- Haskell-Cafe - Haskell.org

control-timeout with gtk
by Marco Túlio Gontijo e Silva 20 Sep '08

20 Sep '08

Hello, I've written a simple sequencer[0] using gtk2hs and control-timeout[1]. In GHCi it works fine, but when I compile it, the timeouts generated by control-timeout are not executed. Actually, when I use the keyboard a lot, at some time they got executed. I thought it could be that these two packages are incompatible, but the I wondered why they work in GHCi. Any ideas? Greetings. 0: http://marcot.iaaeee.org/capoeira 1: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/control-timeout -- marcot Página: http://marcotmarcot.iaaeee.org/ Blog: http://marcotmarcot.blogspot.com/ Correio: marcot(a)riseup.net XMPP: marcot(a)jabber.org IRC: marcot(a)irc.freenode.net Telefone: 25151920 Celular: 98116720 Endereço: Rua Turfa, 639/701 Prado 30410-370 Belo Horizonte/MG Brasil

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ANNOUNCE: Extensible and Modular Generics for the Masses (EMGM) 0.1
by Sean Leather 20 Sep '08

20 Sep '08

Extensible and Modular Generics for the Masses ============================================== Extensible and Modular Generics for the Masses (EMGM) is a library for generic programming in Haskell using type classes. This is the initial release of a maintained library for EMGM. Other versions have previously existed in various states from various sources. We plan to continue updating and maintaining this version. Visit the home page: http://www.cs.uu.nl/wiki/GenericProgramming/EMGM Features -------- The primary features of EMGM include: * Datatype-generic programming using sum-of-product views * Large collection of ready-to-use generic functions * Included support for standard datatypes: lists, Maybe, tuples * Easy to add support for new datatypes * Type classes make writing new functions straightforward in a structurally inductive style * Generic functions are extensible with ad-hoc cases for arbitrary datatypes * Good performance of generic functions The features of this distribution include: * The API is thoroughly documented with Haddock * Fully tested with QuickCheck and HUnit * Program coverage ensures that all useful code has been touched by tests * Tested on both Mac and Windows systems Requirements ------------ EMGM has the following requirements: * GHC 6.8.1 - It has been tested with versions 6.8.3 and 6.9.20080916. * Cabal library 1.2.1 - It has been tested with versions 1.2.3 and 1.4.0.1. Download & Source ----------------- Use caball-install: cabal install emgm Get the package: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/emgm Get the source: svn checkout https://svn.cs.uu.nl:12443/repos/dgp-haskell/EMGM Examples -------- Check out the examples: https://svn.cs.uu.nl:12443/viewvc/dgp-haskell/EMGM/examples/ Bugs & Support -------------- Report issues or request features: http://code.google.com/p/emgm/ Discuss EMGM with the authors, maintainers, and other interested persons: http://www.haskell.org/mailman/listinfo/generics Credits ------- The research for EMGM originated with Ralf Hinze. It was extended with work by Bruno Oliveira and Andres Löh. More details of the library functionality were explored by Alexey Rodriguez. We are very grateful to all of these people for the foundation on which this library was built. The current authors and maintainers of EMGM are: * Sean Leather * José Pedro Magalhães * Alexey Rodriguez * Andres Löh

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performance of map reduce
by Manlio Perillo 20 Sep '08

20 Sep '08

Hi again. In http://book.realworldhaskell.org/read/concurrent-and-multicore-programming.… there is a map reduce based log parser. I have written an alternative version: http://paste.pocoo.org/show/85699/ but, with a file of 315 MB, I have [1]: 1) map reduce implementation, non parallel real 0m6.643s user 0m6.252s sys 0m0.212s 2) map reduce implementation, parallel with 2 cores real 0m3.840s user 0m6.384s sys 0m0.652s 3) my implementation real 0m8.121s user 0m7.804s sys 0m0.216s What is the reason of the map reduce implementation being faster, even if not parallelized? It is possible to implement a map reduce version that can handle gzipped log files? [1] These tests does not consider the "first run". For the first run (no data in OS cache), I have (not verified): 1) map reduce implementation, parallel with 2 cores real 0m3.735s user 0m6.328s sys 0m0.604s 2) my implementation real 0m13.659s user 0m7.712s sys 0m0.360s Thanks Manlio Perillo

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Hackage and HaXml situation
by John Goerzen 20 Sep '08

20 Sep '08

Hi everyone, We've got a bit of a problem in the community regarding HaXml. This has been brought up before, but never definitively resolved, and it's causing more and more trouble. Over at the HaXml homepage [1], it is stated that 1.13.3 is the most recent stable version of HaXml. I believe that Malcolm Wallace, HaXml maintainer, has confirmed this. Over at the HaXml hackage page [2], it would appear as if 1.19.4 is the most recent stable version of HaXml. There is no provision made to mark some versions as unstable, and to make 1.13.3 the default version downloaded from Hackage. There are API incompatibilities between 1.13.3 and 1.19.4 that make it impossible for my programs to support both. This is becoming a larger and more fragmenting problem. Arch Linux looked at the Hackage package and bundled up 1.19.4. Debian looked at the HaXml website and bundles 1.13.3. So we have a situation where I can write software that builds easily with one or the other, but not both. We really need a fix for this. Some things we could do would be: 1) Make a policy that Hackage is for the "most stable branch" of software only, and remove the 1.19.x series of HaXml. 2) Malcolm could decide to bless 1.19.x as stable, fixing the instant problem, but this would still leave the problem open for the future. 3) Add support to Hackage for labeling branches, and present the stable versions by default. Thoughts? [1] http://www.cs.york.ac.uk/fp/HaXml/ [2] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HaXml

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lazy strings and parallel read
by Manlio Perillo 20 Sep '08

20 Sep '08

Hi. After having read http://book.realworldhaskell.org/read/concurrent-and-multicore-programming.… I have a doubt about Data.ByteString.Lazy. Why getContents function don't use pread (or an emulation, if not available)? Thanks Manlio Perillo

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A round of golf
by Creighton Hogg 19 Sep '08

19 Sep '08

Hey Haskell, So for a fairly inane reason, I ended up taking a couple of minutes and writing a program that would spit out, to the console, the number of lines in a file. Off the top of my head, I came up with this which worked fine with files that had 100k lines: main = do path <- liftM head $ getArgs h <- openFile path ReadMode n <- execStateT (countLines h) 0 print n untilM :: Monad m => (a -> m Bool) -> (a -> m ()) -> a -> m () untilM cond action val = do truthy <- cond val if truthy then return () else action val >> (untilM cond action val) countLines :: Handle -> StateT Int IO () countLines = untilM (\h -> lift $ hIsEOF h) (\h -> do lift $ hGetLine h modify (+1)) If this makes anyone cringe or cry "you're doing it wrong", I'd actually like to hear it. I never really share my projects, so I don't know how idiosyncratic my style is.

