- Haskell - Haskell.org

proposal about declaring language version and language extensions used in specific module
by Bulat Ziganshin 10 Aug '05

10 Aug '05

Hello all. i have a proposal about declaring language version and language extensions used in specific module PROBLEM: 1) Haskell 2.0 may be slightly incompatible with Haskell 98. at least, new records extension will, afaik, lead to that H'98 (old) programs can't be compiled with Haskell 2.0 compilers. this will create a problem if we want to use the same extensions (hs, lhs, chs) both for Haskell 98 and Haskell 2.0 programs 2) currently language extensions needed to compile specific module are coded via {-# OPTIONS ... #-} pragma. that is both compiler-specific and encodes information, essential for compiling module, as some optional pragma/comment SOLUTION: use specific language keyword to declare language version and used extensions. i will give several examples of this statement: haskell 98 with template haskell, implicit parameters haskell 2.0 haskell 2.0 with generics haskell 2.1 so, general syntax will be: "haskell" version ["with" extension [,extension]*] where version = "98" | "2.0" | ... extension = "template haskell" | "implicit parameters" | ... such statement will declare base language version and additional extensions to this language version, used in the module. it must be first statement in the module. if this statement is not given, then "haskell 98" is supposed using this statement will give exact meaning to each module, despite all the future Haskell versions and extensions we will encounter and will simplify adding new language extensions that is not backward-compatible ps: as variants of this idea we can standartize OPTIONS and several other pragmas in the language itself or, better, add new language keyword to replace these pseudocomments. how about this: option "haskell 2.0" option "inline main, sub1, sub2" option "ghc-rule ..." -- GHC-specific options starts with "ghc-" -- Best regards, Bulat mailto:bulatz@HotPOP.com

3 3

Let's stay in touch
by M O 09 Aug '05

09 Aug '05

Hi I'm updating my address book. Please click on the link below and enter your contact info for me: http://www.ringo.com/i.html?i=116036757x738164&homeEmail=haskell%40haskell.… I'm using a new, free service where I put in my contact info for you, you put in your contact info for me, and everyone stays up to date automatically. It's surprisingly easy and useful. Thanks for your help. M O Don't receive any more invitations from M O: http://www.ringo.com/friends/invite/block.html?memberId=116036757&email=has… Don't receive any more invitations from anyone: http://www.ringo.com/friends/invite/block.html?email=haskell%40haskell.org

