July 2003 - Haskell - Haskell.org

How overload operator in Haskell?
by Liu Junfeng 04 Jul '03

04 Jul '03

I've learned haskell months, but I still can't understand the type system very well. I can only write: ----------------------------------- type Vector = [Double] vadd,vsub :: Vector->Vector->Vector v1 `vadd` v2 = zipWith (+) v1 v2 v1 `vsub` v2 = zipWith (-) v1 v2 svmul :: Double->Vector->Vector s `svmul` v = map (s*) v ------------------------------------ Tough it works, it is not convenient to use. How to create a Vector type and overload mathematical operators? Thanks for your help. 　　　　　　　　Liu Junfeng 　　　　　　　　liujf(a)softhome.net 　　　　　　　　　　2003-07-04

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Announce: hircules 0.2 released
by Jens Petersen 04 Jul '03

04 Jul '03

I am happy to annouce the release of Hircules 0.2. Hircules is an IRC client written in Haskell, using gtk2hs for its user interface. Since the last release about 2 months ago, many bugs have been fixed and some new features added. A brief list of the main changes is appended below. See the Hircules webpage <http://haskell.org/hircules/> for more details. A source tarball and an i386 rpm built for Linux with glibc-2.3 and gtk2-2.2 are available for download from the project site page <http://sourceforge.net/projects/hircules/>. Going forward I want to make an effort to keep the cvs repository more up-to-date with more regular commits. The TODO list contains the list of features I would like to implement in coming versions. Enjoy, and as usual please send your feedback and comments, Jens * Summary of changes in Hircules 0.2 - Whitespace in messages received, sent and displayed is now preserved. - Hircules can now be run from a non-tty. - The "/me" command has been added for writing ctcp actions. - CTCP actions are now displayed properly. - When users join a channel their "user@host" info is also displayed now. - MODE changes should now be displayed properly. - All channels are now logged to a log file in ".hircules/log/". - Channel tabs are now numbered. - Channel members are now tracked per channel. Quit and nick-change messages only appear now in the relevant channel. - Private messages and public messages addressed to one (with "nick: ...") cause an audible beep. - Public messages addressed to one appear also in a new virtual Alert channel. - /join can now also be used to display a hidden channel or start a private conversation.

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RE: foldr in terms of map
by Hal Daume 03 Jul '03

03 Jul '03

Hi, quick reply :)...i've reordered some of what you've said (i hope you don't mind!) > However the monad is defined, sequence_ has to process the entire list > before anything can be determined about the result. The entire result > of (>>) depends upon both arguments, whereas you can deduce the head > of the result of (:) based solely upon the first argument. yes, thanks. > Yeah, but given that sequence_ is essentially the direct monadic > translation of fold: > that might be considered cheating (i.e. we can implement fold using > only map and fold, although the fold can be "disguised"). > <snip> > As mentioned above, sequence_ etc are essentially the embodiment of > primitive recursion. Is this true in the same way that the statement 'foldr' is the embodiment of PR is true? That is, can you write foldr in terms of sequence_? It doesn't seem possible: you seem to also need the map in order to get the list lifted into the monadic world and I don't think you can even write map in terms of sequence_ (am I wrong?). > An implementation using a simple state-transformer monad (s -> (a, s)) > wouldn't look significantly different to one using IORef/STRef. True :). Thanks for your comments! - Hal

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haskell-mode and xemacs
by Christian Maeder 03 Jul '03

03 Jul '03

Hi, whenenver I open a haskell file (*.hs) with my xemacs I get an annoying warning in a splitted window: (1) (error/warning) Error in `post-command-hook' (setting hook to nil): (void-variable imenu--index-alist) The modes (Haskell Font Ind Doc) seem to work, though. My xemacs has [version 21.4; April 2001]. I use this xemacs as it was installed without any customizations under Suse Linux 2.4.19-4GB on i686. I've found write-protected /usr/share/xemacs/xemacs-packages/lisp/haskell-*.elc files. The haskell-mode.elc does not seem to be the latest version: (defconst haskell-version "1.43" (#$ . 632)) I don't know how to include the *.el files of the latest haskell mode in xemacs. (I've no problems with GNU emacs.) The problem occurs on several laptops. Does someone has an advice? Thanks Christian

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Template Haskell - read function
by Thomas L. Bevan 02 Jul '03

02 Jul '03

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Hi, I'm struggling to write the following function in Template Haskell. I have a phantom type, >data HTMLPage a = HTMLPage FilePath This type points to an HTML file, where 'a' refers to the return type of the posted form. So, mostly, I would expect types like HTMLPage (String, String, Date, Int) Where this represented a form the returned someone's first name, last name, dob and age. So, I want a function that will take the returned name/value pairs and cast them into the matching type. eg. >readForm :: Read a => [(String,String)] -> HTMLPage a -> a Can anyone give me any pointers or am I better off just abandoning this approach? Tom -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.2 (GNU/Linux) iD8DBQE/AsiKYha8TWXIQwoRAvzuAKDPHD4oztAPLFShEWsLuclT7BaImwCfSB8e 9c2aTbTCSpVwabXVsht62DM= =orX3 -----END PGP SIGNATURE-----