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An interesting curiosity
by Andrew Coppin 19 Sep '08

19 Sep '08

I'm currently reading a C++ tutorial. (Don't ask!) Of course, C++ is that crazy language where *assignment* is actually an *operator*. Sick, sick people... The tutorial pointed out that "x = 4" is a legal assignment, but "4 = x" is not. "Obviously that would be silly." Now it occurs to me... in Haskell, "4 = x" is actually a perfectly legal thing to say. (!) It's pretty pointless, but it's legal. For example: foo x = let 4 = x in 2*x which, as best as I can tell, does exactly the same thing as (2*). In a similar vein, one might write foo x = do let 4 = x return (2*x) which also does more or less the same as (return). However, consider the following: foo :: Int -> Maybe Int foo x = do 4 <- return x return (2*x) This, it turns out, is an extremely odd way of checking whether x == 4. All of this works of course because in Haskell, "=" is not an assignment, it's a definition, and the RHS is not a variable, it's a pattern. And "4" is a perfectly legitimate pattern. Now, if only I could find a use for all this that borders on "useful"...! ;-)

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Re: Strongly Typed Memory Areas
by oleg＠okmij.org 19 Sep '08

19 Sep '08

John Van Enk wrote: > Was Iavor/Mark's paper ever implemented as a GHC extension? > "Strongly Typed Memory Areas" It turns out most of the functionality is already available in Haskell: Lightweight static resources, for safe embedded and systems programming http://okmij.org/ftp/Haskell/types.html#ls-resources Chung-chieh Shan's TFP2007 talk specifically contrasted code from Iavor/Mark's paper with Haskell code. Strongly typed memory areas are possible in Haskell today.

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Specifying a function type in a where clause.
by Rob deFriesse 19 Sep '08

19 Sep '08

I would like to know why I'm getting a particular compile time error message. In this program, I am specifying a function type on /combs'/ in the where clause: -------- module Main where import List (delete) combs :: Eq a => [a] -> Int -> [[a]] combs l 1 = map (\x -> [x]) l combs l n = foldl combs' [] l where combs' :: Eq a => [[a]] -> a -> [[a]] combs' acc x = let sl = delete x l in (map (\i -> x:i) $ combs sl (n-1)) ++ acc main = do print $ combs ["a","b","c","d"] 3 -------- I get this error message from GHC 6.8.3: cafe1.hs:9:43: Couldn't match expected type `a1' against inferred type `a' `a1' is a rigid type variable bound by the type signature for `combs'' at cafe1.hs:8:23 `a' is a rigid type variable bound by the type signature for `combs' at cafe1.hs:5:12 Expected type: [a1] Inferred type: [a] In the second argument of `delete', namely `l' In the expression: delete x l I don't understand why I'm seeing this. The type /a/ is the same type in /combs/ and /combs'/. I realize I can remove the type specification from /combs'/, and the code will compile. However, I'd like to get a better understanding of why GHC objects to this. Thank you, Rob.

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Re: HStringTemplate to generate html tables
by Sterling Clover 19 Sep '08

19 Sep '08

Hmm... an untested suggestion... <table> $cells:{ row | <tr>$row:{ cell | <td>$cell$</td>}$</tr>}$ </table> (i.e. nested iteration) or to encompass the table call inside, $cells:{ row | <tr>$row:{ cell | <td>$cell$</td>}$</tr>}:{<table>$it$</table>}$ Regards, Sterl. On Thu, Sep 18, 2008 at 8:16 AM, Thomas Hartman < thomashartman1(a)googlemail.com> wrote: > I am trying to use HStringTemplate for generating html tables, for use > in http://happstutorial.com:5001 > > The best I could do is below. Seems awfully kludgy. > > Can someone suggest a better way, which keeps me inside the > StringTemplate way of doing things? (Eg, no suggestions to use HTML.* > libraries, which are nice for some things but are a higher barrier to > entry than just jumping in and writing html.) > > Some kind of map structure... for each... ? > > Thanks for any help! > > thomas. > > {-# LANGUAGE NoMonomorphismRestriction #-} > import Text.StringTemplate > import Control.Applicative > import qualified Text.PrettyPrint as PP > > main = putStrLn . PP.render . toPPDoc $ tableFromCells cells > cells = [["mee","mah","moh"],["fee","fah","foh"]] > > -- This does generate an html table, but seems horribly obfuscated. > -- is there machinery for doing things like html table generation > -- in some better way than this hack where </end><start> > -- tags are used as a separator? > -- I would imagine something like > -- newSTMP "$cells:{<td>$it$</td>}:{<tr>$it</tr>}:{<table>$it</table>}" > -- which works a bit like a list comprehension > tableFromCells cells = setAttribute "cells" cells . optInsertTmpl > [("separator","</td><td>")] > $ newSTMP "$cells:\ > \{<tr><td>$it$</td></tr>}:\ > \{<table>$it$</table>}$"

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HStringTemplate to generate html tables
by Thomas Hartman 19 Sep '08

19 Sep '08

I am trying to use HStringTemplate for generating html tables, for use in http://happstutorial.com:5001 The best I could do is below. Seems awfully kludgy. Can someone suggest a better way, which keeps me inside the StringTemplate way of doing things? (Eg, no suggestions to use HTML.* libraries, which are nice for some things but are a higher barrier to entry than just jumping in and writing html.) Some kind of map structure... for each... ? Thanks for any help! thomas. {-# LANGUAGE NoMonomorphismRestriction #-} import Text.StringTemplate import Control.Applicative import qualified Text.PrettyPrint as PP main = putStrLn . PP.render . toPPDoc $ tableFromCells cells cells = [["mee","mah","moh"],["fee","fah","foh"]] -- This does generate an html table, but seems horribly obfuscated. -- is there machinery for doing things like html table generation -- in some better way than this hack where </end><start> -- tags are used as a separator? -- I would imagine something like -- newSTMP "$cells:{<td>$it$</td>}:{<tr>$it</tr>}:{<table>$it</table>}" -- which works a bit like a list comprehension tableFromCells cells = setAttribute "cells" cells . optInsertTmpl [("separator","</td><td>")] $ newSTMP "$cells:\ \{<tr><td>$it$</td></tr>}:\ \{<table>$it$</table>}$"

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Real World HAppS: Cabalized, Self-Demoing HAppS Tutorial (Version 3)
by Thomas Hartman 18 Sep '08

18 Sep '08

I pushed a new version of happs-tutorial to the online demo at http://happstutorial.com:5001 This is also on hackage: cabal install happs-tutorial. (now version 3.) or darcs get http://code.haskell.org/happs-tutorial for the latest The demo/tutorial has the same basic functionality as the last release -- just a login form essentially -- but a lot more bling now. Like menu link items that change colors when the page is clicked. Also the login form that gives sane error messages. The focus for this release was on explaining how I used StringTemplate with HAppS. Hopefully in version 4 I'll finally get to State and Macid! And hopefully some functionality that actually does something beyond just showing what users have created logins :) Thomas