1 0

Haskell Weekly News: August 9, 2005
by John Goerzen 09 Aug '05

09 Aug '05

Haskell Weekly News: August 9, 2005 Greetings, and thanks for reading the second issue of HWN, a weekly newsletter for the Haskell community. Each Tuesday, new editions will be posted (as text) to [1]the Haskell mailing list and (as HTML) to [2]The Haskell Sequence. 1. http://www.haskell.org/mailman/listinfo/haskell 2. http://sequence.complete.org/ Discussion Practical Monads. Paul Moore started a [3]discussion about Monads and resources for learning about them. Quite a few readers responded with suggestions. 3. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/7964 STRef vs. IORef. Srinivas Nedunuri started a [4]discussion by asking when to STRef and when to use IORef. Iavor Diatchki posted a helpful [5]example, and many other helpful answers were posted as well. 4. http://thread.gmane.org/gmane.comp.lang.haskell.general/11895 5. http://article.gmane.org/gmane.comp.lang.haskell.general/11901 Parsing Foreign Languages. The The ParsingForeignLanguagesInHaskell wiki page was the subject of a short [6]discussion on the libraries mailing list. If you have any further information or would like to join or start a project to parse a particular language, see the [7]wiki page. 6. http://thread.gmane.org/gmane.comp.lang.haskell.libraries/3572 7. http://www.haskell.org/hawiki/ParsingForeignLanguagesInHaskell Haskell Toolchain Cabal was again a hot topic this week. There were discussions about [8]data directories, [9]running on Windows 98, and [10]package description fields in general. 8. http://thread.gmane.org/gmane.comp.lang.haskell.libraries/3594 9. http://thread.gmane.org/gmane.comp.lang.haskell.libraries/3575 10. http://thread.gmane.org/gmane.comp.lang.haskell.libraries/3566 Darcs Corner Darcs in FreshMeat. David Roundy is [11]looking for volunteers to maintain the Darcs [12]entry at FreshMeat.net. It wouldn't require much time, but the ability to summarize changes at release time. 11. http://article.gmane.org/gmane.comp.version-control.darcs.user/8034 12. http://freshmeat.net/projects/darcs/ Binary files and line endings. Phil Brooke [13]asked how darcs handles line endings and binary files. 13. http://thread.gmane.org/gmane.comp.version-control.darcs.user/8017 Uniqueness of patch names. On #darcs this week, a discussion about the uniqueness of low-level patch names in darcs. The consensus seemed to be that darcs needs an additional better-than-1-second component to patch names to eliminate a situation in which collisions can arise. New Releases * Simon Marlow announced the [14]release of Haddock version 0.7. Highlights of this version include improvements for linking across different packages, bug fixes, collapsable trees in HTML, and support for new output formats. * Einar Karttunen has released [15]hsgnutls 0.1, a Haskell binding for the GnuTLS SSL/TLS library. * John Goerzen [16]announced the release of a preliminary, but working, binding to OpenLDAP from Haskell. 14. http://article.gmane.org/gmane.comp.lang.haskell.general/11903 15. http://www.cs.helsinki.fi/u/ekarttun/hsgnutls/ 16. http://article.gmane.org/gmane.comp.lang.haskell.general/11932 Quotes of the Week <CosmicRay> "Oh Lord, bless this thy holy IO monad, and use it for thy purposes that it may smash Java to tiny bits..." (with apologies to monty python) <Pseudonym> If I ever write a GUI library for Haskell, I'm going to call it pointlesstif. About Haskell Weekly News Want to continue reading HWN? Please help us create new editions of this newsletter. Please see the [17]contributing information, or send stories to hwn -at- complete -dot- org. There is also a Darcs repository available. 17. http://sequence.complete.org/hwn-contrib

1 0

do notation and type synonym
by mt 08 Aug '05

08 Aug '05

hi, first one : i've read in different places that "there is nothing magic with do" and that do { x <- m ; return x } is the *same* as m >>= \x -> return x however i've came up with : m :: Maybe Int m = return 4 check5 = do { 5 <- m ; return 0 } check5' = m >>= \5 -> return 0 the first check5 is ok and return Nothing while the second check5' will raise an "Exception: Non-exhaustive patterns" (which is what i've expected). the sentence "there is nothing magic with do" with its reason are important for me to understand monads and do notation, so maybe someone can rephrase "there is nothing magic with do" and explain the exposed behavior? second one: is it possible to write something like type Point = (Float, Float) type Vector = (Float, Float) and still have type safety (i.e. not be able to mix Point and Vector) (and still have (*,*) without constructor)? thanks, minh thu

5 5

Re: [Haskell] Partially applied type class functions
by Paul Govereau 06 Aug '05

06 Aug '05

Thanks alot for the clarifications. If I understand correctly, I ran into the monomorphism restriction because of the constraint on the type variable. The Haskell report says this in Section 4.5.5: In addition, the constrained type variables of a restricted declaration group may not be generalized in the generalization step for that group. Thus, this is perfectly fine: > f1 :: Int -> a -> String > f1 i a = "" > x = let g = f1 1 in (g 1, g "a") Whereas this definition, with the additional (Show a) constraint, runs in to the monomorphism restriction: > f2 :: Show a => Int -> a -> String > f2 i a = show a > y = let g = f2 1 in (g 1, g "a") No instance for (Num [Char]) arising from the literal `1' at Syntax.lhs:16:24 Probable fix: add an instance declaration for (Num [Char]) In the first argument of `g', namely `1' In the definition of `y': y = let g = f2 1 in (g 1, g "a") I think my original program was a case similar to this last one. Thanks again for the quick response. Paul On Aug 05, Roberto Zunino wrote: > Paul Govereau wrote: > [snip] > >>instance AbSyn Constraint where > >> subst e n constr = > >> let sub = subst e n -- :: AbSyn a => a -> a > >> in case constr > >> of Zero expr -> Zero (sub expr) > >> AndL cs -> AndL (sub cs) > > > >It looks sort of like sub is being monomorphised -- or something? > > I think you just hit the monomorphism restriction. For the details, see: > http://www.haskell.org/onlinereport/decls.html - Sect. 4.5.5 > http://haskell.org/hawiki/MonomorphismRestriction > > Possible solutions for your specific case are: > > 1) add an explicit type signature for sub > > let sub :: AbSyn a => a -> a > sub = subst e n > in ... > > 2) eta-expand sub, so its definition becomes a function binding > > let sub c = subst e n c > in ... > > > Regards, > Roberto Zunino. >

