June 2003 - Haskell - Haskell.org

RE: Language extension proposal
by Hal Daume 26 Jun '03

26 Jun '03

I'm not sure this is really necessary (at least syntax-wise). We can do something like: > data T a > class Trait a where { trait :: T a -> Int } > > instance Trait Int where { trait _ = 0 } > instance Trait Char where { trait _ = 1 } As far as I can tell with the various --ddump-* flags, the compiler hasn't yet figured out that the argument to trait is useless (i.e., it keeps it in there). However, from a sanity point of view, I much prefer to write my "useless arguments" like this; that way at least *I* know that they'll never be evaluated. I suppose the only thing that could go wrong is someone writing: > instance Foo Bool where { trait x = x `seq` 1 } in which case it would be wrong for the compiler to get rid of this argument. I suppose that your solution gets rid of this problem by relizing that seq doesn't have absent demand on its argument (quite the opposite, in fact). Of course, the powers that be can weight in on this, and I'm sure that you're aware of the phantom type solution, but I figured I'd post anyway so that others can get a look at types like this for their own benefit... -- Hal Daume III | hdaume(a)isi.edu "Arrest this man, he talks in maths." | www.isi.edu/~hdaume > -----Original Message----- > From: haskell-admin(a)haskell.org > [mailto:haskell-admin@haskell.org] On Behalf Of Andrew J Bromage > Sent: Tuesday, June 24, 2003 9:18 PM > To: haskell(a)haskell.org > Subject: Language extension proposal > > > G'day all. > > I've been toying with an idea for a small language extension, and I > was curious what you all think of this. > > As a motivating example, consider the following standard typeclass: > > class Bounded a where > min :: a > max :: a > > This is an example of what I refer to as a "traits typeclass" (by > analogy to traits classes, well-known to C++ programmers). The idea > is that you want to specify certain properties/operations which depend > on a _type_, rather than any particular value of that type. > > Another example is floating point format information, like the > information in C's <float.h>. One might implement this as: > > class (Bounded a) => FloatTraits a where > epsilon :: a -- OK > mantissaDigits :: Int -- Not OK! > > For obvious reasons, the declaration of mantissaDigits is not valid > Haskell. Instead, you might do something like this: > > class FloatTraits a where > mantissaDigits :: a -> Int > > instance FloatTraits Float where > mantissaDigits _ = 24 > > instance FloatTraits Double where > mantissaDigits _ = 53 > > and invoke it as, say: > > mantissaDigits (undefined :: Float) > > A decent strictness analyser can tell if an argument to some function > is always unused (in GHC core terminology, this means its demand is > "absent") and thus avoid passing the argument at all. However, it is > currently impossible to tell if this is true all instances of a > typeclass. What I would like is some annotation that I can > use to tell > my compiler that this argument of this method is always unused, no > matter what instance it resolves to. > > My proposal is to grab a new punctuation symbol (I will use '~' as > an example) to sit alongside the GHC extension use of '!'. Where '!' > means that an argument may be evaluated before calling a function, '~' > means than an argument may be ignored by the callee. So in the above > example, I can write: > > class FloatTraits a where > mantissaDigits :: ~a -> Int > > Now this marker is not as simple to implement as '!'. The strict > marker is advisory; if you stripped all '!'s from your program, it > would still have the same static semantics. Not so with '~', which > must be statically checked. > > However, the static analysis is pretty trivial, except for the > problem of inference. Basically, if a variable has absent demand, > it a) can't be pattern matched on (unless the pattern is a variable, > of course), and b) can't be passed as an argument to a function > (including a data constructor) which doesn't also treat that argument > as having absent demand. > > I therefore propose that the marker not be inferred for top-level > definitions, or at least for exported definitions. If you declare it, > it's checked, but otherwise it's not. In addition, any calls that you > pass a variable with absent demand to must themselves have the marker > declared explicitly in their type signatures. > > There may also be an argument for extending '~' to data constructors, > in the same way that H98 uses '!'. This would allow parameters with > absent demand to be stored in data structures. I can't think of a > use for this right now, but it shouldn't be too much harder to add. > > What do you all think? Useful? Not worth the effort? > > Cheers, > Andrew Bromage > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell >