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Re: [Haskell-cafe] Re: Float instance of 'read'
by Richard A. O'Keefe 18 Sep '08

18 Sep '08

On 18 Sep 2008, at 3:20 am, Mauricio wrote: > Agree about the answer, not about the question. The > correct one would be "is it possible to change haskell > syntax to support the international notation (not any > locally sensitive one) for decimal real numbers? Would > a change in 'read' be a good first step?" For some sense of "possible", the answer is clearly yes. However, it is perhaps misleading to call commas "THE international notation". The plain fact of the matter is that whatever any standards body may say, there are MANY notations for numbers. You might as well ask "is it possible to change Haskell syntax to support only the *real* Arabic digits ... for ... numbers". The Arabic script is used in many countries, so it's international. I don't read Arabic letters at all, but to me numbers written in Arabic script are no weirder than numbers using commas instead of dots. I would draw readers' attention to http://engineers.ihs.com/news/2006/nist-decimal-international.htm which says that (1) China, India, and Japan use decimal points, not commas. (2) There was a resolution of the CGPM in 2003 "endorsing the use of the point on the line as a decimal sign". (3) In June 2006, ISO agreed to allow the use of dots as decimal points in international standards. As it happens, the decimal point character I greatly prefer is the traditional British raised dot, but I can live with a low dot. I am 100% behind internationalised input and output, but allowing commas in numbers inside programming languages that already use commas for other purposes is, let's be polite, not a terribly good idea. You could probably get away with it in Lisp, but how many numbers are there in (1,2,3)? One thing that definitely is missing from Haskell numbers, and which I greatly miss, is a "thousands separator". Ada got around the dot/comma/apostrophe/whatever issue for thousands separators by using the underscore. For an unsigned decimal integer, this means allowing [0-9](_?[0-9])* instead of [0-9]+. It works really well, despite not being ANYBODY's cultural convention. > > > I know this looks difficult, but I'm sure it deserves at > least some thinking. We have previous examples for > less important issues: ghc does accept Windows end > of line conventions, the minus and sign needs special > syntax, and '.' can be used as decimal separator even > if it's use as function notation. > > Best, > Maurício > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > ==================================================================== > Auto Generated Mail > Footer If this email is requesting your > password then it is a HOAX. > DO NOT reply to the email. > Validate this footer at the > Otago University web site keyword search: information security phish > ==================================================================== > > > -- If stupidity were a crime, who'd 'scape hanging?

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Strongly Typed Memory Areas
by John Van Enk 18 Sep '08

18 Sep '08

Was Iavor/Mark's paper ever implemented as a GHC extension? "Strongly Typed Memory Areas" http://galois.com/down/2006_ID_Strongly.pdf -- /jve

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Exceptions
by Adrian Neumann 18 Sep '08

18 Sep '08

Hello, I think it'd be nice if the compiler could warn me if there are any exceptions which I'm not catching, similar to checked exceptions in Java. Does anyone know of a possibility to do that in Haskell? Adrian

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ANN: pandoc 1.0.0.1
by John MacFarlane 18 Sep '08

18 Sep '08

I'm pleased to announce the release of pandoc version 1.0.0.1. Pandoc aspires to be the swiss army knife of text markup formats: it can read markdown and (with some limitations) HTML, LaTeX, and reStructuredText, and it can write markdown, reStructuredText, HTML, DocBook XML, OpenDocument XML, ODT, RTF, groff man, MediaWiki markup, GNU Texinfo, LaTeX, ConTeXt, and S5. Pandoc's markdown syntax includes extensions for LaTeX math, tables, definition lists, footnotes, and more. HackageDB: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/pandoc User's guide: http://johnmacfarlane.net/pandoc/README.html Examples: http://johnmacfarlane.net/pandoc/examples.html Interactive demo: http://johnmacfarlane.net/pandoc/trypandoc/ Haddock docs: http://johnmacfarlane.net/pandoc/doc/pandoc/ Repository: http://pandoc.googlecode.com/ Bug tracker: http://code.google.com/p/pandoc/issues/list Mailing list: http://groups.google.com/group/pandoc-discuss Some highlights of this release: + New GNU Texinfo writer (contributed by Peter Wang) + New OpenDocument XML writer (contributed by Andrea Rossato) + New ODT (OpenOffice document) writer + New MediaWiki markup writer + New delimited code block syntax + Handy generic functions for querying and transforming documents without lots of boilerplate (thanks to Andrea Rossato) + Cleaner build system: pandoc can now be built as a regular Cabal package + New dependencies: utf8-string and zip-archive + Code is -Wall clean + New Windows installer + Better support for math, including display math + Better HTML sanitizing for use in web applications + Many minor improvements and bug fixes (see changelog for details) Pandoc can optionally be compiled with support for + syntax highlighting of delimited code blocks, using the highlighting-kate library (over 50 languages are supported) (specify -fhighlighting) + automatically generated citations and bibliography, using Andrea Rossato's hs-citeproc library (specify -fciteproc) I am particularly excited about Rossato's experimental citation support. It's basically a BibTeX-like system that one can use in any of pandoc's output formats. So you can have automatically generated citations in a blog post, a wiki page, or even a man page! It's not yet complete, but it's far enough along for those with an adventurous spirit to use. I am very grateful to everyone who contributed bug reports and code, and especially to Andrea Rossato and Peter Wang for their major contributions.

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announcing darcs 2.0.3pre1
by Eric Kow 18 Sep '08

18 Sep '08

Hi everybody, I am very pleased to announce the first pre-release of darcs 2.0.3, available at http://www.darcs.net/darcs-2.0.3pre1.tar.gz This pre-release includes a few major bug fixes and a handful of interesting features. I expect darcs 2.0.3 to be more reliable than its predecessors. We have better HTTP support, safer handling of the pending patch and a more solid implementation of common flags (for example, darcs get --to-match or darcs diff --patch). For more details, see the attached list. So what are we working on to get darcs 2.0.3 out the door? Aside from the usual testing, I hope that we can round out our improved reliability with a few small user interface enhancements, mostly to help users avoid some common pitfalls of real world version control: - issue53: detection of Windows reserved device names - issue971: hashed darcs check - issue1003: more tolerant darcs-transfer mode - issue1039: detection of seemingly unrelated repositories (see http://bugs.darcs.net for details) In the meantime, please give darcs 2.0.3pre1 a try and let us know how things go! Thanks, -- Eric Kow <http://www.nltg.brighton.ac.uk/home/Eric.Kow> PGP Key ID: 08AC04F9

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Ajax-Haskell framework?
by Greg Meredith 18 Sep '08

18 Sep '08

Haskellians, Is there an Ajax-Haskell framework? In case there are many, is there a preferred one? Experiences people would like to share? Best wishes, --greg -- L.G. Meredith Managing Partner Biosimilarity LLC 806 55th St NE Seattle, WA 98105 +1 206.650.3740 http://biosimilarity.blogspot.com

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Haskell Weekly News: Issue 85 - September 13, 2008
by Brent Yorgey 18 Sep '08