1 0

Partially applied type class functions
by Paul Govereau 06 Aug '05

06 Aug '05

Hello, I have encountered a type error that I find quite puzzling. I would appreciate it if someone could help me understand what is going wrong. Here is the program: > data Expr = Var String | Const Int > data Constraint = Zero Expr | AndL Constraint > class AbSyn a where > subst :: Expr -> String -> a -> a > instance AbSyn Expr where subst e n expr = expr > instance AbSyn Constraint where > subst e n constr = > let sub = subst e n -- :: AbSyn a => a -> a > in case constr > of Zero expr -> Zero (sub expr) > AndL cs -> AndL (sub cs) GHC 6.4 (as well as Hugs March 2005) produces this error: Couldn't match `Constraint' against `Expr' Expected type: Constraint Inferred type: Expr In the application `sub cs' In the first argument of `AndL', namely `(sub cs)' If I replace the last line with this one, everything is fine: AndL cs -> AndL (subst e n cs) It looks sort of like sub is being monomorphised -- or something? Thanks for having a look, Paul

5 4

unsafePerformIO and optimizations
by Wolfgang Jeltsch 06 Aug '05

06 Aug '05

Hello, http://haskell.org/ghc/docs/latest/html/libraries/base/ System.IO.Unsafe.html#v%3AunsafePerformIO talks about what optimizations you should disable if you apply unsafePerformIO to an action which contains side effects. These are: * inlining of functions which call unsafePerformIO * common subexpression elimination (on the module which contains the unsafePerformIO application, if I understand correctly) * let-floating Alas, the documentation is very terse, in my opinion, and I don't understand these things fully. First, what is the problem with inlining a function call? Say, I have a function f defined as follows: f :: Char -> () f c = unsafePerformIO (putChar c) If inlining is allowed, the expression f '*' becomes unsafePerformIO (putChar '*'). Is this a problem? Or is inlining dangerous in other situations? On the other hand, I can see that inlining non-functions could cause harm. Say I have something like this: u :: () u = unsafePerformIO (putChar '*') Now, inlining u would transform (u,u) to (unsafePerformIO (putChar '*),unsafePerformIO (putChar '*')) which could result in putChar '*' being executed multiple times. So why does the library documentation only talk about disabling inlining of functions? I understand that common subexpression elimination could cause harm. It could transform the expression (unsafePerformIO (putChar '*),unsafePerformIO (putChar '*')) to let u = unsafePerformIO (putChar '*') in (u,u), couldn't it? This would result in the I/O action be performed at most once which is not what was intended. But couldn't the same thing also happen if I use the expression (f '*',f '*'), probably in a different module? Does this mean that I have to use -fno-cse not only in the module which contains the unsafePerformIO application but also in every other module which uses unsafePerformIO indirectly via f or functions using f? Does let-floating only have to be disabled in the module which uses unsafePerformIO? Say, I have f defined as above and have the following definition in a module different from the one f is defined in: g :: Int -> () g n = f '*' Couldn't it be that the asterisk is output only once instead of every time, g is called? I would be thankful for any help. Best regards, Wolfgang

1 0

ANN: Haskell bindings to OpenLDAP
by John Goerzen 06 Aug '05

06 Aug '05

Hello, The first preliminary (but working!) version of my bindings to OpenLDAP from Haskell is now available from: darcs get --partial http://darcs.complete.org/ldap-haskell This implements searching, adding, modifying, and deleting data in LDAP directories. As such, it fully supports the base operations that are most important to LDAP programs. It doesn't yet support all the features in the C API, and also doesn't yet support lazy reads of the search result set. These things will be coming. Comments, questions, and patches are welcome. -- John