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Suffix Tree
by Markus.Schnell＠infineon.com 02 Jul '03

02 Jul '03

Does anybody know of a suffix tree implementation for Haskell? Are there algorithms for a (lazy) functional setting? My reference is Dan Gusfield: "Algorithms on Strings, Trees, and Sequences: Computer Science and Computational Biology". Thanks, Markus -- Markus Schnell Infineon Technologies AG, CPR ET Tel +49 (89) 234-20875

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Re: Ledit (was Re: [Caml-list] how to use a module) (fwd)
by Hal Daume III 01 Jul '03

01 Jul '03

There was some discussion about something like this a while ago...would this solve our problems? -- Hal Daume III | hdaume(a)isi.edu "Arrest this man, he talks in maths." | www.isi.edu/~hdaume ---------- Forwarded message ---------- Date: Tue, 1 Jul 2003 14:49:22 -0400 From: Neel Krishnaswami <neelk(a)alum.mit.edu> To: Matt Gushee <mgushee(a)havenrock.com> Cc: caml-list(a)pauillac.inria.fr Subject: Re: Ledit (was Re: [Caml-list] how to use a module) Matt Gushee writes: > > Okay, then, I have some questions about ledit: > > How do you use it? I have actually tried ledit 2 or 3 times, but if > I invoke the 'ledit' executable that is created, e.g. > > $ ./ledit > > I get ... something ... some sort of shell-like environment, I > guess. But all it does is echo whatever I type. And when I read your > post, I downloaded it again to be sure, but the same thing happened > again. I also tried making a custom toplevel with ledit, but the > result seems to be identical to the plain ledit executable. Do this $ ledit ocaml in order to get line editing in the ocaml toplevel. The really cool thing about ledit is that it adds line editing to *any* program. -- Neel Krishnaswami neelk(a)alum.mit.edu ------------------- To unsubscribe, mail caml-list-request(a)inria.fr Archives: http://caml.inria.fr Bug reports: http://caml.inria.fr/bin/caml-bugs FAQ: http://caml.inria.fr/FAQ/ Beginner's list: http://groups.yahoo.com/group/ocaml_beginners

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foldr in terms of map
by Hal Daume 01 Jul '03

01 Jul '03

Kind of a random question for today :). We all know that we can write map in terms of foldr: > map f = foldr ((:) . f) [] as I understand it, this is essentially because foldr encapsulates all primitive recursive functions and since map is primitive recursive, we can implement it in terms of a fold. one thing that is interesting to note is that if we are also given references, a function to sequence monadic actions ('sequence_') and reverse, we can write foldr in terms of map (I do this in the IO monad, but as long as you have refs, it doesn't really matter -- if you used a "better" monad, you could replace the unsafePerformIO with something which doesn't sound so bad, but in this case, the operation is safe): > foldr f z l = unsafePerformIO $ do > v <- newIORef z > sequence_ $ map (modifyIORef v . f) (reverse l) > readIORef v Of course, we can postulate instead of sequence_, rsequence_, defined as: > rsequence_ :: Monad m => [m a] -> m () > rsequence_ [] = return () > rsequence_ (x:xs) = do rsequence_ xs; x; return () it's probably also possible to do it by storing in the reference a function which we compose, but that's not interesting here. now, when we're in the IO monad, the difference between the real foldr and the simulated one is that the simulated one cannot deal with infinite inputs. for instance: > head $ foldr (:) [] [1..] should return 1, which it does for the real version. the simulated one hangs. one might like to attribute this to the fact that IO is a strict monad, but that doesn't seem to be all -- we would also need to be able to read and write references lazily in order to get this to work completely. Q1: is this correct? is there a way to "fix" the above definition or to use a different monad to get laziness? i think not, but can't prove it. now, if we just limit ourselves to finite inputs, things look a lot better. however, this brings up really the main question i have. foldr (in its original form) is in some sense quintessentially(sp?) PR on lists, whereas map is not. however, it seems that the combination map+rsequence_+references is enough to give you full PR power. what more can we say about this? clearly references play the largest role here, but it also seems impossible to remove this dependence on the sequencing operation. clearly there is some class of algorithms weaker that primitive recursive into which both map and rsequence_ fall. these two functions are in some sence counterparts to eachother: map retains list structure and only modifies elements, which rsequence_ destroys list structure and doesn't modify (just "runs") elements. is there some sor tof theory that talks about this relationship? and why is it that when put together, these things are strong enough (plus refs) to subsume primitive recursiveness? - Hal p.s., responses delayed until after ICFP are equally welcome :) -- Hal Daume III | hdaume(a)isi.edu "Arrest this man, he talks in maths." | www.isi.edu/~hdaume

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