18 Sep '08

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20080913 Issue 85 - September 13, 2008 --------------------------------------------------------------------------- Welcome to issue 85 of HWN, a newsletter covering developments in the [1]Haskell community. Announcements citeproc-hs. Andrea Rossato [2]announced the first release of [3]citeproc-hs, a Haskell implementation of the [4]Citation Style Language. citeproc-hs adds a Bibtex-like citation and bibliographic formatting and generation facility to [5]Pandoc. Twidge. John Goerzen [6]announced the release of [7]Twidge, a command-line Twitter and Identi.ca client. Real World HAppS: Cabalized, Self-Demoing HAppS Tutorial (Version 3). Thomas Hartman [8]announced a new version of [9]happs-tutorial, with a correspondingly updated [10]online demo. generic list functions fixed. Jim Apple reported that genericTake, genericDrop, and genericSplitAt have [11]been fixed so they are now total functions (they used to fail on negative integer inputs, unlike their ungeneric counterparts). The Monad.Reader (13) - Call for copy. Wouter Swierstra [12]announced a call for copy for Issue 13 of the [13]Monad.Reader. The deadline for submitting articles is February 13, 2009. Heads Up: code.haskell.org is upgrading to darcs 2. Duncan Coutts [14]announced that /usr/bin/darcs on code.haskell.org will soon be upgraded to version 2. Most users should be unaffected as darcs 2 works just fine with repositories in darcs 1 format, and has been extensively tested for correctness. Discussion packages and QuickCheck. Conal Elliott [15]asked what methods of organization people use to package up QuickCheck tests for their libraries. Hackage needs a theme song!. Jason Dagit wrote a [16]theme song for Hackage! Jobs Gamr7. Lionel Barret De Nazaris [17]announced that [18]Gamr7, a startup in France focused on procedural city generation for the game and simulation market, is looking for a senior developer/technical director. senior role at Credit Suisse. Ganesh Sittampalam [19]announced that [20]Credit Suisse is seeking to recruit an expert in functional programming for a senior role in the Global Modelling and Analytics Group (GMAG) in the Securities Division. Blog noise [21]Haskell news from the [22]blogosphere. * Douglas M. Auclair (geophf): [23]What is declarative programming?. * John Goerzen (CosmicRay): [24]New Twitter Client: Twidge. * Mark Jason Dominus: [25]Return return. Mark's mind is blown by the code "return return" appearing in a paper by Mark Jones. * Eric Kow (kowey): [26]darcs weekly news #3. * Ketil Malde: [27]The FastQ file format for sequences. * >>> Nathan Sanders: [28]Real World Haskell. Nathan experiments with porting some of his Python code to Haskell. * Andrea Vezzosi (Saizan): [29]Results from GSoC. Andrea's GSoC work on a high-level dependency framework for Cabal is going to be released in a separate package for now, [30]hbuild. * >>> Ricky Clarkson: [31]An IRC Bot in Haskell, 20% code, 80% GRR. Ricky shares his experiences, frustrations, and eventual success writing a simple IRC bot in Haskell. * >>> Yaakov Nemoy: [32]Xmonad 0.8 released. * Luke Palmer: [33]The problem with Coq. ...is, according to Luke, that it doesn't have a nice graphical interface. * >>> James Cowie: [34]Haskell! yes no?. James dives into learning some Haskell. Verdict so far: a "very nice language". Quotes of the Week * EvilTerran: this is hard to express in this type system. i'm going to make my own type system instead! About the Haskell Weekly News New editions are posted to [35]the Haskell mailing list as well as to [36]the Haskell Sequence and [37]Planet Haskell. [38]RSS is also available, and headlines appear on [39]haskell.org. To help create new editions of this newsletter, please see the information on [40]how to contribute. Send stories to byorgey at cis dot upenn dot edu. The darcs repository is available at darcs get [41]http://code.haskell.org/~byorgey/code/hwn/ . References 1. http://haskell.org/ 2. http://article.gmane.org/gmane.comp.lang.haskell.cafe/44471 3. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/citeproc%2Dhs 4. http://xbiblio.sourceforge.net/csl/ 5. http://johnmacfarlane.net/pandoc/ 6. http://article.gmane.org/gmane.comp.lang.haskell.cafe/44453 7. http://software.complete.org/twidge 8. http://article.gmane.org/gmane.comp.lang.haskell.cafe/44410 9. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/happs-tutorial 10. http://happstutorial.com:5001/ 11. http://hackage.haskell.org/trac/ghc/ticket/2533 12. http://article.gmane.org/gmane.comp.lang.haskell.general/16420 13. http://www.haskell.org/haskellwiki/The_Monad.Reader 14. http://article.gmane.org/gmane.comp.lang.haskell.general/16419 15. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/44259 16. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/44450 17. http://article.gmane.org/gmane.comp.lang.haskell.cafe/44211 18. http://www.gamr7.com/ 19. http://article.gmane.org/gmane.comp.lang.haskell.general/16422 20. http://www.credit-suisse.com/ 21. http://planet.haskell.org/ 22. http://haskell.org/haskellwiki/Blog_articles 23. http://logicaltypes.blogspot.com/2008/09/what-is-declarative-programming.ht… 24. http://changelog.complete.org/posts/752-New-Twitter-Client-Twidge.html 25. http://blog.plover.com/prog/springschool95.html 26. http://koweycode.blogspot.com/2008/09/darcs-weekly-news-3.html 27. http://blog.malde.org/index.php/2008/09/09/the-fastq-file-format-for-sequen… 28. http://sandersn.com/blog/index.php?title=real_world_haskell 29. http://vezzosi.blogspot.com/2008/09/even-if-this-blog-has-been-silent-since… 30. http://hackage.haskell.org/trac/hackage/wiki/HBuild 31. http://rickyclarkson.blogspot.com/2008/09/irc-bot-in-haskell-20-code-80-grr… 32. http://loupgaroublond.blogspot.com/2008/09/xmonad-08-released.html 33. http://luqui.org/blog/archives/2008/09/07/the-problem-with-coq/ 34. http://www.jcowie.co.uk/2008/09/haskell-yes-no/ 35. http://www.haskell.org/mailman/listinfo/haskell 36. http://sequence.complete.org/ 37. http://planet.haskell.org/ 38. http://sequence.complete.org/node/feed 39. http://haskell.org/ 40. http://haskell.org/haskellwiki/HWN 41. http://code.haskell.org/~byorgey/code/hwn/

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a question about Database.HDBC.ODBC
by Changying Li 18 Sep '08

