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Re: ANNOUNCE: Draft TOC of Haskell in a Nutshell
by Ketil Malde 09 Jan '01

09 Jan '01

Brook Conner <nellardo(a)concentric.net> writes: > Do you think the cookbook would be better as its own book? Perhaps it could? I think it's a natural component in a "nutshell" book, but it seems clear that there's tons of example programs that would be immensely useful as well-documented howtos. [Cool graphics hacks] > I figured those kinds of examples showed off the power of Haskell. Sounds good to me. -kzm -- If I haven't seen further, it is by standing in the footprints of giants

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Haskell 98 (Library) Report: contradiction about fail :: String -> IO a
by qrczak＠knm.org.pl 09 Jan '01

09 Jan '01

The Haskell 98 Report says that fail in instance Monad IO invokes error. The Library Report says that fail in instance Monad IO raises a user error. I would let it raise a user error, to be able to use fail in exception monads where strings suffice as the error type. -- __("< Marcin Kowalczyk * qrczak(a)knm.org.pl http://qrczak.ids.net.pl/ \__/ ^^ SYGNATURA ZASTÊPCZA QRCZAK

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Just for fun
by Anton Moscal 09 Jan '01

09 Jan '01

Hello! This is well-known definition of the existential quantification through universal: (E x.P(x)) <=> A y.(A x.P (x) => y) => y I try to translate in to Haskell. The following program can be compiled by "ghc -fglasgow-exts ..." and works correctly: ---------------------------------------- import Char type E a = forall t . (forall x . a x -> t) -> t newtype Pair a b = MkPair (b, (b -> a)) newtype App a = MkApp (E (Pair a)) mk_app :: (a -> b) -> a -> App b mk_app f v = MkApp (\g -> g (MkPair (v, f))) app_s = mk_app Char.ord '1' app_id = mk_app (\x->x) 239 eval :: App a -> a eval (MkApp g) = g (\(MkPair (x, f)) -> f x) main = print (map eval [mk_app (\x->x) 239, mk_app Char.ord '1']) --------------------------------------- Regards, Anton Moscal

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Defaults in datatype definitions
by Johannes Waldmann 09 Jan '01

09 Jan '01

Below are some suggestions for extensions to Haskell records. These are just naive ideas. Of course the extensions should integrate nicely w.r.t. the existing syntax, type system, and module system. Perhaps you can point me to opinions and research on what has already been done, or to why it should not, or can not. 1) I would like to write down default values for record components data Foo = Foo { x :: Bar; y :: Baz; x = some_expression } We can do this for methods in type classes, so why not here. 2) With classes, we can do even more: in the default method, we can access other methods. This suggests that the above example should in fact read data Foo = Foo { x :: Bar; y :: Baz; x = \ self -> some_expression } where `self' might occur in some_expression. that is, the default value of `me.x ' gets computed by applying a function to `me'. 3) I want to be able, in modules that use the type, to add more components (with defaults) to the type: This would avoid lengthy recomputations (at the cost of additional space). Imagine the difference between data Tree a = Node { key :: a, children :: [ Tree a ] } size t = 1 + sum (map size (children t)) and data Tree a = Node { key :: a, children :: [ Tree a ] , size :: Integer , size = \ self -> 1 + sum (map size (children self)) } when you have one large tree and frequently need sizes of subtrees. (by the way, what type does `size' have now - do we write `Integer' or `Tree a -> Integer'?) yes, I can do this manually, replacing `Node' by a smart constructor that computes and stores the size. What I don't like about this is that I have to know in advance (i. e. at the moment I define the type) what functions I want to be "cached" this way. Imagine `size' as some more complicated function, only defined many modules later. Of course I see that this would be difficult to implement w.r.t. separate compilation. But I would be happy with a compiler that ignores the "cache" specs during single-module compilations, but honors them when doing a full rebuild of a set of modules. I imagine that inter-module inlining behaves similar. -- -- Johannes Waldmann ---- http://www.informatik.uni-leipzig.de/~joe/ -- -- joe(a)informatik.uni-leipzig.de -- phone/fax (+49) 341 9732 204/252 --

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Re: ANNOUNCE: Draft TOC of Haskell in a Nutshell
by Ketil Malde 09 Jan '01

09 Jan '01

Brook Conner <nellardo(a)concentric.net> writes: > Inspired by the recent discussions of what kinds of books would encourage the > spread of Haskell, I whipped up a draft table of contents for "Haskell in a > Nutshell." Let's see. It all depends on what you put into the chapters, of course, but you seem to jump directly from simple concepts into Haskell for web servers. Wouldn't it be better to push this further out, until at least some of the syntax has been explored? >From the headlines, one might get the impression that type declarations are only important when the compiler can't figure it out itself - I'm sure the actual text will give a different view, no? The cookbook looks great, and it's certainly something I'd buy the book for. I can, of course, think of many kinds of example programs I'd like to see - how about a GUI oriented program, for instance? Perhaps you could finish it off with a "real" application, say a GUIed JPEG viewer, where you sort of string it all (the JPEG codec, a C library interface for something or other, and GUI stuff, etc.) together into a whole? On the whole, I think it looks good. Best of luck! -kzm -- If I haven't seen further, it is by standing in the footprints of giants

