network.So > cket.html#v%3Ainet_ntoa > > Clearly it should be pure. To avoid breaking the interface, we could > add: > > showHostAddress :: HostAddress -> String > > and deprecate inet_ntoa? > > Cheers, > Simon I haven't followed the thread on this carefully but as a pragmatic approach, how about: hostAddress :: HostAddress -> (Word8,Word8,Word8,Word8) inetAddress :: (Word8,Word8,Word8,Word8) -> HostAddress then there shouldn't be any errors to deal with as all the addresses are valid in both representations. And if you want to show the address you can always do: showHostAddress x = concat $ intersperse "." (map show [first x, second x, third x, fourth x]) where first (a,_,_,_) = a second (_,b,_,_) = b third (_,_,c,_) = c fourth (_,_,_,d) = d Dominic.

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resolving missing class instances @ compile time
by Greg Buchholz 13 May '05

13 May '05

Here's a little quirk I ran into recently. While making a little vector data type in class Num (code below), I didn't implement an instance of "fromInteger" (thinking I didn't need it). Well as you can probably guess, it turns out I did need it, and subsequently got a run time exception. Which surprised me a little, since it seems like it could have been caught at compile time. (Although I did ignore a warning). This has probably been answered a thousand times, but could someone point me in the direction of documentation explaining why it compiled? Thanks, Greg Buchholz --uncomment out "fromInteger" to get a working program instance Num Vec where (V a b c) + (V x y z) = (V (a+x) (b+y) (c+z)) (V a b c) - (V x y z) = (V (a-x) (b-y) (c-z)) --fromInteger 0 = V 0.0 0.0 0.0 instance Eq Vec where (V a b c) == (V x y z) = (a==x) && (b==y) && (c==z) data Vec = V !Double !Double !Double deriving Show main = print $ sum [(V 1.0 2.0 3.0), (V 4.0 5.0 6.0)]

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