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CHARME 2003
by Tronci Enrico 25 Jun '03

25 Jun '03

---------------------------------------------------------------------- We apologize if you receive multiple copies of this announcement. ---------------------------------------------------------------------- ********************************************************************** C A L L F O R P A R T E C I P A T I O N ********************************************************************** CHARME 2003 The 12th Advanced Research Working Conference on: Correct Hardware Design and Verification Methods 21-24 October 2003, University of L'Aquila, L'Aquila, Italy http://www.di.univaq.it/charme2003 Co-sponsored by the IFIP TC10/WG10.5 Working Group on Design and Engineering of Electronic Systems Sponsored by: IBM, INTEL, University of L'Aquila The technical program is available at <http://www.di.univaq.it/charme2003>. Information about registration, travel and accomodation will be soon available at <http://www.di.univaq.it/charme2003>.

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type inference visualization
by Hugo Simoes 25 Jun '03

25 Jun '03

Hi I did a Web application for the visualization of the type inference process for the Simple Type System and pure ML. You can "play" with it at: http://www.ncc.up.pt/typetool I think it can be useful to Haskell programmers. Comments are welcome, Best regards Hugo Simões

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yet another very simple overlapping instance example
by Razvan Musaloiu-E. 25 Jun '03

25 Jun '03

Hi! Can somebody explain to me why ghc/hugs fails to compile the following Haskell program? As long as MyInt is not an instance of Num the compilations should succed... but it don't. :-( Why? $ cat multi.hs data MyInt = MyInt Int deriving Show class Op_plus a b where plus :: a -> b -> Int instance Op_plus MyInt MyInt where (MyInt a) `plus` (MyInt b) = a + b instance (Num a) => Op_plus a MyInt where i `plus` (MyInt b) = i + b instance (Num a) => Op_plus MyInt a where (MyInt b) `plus` i = i + b $ ghci \ -fglasgow-exts \ -fallow-overlapping-instances \ -fallow-undecidable-instances multi.hs ___ ___ _ / _ \ /\ /\/ __(_) / /_\// /_/ / / | | GHC Interactive, version 5.04.2, for Haskell 98. / /_\\/ __ / /___| | http://www.haskell.org/ghc/ \____/\/ /_/\____/|_| Type :? for help. Loading package base ... linking ... done. Loading package haskell98 ... linking ... done. Compiling Main ( multi.hs, interpreted ) multi.hs:1: Warning: No 'main' defined in module Main Overlapping instance declarations: multi.hs:9: Op_plus a MyInt multi.hs:12: Op_plus MyInt a Failed, modules loaded: none. Prelude> Thanks and regards, Razvan ME

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Announce: buddha 0.8 released
by Bernard James POPE 25 Jun '03

25 Jun '03

Announcing the release of buddha version 0.8 -------------------------------------------- www.cs.mu.oz.au/~bjpop/buddha Buddha is a declarative debugger for Haskell 98. It is based on program transformation and relies on GHC version 5.04 or greater (but not version 6 yet). It also needs GHC's libraries compiled for profiling. New in this release: - much improved support for debugging with exceptions If your program calls error, exhausts the stack, calls a class method which is missing, has a pattern match failure etc etc, and you don't catch the exception, buddha will notice this and start debugging. You can even interrupt a long running program with control-C (SIG INT) and buddha will start debugging with whatever computation was done up to that point. Features still missing: - record syntax, - some standard libraries (notably: Ratio, Numeric, Directory, Time), - mutually recursive modules, - deriving Ix. Platforms supported: - tested on linux - (should work in any unix-like environment), - not tested on Windows (sorry). Note that buddha does not yet work with GHC version 6.0. This will be fixed in coming releases. Cheers, Bernie.