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Re: ANNOUNCE: Draft TOC of Haskell in a Nutshell
by Bill Halchin 08 Jan '01

08 Jan '01

Yes! I agree with Manuel about the IO. It looks bad being towards the end. Regards, Bill Halchin >From: "Manuel M. T. Chakravarty" <chak(a)cse.unsw.edu.au> >Reply-To: chak(a)cse.unsw.edu.au >To: nellardo(a)concentric.net >CC: haskell(a)haskell.org >Subject: Re: ANNOUNCE: Draft TOC of Haskell in a Nutshell >Date: Tue, 09 Jan 2001 13:43:59 +1100 > >Brook Conner <nellardo(a)concentric.net> wrote, > > > You can find it in PDF form at > > http://nellardo.com/lang/haskell/hianTOC.pdf (it should have the fonts > > embedded - OReilly uses a couple of oddballs) and in ASCII text at > > http://nellardo.com/lang/haskell/haskell-in-a-nutshellTOC.txt. > >Makes sense to me. However, I would move IO further to the >front. For any "real life" programming, IO is essential >and, I think, it is bad for the image of Haskell to treat it >as something dirty that is relegated to end of every (at >least, many) introduction of Haskell. > >I have just given an Introduction to Computing course using >Haskell and written a set of lecture notes, which introduce >a couple of concepts (including IO) differently than >commonly done. The whole stuff is at > > http://www.cse.unsw.edu.au/~cs1011/lectures/index.html > >Feel free to get inspiration from the material (except Weeks >8 - 10, which weren't given by me). You will find IO in >Week 5. Note that I haven't used the word monad a single >time. IMHO, monads are an advanced concept, IO is not. >It's like you can introduce recursion without discussing >stacks. If you understand stacks, you can gain a deeper >understanding of recursion, but if you hear about recursion >for the first time, the mention of stacks will only distract >the average student. > > > The middle part is a bit different - a short "cookbook" of > > semi-practical programs addressing fairly common programming tasks - > > some of the kinds of things you'd typically do with Perl or C++. I think > > it needs more "recipes" of course, but I'd like to know if the general > > idea is working. > >I think, that this is a good idea. > >Cheers, >Manuel > >_______________________________________________ >Haskell mailing list >Haskell(a)haskell.org >http://www.haskell.org/mailman/listinfo/haskell _________________________________________________________________ Get your FREE download of MSN Explorer at http://explorer.msn.com

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Re: Extending the do-notation
by Sebastien Carlier 08 Jan '01

08 Jan '01

> import Monad > ... > do y <- liftM unzip m1 Thanks. I'm constantly amazed by the number of tricks one has to know before he can write concise code using the do-notation (among other things, I used to write "x <- return $ m" instead of "let x = m"). Is there a paper demonstrating the most common coding styles for do-notation ? Another question concerning the do-notation: I noticed that most parts of ghc do not use it. Is it because the code was written before the notation was available, because the do-notation is too weak to express these parts, or for another fundamental reason ?

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2nd CfP: ESSLLI 01 Student Session
by Kristina Striegnitz 08 Jan '01