18 Sep '08

I want to use hdbc to connect my mysql test db. so I set up my /etc/odbcinst.ini : [MySQL] Description = ODBC Driver for MySQL Driver = /usr/lib/libmyodbc.so Setup = /usr/lib/libodbcmyS.so FileUsage = 1 and /etc/odbc.ini: [test] Description = MySQL database test Driver = MySQL Server = localhost Database = test Port = 3306 User = root Socket = /tmp/mysql.sock Option = Stmt = user chylli@localhost must use password to connect mysql. other users needn't. I connect db like this: Prelude Database.HDBC.ODBC Database.HDBC> handleSqlError (connectODBC "DSN=test") *** Exception: user error (SQL error: SqlError {seState = "[\"HY000\"]", seNativeError = -1, seErrorMsg = "connectODBC/sqlDriverConnect: [\"1045: [unixODBC][MySQL][ODBC 3.51 Driver]Access denied for user 'chylli'@'localhost' (using password: NO)\"]"}) Prelude Database.HDBC.ODBC Database.HDBC> handleSqlError (connectODBC "DSN=test;UID=chylli") Prelude Database.HDBC.ODBC Database.HDBC> in another term: mysql> show processlist; +----+-------------+-----------+------+---------+------+-------+------------------+ | Id | User | Host | db | Command | Time | State | Info | +----+-------------+-----------+------+---------+------+-------+------------------+ | 31 | chylli¶è | localhost | test | Sleep | 1483 | | NULL | | 43 | root | localhost | test | Sleep | 116 | | NULL | my question is: 1. why not HDBC.ODBC use configuration in /etc/odbc.ini ? if is use, then connectODBC "DSN=test" should use root as user without password and it should succeed. 2. when using UID=chylli, why the user in 'show processlist' is not chylli but 'chylli¶è' ? if it use account 'chylli', that connection should fail. but in fact it succeed !!! -- Thanks & Regards Changying Li

2 2

Re: [Haskell-cafe] Re: Python's big challenges, Haskell's big advantages?
by Richard A. O'Keefe 18 Sep '08

18 Sep '08

It may be of interest that although Erlang has been doing lightweight concurrency for >20 years, - you can choose whether you want to use an SMP version that has as many schedulers as there are cores (plus internal locking as needed) or a non-SMP version with one scheduler (and no internal locking); both versions are standard and it's only a performance issue, not a semantics issue - performance sometimes goes one way, sometimes the other - there was a "one UNIX process per Erlang process" implementation; I have a copy. The community interest in it was, shall we say, massively underwhelming. It might also be interesting to note that the experimental operating system K42 from IBM does _all_ user-visible threading in user-land. This includes thread switching and even I/O blocking and unblocking; all done in user-land. I don't think we've begun to explore all the options yet.

1 0

about openTempFile
by Manlio Perillo 18 Sep '08

18 Sep '08

Hi. After reading the chapter about IO in the "Real Word Haskell" book, I have noted that there is no support for opening a temporary file that will be automatically removed at program termination. The Python tempfile module, as an example, implements a wrapper around mkstemp function that does exactly this, and the code is portable; on Windows it uses O_TEMPORARY_FILE flag, on POSIX systems the file is unlink-ed as soon as it is created (but note that the code is not signal safe - well, many functions in the Python standard library are not signal safe). There are reasons why GHC library does not implement this? The Python version also set the FD_CLOEXEC, O_NOINHERIT and O_NOFOLLOW flags (where available). The GHC version, instead, set the O_NOCTTY flag. Manlio Perillo

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Re: [Haskell-cafe] Re: Python's big challenges, Haskell's big advantages?
by Donn Cave 18 Sep '08

18 Sep '08

Quoth Jonathan Cast <jonathanccast(a)fastmail.fm>: ... | Say what? This discussion is entirely about performance --- does | CPython actually have the ability to scale concurrent programs to | multiple processors? Well, ostensibly the discussion also has something to do with Haskell. On that premise, may I say I hope we're not inspired by Python's example to treat concurrency as a religious matter. The C Python community is forced to take the position that interpreter- level threading is not worth the price, because they don't have it and can't conveniently get it. I doubt we'll ever know for sure whether they're right or wrong, since Python comes with other issues that could also take blame or credit for its success at large scale applications. The Haskell community seems to be convinced as a whole that user-level threads are the way to go, but at most that's just a majority, not a consensus. Who cares!? Given the ability to take either path, Haskell programmers will solve our religious argument - we'll know who gets to heaven with working, high performance, reliable applications. That is, assuming that programmers of each persuasion can find Haskell suited to their model. As far as I know, though, that isn't a really sure thing. I've read things like "thread RTS can't/shouldn't support fork" and "would like to be able to get rid of the non-threaded RTS", with respect to GHC. At least until there is more empirical evidence that it's a waste of time, Haskell implementations should leave this to the programmer - support processes, OS threads, or user threads without undue work-around burdens. Otherwise we'll be stuck like the Python community is, defending this approach as a religious argument. We have enough of those already. Donn Cave, donn(a)avvanta.com

1 0

How to check if two Haskell files are the same?
by Mauricio 18 Sep '08

18 Sep '08

Hi, I would like to write a Haskell pretty-printer, using standard libraries for that. How can I check if the original and the pretty-printed versions are the same? For instance, is there a file generated by GHC at the compilation pipe that is always guaranteed to have the same MD5 hash when it comes from equivalent source? Thanks, Maurício

8 10

Float instance of 'read'
by Mauricio 18 Sep '08

18 Sep '08

Hi, A small annoyance some users outside english speaking countries usually experiment when learning programming languages is that real numbers use a '.' instead of ','. Of course, that is not such a problem except for the inconsistence between computer and free hand notation. Do you think 'read' (actually, 'readsPrec'?) could be made to also read the international convention (ie., read "1,5" would also work besides read "1.5")? I'm happy to finaly use a language where I can use words of my language to name variables, so I wonder if we could also make that step. Thanks, Maurício

13 20

Predicativity?
by Wei Hu 18 Sep '08

18 Sep '08

Hello, I only have a vague understanding of predicativity/impredicativity, but cannot map this concept to practice. We know the type of id is forall a. a -> a. I thought id could not be applied to itself under predicative polymorphism. But Haksell and OCaml both type check (id id) with no problem. Is my understanding wrong? Can you show an example that doesn't type check under predicative polymorphism, but would type check under impredicative polymorphism? Thanks!

