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Re: [Haskell-cafe] Rewriting filter with foldr
by PR Stanley 30 Sep '07

30 Sep '07

The question is asking for a new definition of filter using foldr. Sorry, I should have mentioned that before. Cheers, Paul At 14:26 30/09/2007, you wrote: >Perhaps a list comprehension better shows the intention of the >filter function: > >filter p xs = [x | x <- xs, p x] > >You can literally read that as "take all x from xs that satisfy p".

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Rewriting filter with foldr
by PR Stanley 30 Sep '07

30 Sep '07

Hi filter :: (a -> Bool) -> [a] -> [a] filter f = foldr (\x -> \xs -> if (f x) then (x:xs) else xs) [] Somehow I feel this could be done more elegantly. What does the list think? Thanks, Paul

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Re: [Haskell-cafe] Rewriting ord (revisited)
by PR Stanley 30 Sep '07

30 Sep '07

The second version looks very neat, certainly, though I am not entirely convinced that it's any more efficient. Still, I may be missing something. The compiler must be one hell of a machine. I wonder if the source code is available to the public. Cheers, Paul At 12:26 30/09/2007, you wrote: >-----BEGIN PGP SIGNED MESSAGE----- >Hash: RIPEMD160 > >I would think that the compiler is smart enough to fuse the two loops. >That doesn't explain the performance difference though. > >Adrian > > >PR Stanley schrieb: > > Hi > > This was my original version plus some modifications as advised by the > > list: > > f c = sum [1 | x <- ['\0'..], x < c] > > > > The following version was sent by a list member: > > f c = length $ takeWhile (<c) ['\0'..] > > > > Now, the sender asserted that the first version was much too slow. I'm > > wondering how the second version is any more efficient than the first. > > Looking at them through my C programmers' eye, as it were, both would > > require at least two loops before returning the ANSI value. > > Any ideas? > > Thanks, Paul > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > >-----BEGIN PGP SIGNATURE----- >Version: GnuPG v1.4.7 (MingW32) >Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org > >iD8DBQFG/4f+11V8mqIQMRsRAxegAJ4t5grdRDrWVWby6tjDZDdxgJ72FQCfcD7W >Mntl7FV2GpCGtpY2yP61kbk= >=l+3X >-----END PGP SIGNATURE-----

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Rewriting ord (revisited)
by PR Stanley 30 Sep '07

30 Sep '07

Hi This was my original version plus some modifications as advised by the list: f c = sum [1 | x <- ['\0'..], x < c] The following version was sent by a list member: f c = length $ takeWhile (<c) ['\0'..] Now, the sender asserted that the first version was much too slow. I'm wondering how the second version is any more efficient than the first. Looking at them through my C programmers' eye, as it were, both would require at least two loops before returning the ANSI value. Any ideas? Thanks, Paul

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Compiling Simple Regex Program
by Andrew Trusty 30 Sep '07

30 Sep '07

Hello, I'm using GHC 6.6.1 under Windows XP and I can't get the following simple program to compile. import Text.Regex main = putStrLn (subRegex (mkRegex "c") "abc" "a") It runs in GHCi just fine but GHC gives the following output when trying to compile: project1.o(.text+0x169):fake: undefined reference to `regexzmcompatzm0zi71_Textz iRegex_mkRegex_closure' project1.o(.text+0x1f7):fake: undefined reference to `regexzmcompatzm0zi71_Textz iRegex_subRegex_closure' project1.o(.rodata+0x4):fake: undefined reference to `regexzmcompatzm0zi71_Textz iRegex_mkRegex_closure' project1.o(.rodata+0x8):fake: undefined reference to `regexzmcompatzm0zi71_Textz iRegex_subRegex_closure' collect2: ld returned 1 exit status

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build problem (X11 under cabal)
by Jens Blanck 30 Sep '07

30 Sep '07

Not being very savvy in building things I got stuck when trying to build xmonad. It needs X11 1.2.2, so I tried to build that. Configuration step went fine (I believe), but buiding failed: > runhaskell Setup.hs build Preprocessing library X11-1.2.2... /usr/lib/gcc/i586-suse-linux/4.1.0/../../../../i586-suse-linux/bin/ld: cannot find -lX11 collect2: ld returned 1 exit status linking Graphics/X11/Types_hsc_make.o failed command was: /usr/bin/ghc-6.6 -lX11 Graphics/X11/Types_hsc_make.o -o Graphics/X11/Types_hsc_make Setup.hs: got error code while preprocessing: Graphics.X11.Types I have tried the ghc command seperately with the flag -L/usr/X11R6/lib which makes it compile. But I cannot figure out how to proceed from here. Jens

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Re: [Haskell] Seemingly impossible Haskell programs
by Roberto Zunino 30 Sep '07

30 Sep '07

Graham Hutton wrote: > Readers of this list may enjoy the following note by > Martin Escardo, which shows how to write a number of > "seemingly impossible Haskell programs" that perform > exhaustive searches over spaces of infinite size, by > exploiting some ideas from topology: > > http://math.andrej.com/2007/09/28/seemingly-impossible-functional-programs/… Very nice! The note shows that you can actually compute (forall p) where p is a *total* predicate over streams of Bool (a.k.a. infinite lazy lists). Indeed, (writing [] for streams) forall :: ([Bool] -> Bool) -> Bool seems impossible to compute. Here's what I understood: (which might be wrong, of course!) This apparent magic relies on the fact that any total predicate over (total) bool streams, i.e. p :: [Bool] -> Bool can only inspect a _finite_ prefix of the input list. More importantly, the length of the inspected prefix has an upper bound which depends _only_ on p and not on the actual input of p. In rough formulas forall p, exists n:nat, forall l:[Bool], forall l':[Bool], p l == p (take n l ++ l') So, with some nice laziness tricks, you can actually check p over "all" the infinite [Bool] space, using only finite time. Zun.

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Order of Declaration
by PR Stanley 30 Sep '07

30 Sep '07

Hi in C type languages a function must be declared before its application. Would I be right in thinking that this isn't the case in Functional languages? For example: transmit :: String -> String transmit = decode . channel . encode channel :: [[Int]] -> [[Int]] channel = id Cheers, Paul

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Text.Html
by Bob 30 Sep '07

30 Sep '07

Hi everybody! Where can I find a documentation or a tutorial for html combinator library Text.Html?

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Rewriting char
by PR Stanley 30 Sep '07

30 Sep '07

Hi Any advice on writing intToChar? I'm not looking for the code; only sound advice on writing the function in good FP style. Thanks, Paul

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