08 Jan '01

!!! Concerns all students in Logic, Linguistics and Computer Science !!! !!! Please circulate and post among students !!! We apologize, if you receive this message more than once. ESSLLI 2001 STUDENT SESSION SECOND CALL FOR PAPERS August 13-24 2001, Helsinki, Finland Deadline: February 18, 2001 http://www.coli.uni-sb.de/~kris/esslli We are pleased to announce the Student Session of the 13th European Summer School in Logic, Language and Information (ESSLLI 2001) organized by the University of Helsinki under the auspices of the European Association for Logic, Language and Information (FoLLI). ESSLLI 2001 will be held at the University of Helsinki in August 2001. We invite submission of papers for presentation at the ESSLLI 2001 Student Session and for appearance in the proceedings. PURPOSE: This sixth ESSLLI Student Session will provide, like the other editions, an opportunity for ESSLLI participants who are students to present their own work in progress and get feedback from senior researchers and fellow-students. The ESSLLI Student Session encourages submissions from students at any level, from undergraduates (before completion of the Master Thesis) as well as postgraduates (before completion of the PhD degree). Papers co-authored by non-students will not be accepted. Papers may be accepted for full presentation (30 minutes including 10 minutes of discussion) or for a poster presentation. The accepted papers will be published in the ESSLLI 2001 Student Session proceedings, which will be made available during the summer school. KLUWER BEST PAPER AWARD: As in previous years, the best paper will be selected by the program committee and will be offered a prize by Kluwer Academic Publishers consisting in 1000 Dfl worth of books. REQUIREMENTS: The Student Session papers should describe original, unpublished work, completed or in progress that demonstrates insight, creativity, and promise. No previously published papers should be submitted. All topics within the six ESSLLI subject areas (Logic, Language, Computation, Logic & Language, Logic & Computation, Language & Computation) are of interest. FORMAT OF SUBMISSION: Student authors should submit an anonymous extended abstract headed by the paper title, not to exceed 5 pages in length exclusive of references and send a separate identification page (see below). Note that the length of the full papers will not be allowed to exceed 10 pages. Since reviewing will be blind, the body of the abstract should omit author names and addresses. Furthermore, self-references that reveal the author's identity (e.g., "We previously showed (Smith, 1991)... ") should be avoided. It is possible to use instead references like "Smith (1991) previously showed...". For any submission, a plain ASCII text version of the identification page should be sent separately, using the following format: Title: title of the submission First author: firstname lastname Address: address of the first author ...... Last author: firstname lastname Address: address of the last author Short summary: abstract (5 lines) Subject area (one of): Logic | Language | Computation | Logic and Language | Logic and Computation | Language and Computation If necessary, the program committee may reassign papers to a more appropriate subject area. The submission of the extended abstract should be in one of the following formats: PostScript, PDF, RTF, or plain text. But note that, in case of acceptance, the final version of the paper has to be submitted in LaTeX format. Please, use A4 size pages, 11pt or 12pt fonts, and standard margins. Submissions outside the specified length and formatting requirements may be subject to rejection without review. The extended abstract and separate identification page must be sent by e-mail to: kris(a)coli.uni-sb.de by FEBRUARY 18, 2001 ESSLLI 2001 INFORMATION: In order to present a paper at ESSLLI 2001 Student Session, at least one student author of each accepted paper has to register as a participant at ESSLLI 2001. The authors of accepted papers will be eligible for reduced registration fees. For all information concerning ESSLLI 2001, please consult the ESSLLI 2001 web site at http://www.helsinki.fi/esslli. IMPORTANT DATES: Deadline for submission of abstracts: February 18, 2001. Authors Notifications: April 17, 2001. Final version due: May 18, 2001. ESSLLI-2001 Student Session: August 13-24, 2001. PROGRAM COMMITTEE: Raffaella Bernardi, University of Utrecht (Logic & Language) Patrick Blackburn, Loria Nancy (Logic & Language) Gilles Dowek, INRIA (Computation) Ruth Kempson, King`s College London (Language) Carsten Lutz, University of Aachen (Logic & Computation) Ani Nenkova, Columbia University (Logic) Ilkka Niemel�, Helsinki University of Technology (Logic & Computation) Malvina Nissim, University of Pavia (Language) Susanne Salmon-Alt, Loria, Nancy (Language & Computation) Jan Schwinghammer, University of the Saarland (Computation) Kristina Striegnitz, University of the Saarland (Chair) Yde Venema, University of Amsterdam (Logic) Shuly Wintner, University of Haifa (Language & Computation) For any question concerning the ESSLLI 2001 Student Session, please, do not hesitate to contact me: Kristina Striegnitz Computational Linguistics, University of the Saarland, Germany phone: +49 - 681 - 302 4503 email: kris(a)coli.uni-sb.de

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A dictionary implementation in Hasekll
by Shlomi Fish 07 Jan '01

07 Jan '01

Does anybody knows of an implementation for an efficient dictionary (such as a hash, an AVL tree, a B-Tree etc. ) in Haskell? Can an array do that? I'd like something that can map any type to any other type based on a comparison function, and perhaps a hashing function. TIA, Shlomi Fish ---------------------------------------------------------------------- Shlomi Fish shlomif(a)vipe.technion.ac.il Home Page: http://t2.technion.ac.il/~shlomif/ Home E-mail: shlomif(a)techie.com The prefix "God Said" has the extraordinary logical property of converting any statement that follows it into a true one.

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Extending the do-notation
by Sebastien Carlier 07 Jan '01

07 Jan '01

Sometimes I need to write code which looks like this: > do x <- m1 > let y = unzip x > ... -- never using x anymore I thinks the following extension to do-notation would be useful: > pat <- exp1 # exp2 ; exp3 would be rewritten as > exp2 >>= ((\pat -> exp3) . exp1) so that the above example could be rewritten more compactly: > do y <- unzip # m1 I think the biggest problem with this extension is the choice of a proper symbol. Does this extension already exist ? (I only checked the Haskell 98 report).

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