3 4

Semantic Domain, Function, and denotational model.....
by Greg Meredith 18 Sep '08

18 Sep '08

Daryoush, Hopefully, the other replies about proving the monad laws already answered your previous question: yes! As for notions of semantic domain and denotational model, these ideas go back quite a ways; but, were given solid footing by Dana Scott<http://en.wikipedia.org/wiki/Dana_Scott>. In a nutshell, we have essentially three views of a computation - Operational <http://en.wikipedia.org/wiki/Operational_semantics> -- computation is captured in a program and rules for executing it - Logical <http://en.wikipedia.org/wiki/Proof-theoretic_semantics> -- computation is captured in a proof and rules for normalizing it - Denotational <http://en.wikipedia.org/wiki/Denotational_semantics> -- computation is captured as a (completely unfolded) mathematical structure In the latter view we think of computations/programs as denoting some (usually infinitary) mathematical object. Our aim<http://en.wikipedia.org/wiki/Domain_theory>is to completely define the meaning of programs in terms of maps between their syntactic representation and the mathematical objects their syntax is intended to denote. (Notationally, one often writes such maps as [| - |] : Program -> Denotation.) For example, we model terms in the lambda calculus as elements in a D-infinity domain or Bohm trees or ... . Or, in more modern parlance, we model terms in the lambda calculus as morphisms in some Cartesian closed category, and the types of those terms as objects in said category. The gold standard of quality of such a model is full abstraction<http://en.wikipedia.org/wiki/Denotational_semantics#Full_abstraction>-- when the denotational notion of equivalence exactly coincides with an intended operational notion of equivalence. In symbols, - P ~ Q <=> [| P |] = [| Q |], where ~ and = are the operational and denotational notions of equivalence, respectively The techniques of denotational semantics have been very fruitful, but greatly improved by having to rub elbows with operational characterizations. The original proposals for denotational models of the lambda calculus were much too arms length from the intensional structure implicit in the notion of computation and thus failed to achieve full abstraction even for toy models of lambda enriched with naturals and booleans (cf the so-called full abstraction for PCF problem<http://en.wikipedia.org/wiki/Programming_language_for_Computable_Functions>). On the flip side, providing a better syntactic exposure of the use of resources -- ala linear logic -- aided the denotational programme. More generally, operational models fit neatly into two ready-made notions of equivalence - contextual equivalence<http://encyclopedia2.thefreedictionary.com/Contextual+equivalence>-- behaves the same in all contexts - bisimulation <http://en.wikipedia.org/wiki/Bisimulation> -- behaves the same under all observations Moreover, these can be seen to coincide with ready-made notions of equivalence under the logical view of programs. Except for syntactically derived initial/final denotational models -- the current theory usually finds a more "home-grown" denotational notion of equivalence. In fact, i submit that it is this very distance from the syntactic presentation that has weakened the more popular understanding of "denotational" models to just this -- whenever you have some compositionally defined map from one representation to another that respects some form of equivalence, the target is viewed as the denotation. Best wishes, --greg Date: Mon, 15 Sep 2008 10:13:53 -0700 From: "Daryoush Mehrtash" <dmehrtash(a)gmail.com> Subject: Re: [Haskell-cafe] Semantic Domain, Function, and denotational model..... To: "Ryan Ingram" <ryani.spam(a)gmail.com> Cc: Haskell Cafe <haskell-cafe(a)haskell.org> Message-ID: <e5b8e9790809151013k53fdc105i88f2f47f1f9b16bd(a)mail.gmail.com> Content-Type: text/plain; charset=ISO-8859-1 Interestingly, I was trying to read his paper when I realized that I needed to figure out the meaning of denotational model, semantic domain, semantic functions. Other Haskell books didn't talk about design in those terms, but obviously for him this is how he is driving his design. I am looking for a simpler tutorial, text book like reference on the topic. Daryoush On Mon, Sep 15, 2008 at 1:33 AM, Ryan Ingram <ryani.spam(a)gmail.com> wrote: > I recommend reading Conal Elliott's "Efficient Functional Reactivity" > paper for an in-depth real-world example. > > http://www.conal.net/papers/simply-reactive > > -- ryan > > On Sun, Sep 14, 2008 at 11:31 AM, Daryoush Mehrtash <dmehrtash(a)gmail.com> wrote: >> I have been told that for a Haskell/Functional programmer the process >> of design starts with defining Semantic Domain, Function, and >> denotational model of the problem. I have done some googling on the >> topic but haven't found a good reference on it. I would appreciate >> any good references on the topic. >> >> thanks, >> >> daryoush >> >> ps. I have found referneces like >> http://en.wikibooks.org/wiki/Haskell/Denotational_semantics which >> talks about semantic domain for "the Haskell programs 10, 9+1, 2*5" >> which doesn't do any good for me. I need something with a more real >> examples. >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> > -- L.G. Meredith Managing Partner Biosimilarity LLC 806 55th St NE Seattle, WA 98105 +1 206.650.3740 http://biosimilarity.blogspot.com

2 2

Mac OS X dylib woes
by John MacFarlane 17 Sep '08

17 Sep '08

I'm hoping some Haskell developers who use Macs can help me with this one. I can install pcre-light just fine using cabal install. But when I try to use it, I get this error: GHCi, version 6.8.2: http://www.haskell.org/ghc/ :? for help :Loading package base ... linking ... done. Prelude> :m Text.Regex.PCRE.Light.Char8 Prelude Text.Regex.PCRE.Light.Char8> compile "h." [] Loading package array-0.1.0.0 ... linking ... done. Loading package bytestring-0.9.0.1 ... linking ... done. Loading package pcre-light-0.3.1 ... can't load .so/.DLL for: pcre (dlopen(libpcre.dylib, 10): image not found) OK, so it can't find the pcre library (which is in /opt/local/lib). I can fix that: export DYLD_LIBRARY_PATH=/opt/local/lib Now it works. But other things are broken! For example, I can't run vim, which looks for a library called libJPEG.dylib and now finds libjpeg.dylib in /opt/local/lib (case-insensitive file system!). The apple website recommends against setting DYLD_LIBRARY_PATH. Instead, they say, the paths to the libraries should be hard-coded into the binary: http://discussions.apple.com/thread.jspa?threadID=1670523&tstart=0 I'm sure others have had the same problem. What's the solution? John

3 3

FRP (was: Semantic Domain, Function, and denotational model)
by John Lato 17 Sep '08

17 Sep '08

I just noticed that the "Simply Efficient Functional Reactivity" paper has been updated since I last looked; I'll have to read it again now. Is the library/code mentioned in the paper released or available anywhere at this time? Conal has left tantalizing hints scattered in various places... Thanks, John Lato

3 2

GHC trouble on Leopard
by Miguel Vilaca 17 Sep '08

17 Sep '08

Hi, I tried to compile some code on Mac Os X (Intel) Leopard. I have GHC 6.8.3 installed - the installer from GHC webpage (GHC-6.8.3- i386.pkg). But when I run make I get this error ghc-6.8.3: could not execute: /Library/Frameworks/GHC.framework/ Versions/608/usr/lib/ghc-6.8.3/ghc-asm The work-around of removing the option -fvia-C pointed somewhere on the web is not an option here due to library dependecies. Any hints on how to solve this? Thanks in advance Miguel Vilaça

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Hyena Status? Re: [Haskell-cafe] Starting Haskell with a web application
by Donnie Jones 17 Sep '08

17 Sep '08

Hello Johan Tibell, Hyena looks very interesting. From the github tracking, you've been working... Maybe a release soon? Also, I saw your slides from the 'Left-fold enumerators' presentation at Galois. Maybe include the slides in the docs/ for a release? Thank you. __ Donnie On Thu, Mar 6, 2008 at 7:40 AM, Johan Tibell <johan.tibell(a)gmail.com> wrote: > > Do you (both) have repos that I could download from? I quite interested > > in both projects, esp. the WSGI clone. > > Yes and no. The code [1] is in my darcs repository but is in an > unusable state until I've fixed my incremental parser (in > Hyena/Parser.hs) which I plan to do next week. I would like the first > release to be nice and polished so I'm trying to not release anything > prematurely. > > 1. http://darcs.johantibell.com/hyena/ > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