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ICFP Programming Contest -- register your email!
by John Hughes 24 Jun '03

24 Jun '03

Planning to join the ICFP Programming Contest this week-end? Register your email address NOW, to receive announcements from the organizing team, including the problem as soon as it is released! Just visit icfpcontest.org and click on "Contest Mailing List". John Hughes

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TPLP special issue call for papers
by Moreno Falaschi 24 Jun '03

24 Jun '03

CALL FOR PAPERS Special Issue of Theory and Practice of Logic Programming on Multiparadigm Languages and Constraint Programming Guest Editors: Moreno Falaschi and Michael Maher Following up on a series of 11 workshops (WFLP) on multiparadigm languages and constraint programming, the journal on Theory and Practice of Logic Programming (TPLP) is planning a special issue on multiparadigm languages and constraint programming to appear in 2005. TOPICS In recent years much research and implementation efforts have been devoted both to multiparadigm languages and constraint programming languages. The aim of this special issue is to attract high-quality research papers on all aspects related to multi-paradigm languages and constraint programming, such as semantics, analysis, extensions, implementations and applications. Papers addressing how to use constraints for integrating different paradigms, or extensions of multiparadigm languages with constraints would be particularly welcome, although papers solely on multiparadigm programming or constraint programming are also of interest. Surveys and comparisons of constraint techniques and multiparadigm languages are also solicited. This special issue should hence stimulate the cross-fertilizing exchange of ideas and experiences among researchers from the different communities interested in the foundations, applications, and combinations of high-level, multiparadigm and constraint programming languages and related areas. Submissions Revised and enhanced versions of papers published at conferences that have not appeared in archival journals are eligible for submission. Papers describing important past research in this area which have not been published in archival journal papers are also solicited. For any doubt regarding the appropriateness of the topic of a paper for this special issue please email either to Moreno Falaschi (falaschi(a)dimi.uniud.it) or to Michael Maher (mjm(a)math.luc.edu) Papers should be written in English, and formatted using the LaTeX style files developed by Cambridge University Press for TPLP. To submit and for further information, please look at: http://www.dimi.uniud.it/~wflp2002/tplp/ Important Dates Submission December 15, 2003 Notification June 1, 2004 Revised paper due: November 1, 2004 Publication 2005

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RE: Typesafe MRef with a regular monad
by Simon Peyton-Jones 24 Jun '03

24 Jun '03

You can't overwrite an entry with a value of a different type, because the keys are typed! Any more than you can overwrite an IORef with a value of a different type. S | -----Original Message----- | From: Ralf Hinze [mailto:ralf@informatik.uni-bonn.de] | Sent: 06 June 2003 15:01 | To: Simon Peyton-Jones; Tim Sweeney; haskell(a)haskell.org; Ashley Yakeley | Subject: Re: Typesafe MRef with a regular monad | | Am Freitag, 6. Juni 2003 15:47 schrieb Simon Peyton-Jones: | > Yes, one *could* use dynamic types. But the check would always succeed! | | Why is that? If I overwrite an entry with a value of a different type, | then the check fails. I am certainly missing something ... | | Cheers, Ralf |

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ANNOUNCE: Cryptographic Library for Haskell
by Dominic Steinitz 23 Jun '03

23 Jun '03

I would like to annouce the first release of the Haskell Cryptographic Library (0.2.0). See http://www.haskell.org/crypto/ReadMe.html for more details. This library collects together existing Haskell cryptographic functions and augments them so that they: · Have common type signatures. · Can be used with the standard mode and padding algorithms (in the case of block mode ciphers). This very initial release contains: · DES · Blowfish · Cipher Block Chaining (CBC) mode · PKCS#5 and nulls padding Dominic Steinitz

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RE: main modules in GHC
by Simon Peyton-Jones 23 Jun '03

23 Jun '03

| >I've implemented a -main-is flag for GHC, so you can nominate either the | >module containing the main function, or the main function name, or both. | | I take it that's not yet a released compiler version? (I'm currently using | 5.4.03, and expect to upgrade at some time after the generics support is | felt to be reasonably stable and also supported by Hugs.) Correct. It'll be in 6.2. Till then, you can use a development snapshot (once a nightly build on the HEAD goes through) S

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