1 0

RE: [Haskell] ANNOUNCE: Haddock version 0.7
by Simon Marlow 05 Aug '05

05 Aug '05

On 05 August 2005 13:57, Adrian Hey wrote: > Does the linking to source still work with this version? > I can't get it to work anyway, but maybe I'm doing something > wrong. All I get for each module is a link to whatever URL > I specify, but without the actual source module name(path) > attached at the end. The semantics of the -s option were changed. Unfortunately, I forgot to document the change. See this message for details: http://www.haskell.org//pipermail/glasgow-haskell-users/2005-May/008394. html Cheers, Simon

1 0

Last call for registration: GPCE'05 (deadline 2 september)
by Eelco Visser 05 Aug '05

05 Aug '05

LAST CALL for REGISTRATION (Deadline Sep 02) >>>>>>>>>>> On-site registration will NOT be available ---------------------------------------------------------------------- 4th International Conference on Generative Programming and Component Engineering (GPCE'05) Sep 29 - Oct 1, 2005, Tallinn (Estonia) http://www.gpce.org/05 Sponsored by ACM SIGPLAN, in cooperation with ACM SIGSOFT co-located with ICFP'05 and TFP'05 ---------------------------------------------------------------------- IMPORTANT DATES * Sep 02, 2005: LATE REGISTRATION DEADLINE * Sep 27-28, 2005: GPCE workshops and tutorials * Sep 29 - Oct 1, 2005: GPCE papers and demos It is recommended that any changes or additions to existing registrations be done before Sep 02. ---------------------------------------------------------------------- SCOPE. Generative and component approaches have the potential to revolutionize software development in a similar way as automation and components revolutionized manufacturing. Generative Programming (developing programs that synthesize other programs), Component Engineering (raising the level of modularization and analysis in application design), and Domain-Specific Languages (elevating program specifications to compact domain-specific notations that are easier to write and maintain) are key technologies for automating program development. ---------------------------------------------------------------------- 3 INVITED SPEAKERS: * Oscar Nierstrasz: Object-oriented Reengineering Patterns * Oege de Moor: The AspectBench Compiler for AspectJ * Bernd Fischer: Certifiable Program Generation 25 TECHNICAL PAPERS 2 DEMONSTRATIONS: * Developing Dynamic and Adaptable Applications with CAM/DAOP: a Virtual Office Application * Metamodeling made easy - MetaEdit+ 2 TUTORIALS T1: Multi-stage Programming in MetaOCaml (Presenters: W.Taha, C.Calcagno) T2: Implementing Domain-Specific Modelling Languages and Generators (Presenter: R.Pohjonen) 3 WORKSHOPS W1: Seventh Young Researchers Workshop (Organizers: D.R.Dechow, D.Foetsch, S.Kiebusch, S.Perugini, M.J.Rutherford, D.Shestakov) W2: Second MetaOCaml Workshop (Organizers: K.Swadi, W.Taha) W3: Graph and Model Transformations Workshop (Organizers: G.Karsai, G.Taentzer) ---------------------------------------------------------------------- PROGRAM COMMITTEE Chairs: * Robert Glück (U. of Copenhagen) * Michael Lowry (NASA) Members: * Don Batory (U. of Texas, USA) * Ira Baxter (Semantic Designs) * Cristiano Calcagno (Imperial College) * Prem Devanbu (U. of California at Davis) * Ulrich Eisenecker (U. of Leipzig) * Tom Ellman (Vassar College) * Robert Filman (NASA) * Zhenjiang Hu (U. of Tokyo) * Patricia Johann (Rutgers U.) * John Launchbury (Galois) * Anne-Françoise Le Meur (U. of Sci. and Tech. Lille) * Hong Mei (Peking U.) * Nicolas Rouquette (NASA) * William Scherlis (CMU) * Yannis Smaragdakis (Georgia Inst. of Tech.) * Walid Taha (Rice U.) * Todd Veldhuizen (Chalmers U. of Tech.) ORGANIZING COMMITTEE General Chair * Eugenio Moggi (Genova U.) Publicity Chair: * Eelco Visser (Utrecht U.) Workshops and Tutorials Chairs * Jeff Gray (U. of Alabama at Birmingham) * Andrew Malton (Waterloo U.) Local Arrangements Chair * Tarmo Uustalu (Inst. of Cybernetics, Tallinn) ----------------------------------------------------------------------

1 0