2 2

Problem with hscurses
by david48 17 Sep '08

17 Sep '08

the getCh funtion is supposed to return an interpreted Key with values like KeyChar c, KeyReturn, KeyBackspace, etc. but in fact, it only ever returns KeyChar c values ! am I doing anything wrong ? Here's an example program : module Main where import UI.HSCurses.Curses import Text.Printf import System.IO main = do hSetBuffering stdout NoBuffering withCurses test1 withCurses f = do initScr echo False cBreak True res <- f endWin return res test1 = do c <- getCh printf "Touche : %s\r\n" (show c) test1

1 1

hircules resurrected (partially ;-)
by wman 17 Sep '08

17 Sep '08

I stumbled upon hircules-0.3 IRC client and decided to take it for a test drive. I managed to get it to compile under ghc-6.8.3/win32, converted it to use Text.Codec.Iconv (the old version of included Iconv.hs should also work), but it seems it doesn't work. It only gives an empty form and according to wireshark doesn't even attempt a connection. If somebody knew hircules in it's days of glory and/or wants to go bug-hunting, i'll gladly provide the modified version, as I probably won't spend more time on it in near future (at least until i get to know gtk2hs better).

1 0

Comparing GADTs for Eq and Ord
by Tom Hawkins 17 Sep '08

17 Sep '08

Hi, How do I compare a GADT type if a particular constructor returns a concrete type parameter? For example: data Expr :: * -> * where Const :: Expr a Equal :: Expr a -> Expr a -> Expr Bool -- If this read Expr a, the compiler has no problem. instance Eq (Expr a) where Const == Const = True (Equal a b) (Equal x y) = a == x && b == y _ == _ = False GHC throws: Couldn't match expected type `a1' against inferred type `a2' `a1' is a rigid type variable bound by the constructor `Equal' at Test.hs:9:3 `a2' is a rigid type variable bound by the constructor `Equal' at Test.hs:9:18 Expected type: Expr a1 Inferred type: Expr a2 In the second argument of `(==)', namely `x' In the first argument of `(&&)', namely `a == x' I believe I understand the reason for the problem; even though Equal has a type Expr Bool, two Equal expressions could have parameters of different types. But I'm not sure how to get around the problem. Any suggestions? Thanks, Tom

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haskell blas bindings: does iomatrix gemv transposing of matrix a?
by Anatoly Yakovenko 17 Sep '08

17 Sep '08

Hey Patric, Thanks for your great work on the blas bidnings. I have a question on gemv. I thought its possible for blas to transpose the input matrix before doing the multiplication. Is it possible to do that with the haskell bindings? Or am I mistaken in how gemv is used Thanks, Anatoly

1 0

Semantic Domain, Function, and denotational model.....
by Daryoush Mehrtash 17 Sep '08

17 Sep '08

I have been told that for a Haskell/Functional programmer the process of design starts with defining Semantic Domain, Function, and denotational model of the problem. I have done some googling on the topic but haven't found a good reference on it. I would appreciate any good references on the topic. thanks, daryoush ps. I have found referneces like http://en.wikibooks.org/wiki/Haskell/Denotational_semantics which talks about semantic domain for "the Haskell programs 10, 9+1, 2*5" which doesn't do any good for me. I need something with a more real examples.

4 10

SYB with class: Bug in Derive.hs module
by Alexey Rodriguez Yakushev 17 Sep '08

17 Sep '08

Hi people (and Ralf and Alex), I found a bug in the SYB with class library when trying to implement generic equality. I am hoping that someone in the Cafe (maybe Ralf) can confirm it is a bug, or maybe show me that I am doing something wrong. I am using the "Scrap your boilerplate with class" library (http:// homepages.cwi.nl/~ralf/syb3/). More precisely, I am using the library distributed by the HAppS project, because it works with GHC 6.8 . You can get the repository as follows: darcs get http://happs.org/HAppS/syb-with-class However, the offending module (Derive.hs) produces broken instances in both distributions. The bug: ---------- Generic equality needs type safe casting when implemented in SYB3, I have tried both the gzipwith variant and using Pack datatypes (geq*.hs in the first distribution). However, both functions loop when applied to a tree value. This loop occurs when the functions try to cast one of the arguments. I have managed to reduce the error to a smaller source file that I send attached. It does the following: > main = print typeReps > > tree = (Bin (Leaf 1) (Leaf 2))::BinTree Int > > data Pack = forall x. Typeable x => Pack x > > packedChildren = gmapQ geqCtx Pack tree > > typeOfPack (Pack x) = typeOf x > > typeReps = map typeOfPack packedChildren Basically the tree is transformed into a list of Pack-ed values and then to a list of type representations. This program loops at "typeOf" when you call "main". The Data instance that Derive generates is as follows: > instance (Data ctx a, > Data ctx (BinTree a), > Sat (ctx (BinTree a))) => > Data ctx (BinTree a) where Note the recursive |Data ctx (BinTree a)| in the context. If I get rid of it (a correct manual instance is also included in the attachment) the example works. I thought of removing this from the context in the Derive source. But maybe I might break some other use cases. So I am asking for help! Should Derive be fixed? How? Cheers, Alexey

3 2

Hackage checks
by Andrea Rossato 16 Sep '08

16 Sep '08

Hi, the Hackage upload script performs some checks on the packages being uploaded. In my case, a library, I need to use the cabal generated Paths_package file to access some locale data stored in a data file directory. This requires to include in "hs-source-dirs" "dist/build/autogen", which doesn't exist in the source tree, since it's generated by cabal in the build process (the inclusion is made necessary by a bug in Cabal-1.2 which doesn't automatically search dist/build/autogen, see [1]). And so Hackage complains: I needed to manually include such an empty directory in the source code archive. Any way out? TIA Andrea [1] http://www.haskell.org/pipermail/libraries/2008-July/010078.html

3 3

Import qualified, inverse of hiding
by Mauricio 16 Sep '08

16 Sep '08

Hi, 'import' allows one to say 'hiding' to a list of names. Is it possible to do the opposite, i.e., list the names I want to import? Something like "import Module showing x"? Thanks, Maurício

4 7

Should `(flip (^^))` work?
by Mauricio 16 Sep '08

16 Sep '08

Hi, I tested the expression below and it doesn't work. Is there some way to achieve that (i.e., turning an expression inside parenthesis into an operator)? 2 `(flip (^^))` (3%4) Thanks, Maurício

5 5

Re: [Haskell-cafe] Haskell Weekly News: Issue 85 - September 13, 2008
by Richard A. O'Keefe 16 Sep '08

16 Sep '08

On 14 Sep 2008, at 10:59 pm, Rafael Almeida wrote: > One thing have always bugged me: how do you prove that you have > correctly proven something? This really misses the point of trying to formally verify something. That point is that you almost certainly have NOT. By the time you get a theorem prover to accept your specification, you have (a) gone through a couple of rounds of redesign (before writing the code!) and now have something really clear (because the prover is too dumb to understand subtle stuff) (b) just done a lot of "testing" on the design that was finally accepted. > I mean, when I write a code I'm formaly > stating what I want to happen and bugs happen. If I try to prove some > part of the code I write more formal text (which generally isn't even > checked by a compiler); I'm sorry? What kind of half-arsed "formal" specification is NOT "checked"? None of the specification tools I've played with (I really wish there were a PVS course I could attend in NZ) can truthfully be described as "not checked". Symbolic model checking tools effectivley _are_ testing tools; what you normally get from them is not a cheery "that's fine boss" but a snarky "you forgot about this possible input didn't you, idiot!" > how can I be sure that my proof doesn't > contain bugs? You can't. What you CAN be sure of is that your previous proof attempts DID contain bugs. Lots of them. At least *those* ones are gone. Let's face it, you can't be *sure* that you aren't a brain in a jar being systematically deceived by a demon. (Read Descartes.) In fact, you can usually be CERTAIN that you haven't proved the validity of your whole system, because you usually haven't tried. Formal methods are a risk reduction tool. You pick some part of the system which has a special need for reliability, isolate it, model it, check the model for consistency, specify operations on it, prove something about them, and you learn a heck of a lot by doing so. What you can't eliminate is the possibility that something nasty is lurking in the BIOS of your computer specifically watching out for your code so it can sabotage it. Several times in my programming career I have encountered perfectly correct code that malfunctioned because of a broken compiler (well ok, in one case it was a broken assembler). > Why would it make my program less error-prone? Is there > any article that tries to compare heavy testing with FM? Lots of them. http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.18.2475 might make a good start. The idea that the FORTEST researchers share is that formal methods can *help* testing. Indeed, QuickCheck basically _is_ an 'automatic tests from specifications' tool, one of many that have been built over the years. If you stop thinking of formal methods as "verifying finished written programs" and more as some mix of "design for checkability" (so that bugs are less likely to be written into the code in the first place) or as "testing for specifications" it may make more sense to you.

5 4

Re: Proofs and commercial code -- was Haskell Weekly News: Issue 85 - September 13, 2008
by Greg Meredith 15 Sep '08

15 Sep '08

Daryoush, One of the subtle points about computation is that -- via Curry-Howard -- well-typed programs are *already* proofs. They may not be the proof you were seeking (there are a lot of programs from Int -> Int), or prove the theorem you were seeking to prove (Ord a => [a] -> [a] is a lot weaker than 'this program sorts lists of ordered types'), but they are witnesses of any type they type-check against. In a system that has principal types, they are proofs of their principal type. In this sense, the utility of proofs is widespread: they are programs. There is a really subtle dance between types that are rich enough to express the theorems you would have your programs be proofs of and the termination of type-checking for that type system. That's why people often do proofs by hand -- because the theorems they need to prove require a greater degree of expressiveness than is available in the type system supported by the language in which the programs are expressed. Research has really been pushing the envelope of what's expressible in types -- from region and resource analysis to deadlock-freedom. Again, in that setting the program is a witness of a property like - won't leak URLs to unsavory agents - won't hold on to handles past their garbage-collect-by date - won't get caught in a 'deadly embrace' (A is waiting for B, B is waiting for A) Sometimes, however, and often in mission critical code, you need stronger assurances, like - this code really does implement a FIFO queue, or - this code really does implement sliding window protocol, or - this code really does implement two-phase-commit-presumed-abort It's harder -- in fact i think it's impossible for bullet item 2 above -- to squeeze these into terminating type-checks. In those cases, you have to escape out to a richer relationship between 'theorem' (aka type) and 'proof' (aka program). Proof assistants, like Coq, or Hol or... can be very helpful for automating some of the tasks, leaving the inventive bits to the humans. In my experience, a proof of a theorem about some commercial code is pursued when the cost of developing the proof is less than some multiple of the cost of errors in the program in the life-cycle of that program. Intel, and other hardware vendors, for example, can lose a lot of business when the implementation of floating point operations is buggy; and, there is a breaking point where the complexity/difficulty/cost of proving the implementation correct is sufficiently less than that of business lost that it is to their advantage to obtain the kind of quality assurance that can be had from a proof of correctness. One other place where proofs of correctness will be pursued is in mathematical proofs, themselves. To the best of my knowledge, nothing mission-critical is currently riding on the proof of the 4-color theorem. The proof -- last i checked -- required programmatic checking of many cases. Proving the program -- that implements the tedious parts of the proof -- correct is pursued because of the culture and standard of mathematical proof. Best wishes, --greg Date: Sat, 13 Sep 2008 22:24:50 -0700 From: "Daryoush Mehrtash" <dmehrtash(a)gmail.com> Subject: Re: [Haskell-cafe] Haskell Weekly News: Issue 85 - September 13, 2008 To: "Don Stewart" <dons(a)galois.com> Cc: Haskell Cafe <haskell-cafe(a)haskell.org> Message-ID: <e5b8e9790809132224u3d29b858j5fda8e41b34566eb(a)mail.gmail.com> Content-Type: text/plain; charset="iso-8859-1" What I am trying to figure out is that say on the code for the IRC bot that is show here http://www.haskell.org/haskellwiki/Roll_your_own_IRC_bot/Source What would theorem proofs do for me? Daryoush -- L.G. Meredith Managing Partner Biosimilarity LLC 806 55th St NE Seattle, WA 98105 +1 206.650.3740 http://biosimilarity.blogspot.com

3 2

Re: [Haskell-cafe] ask
by Richard A. O'Keefe 15 Sep '08

15 Sep '08

On 15 Sep 2008, at 12:51 pm, Daniel Fischer wrote: > Am Montag, 15. September 2008 02:24 schrieb Cetin Sert: >> Hi why do I get? > > Buffering. For compiled programmes, stdin and stdout are line- > buffered by > default, so the output doesn't appear until the program finishes. > Either put > hSetBuffering stdout NoBuffering > at the top of main or, better IMO, > insert a > hFlush stdout > into ask before the readLn. > And in eval, putStrLn would be better, I think. Is this the problem we've had before, where the example in the Haskell 98 Report could not possibly work unless Haskell followed the same rule as C (read from stdin *automatically* flushes stdout), and the example works in several other Haskell systems, but not in GHC? Or is this a different problem?

1 0

ask
by Cetin Sert 15 Sep '08

15 Sep '08

Hi why do I get? cetin@linux-d312:~/lab/exp/1> ./eq 23 23 3 a = b = c = n1-0.8457820374040622n2-0.1542179625959377 when I run import System.IO main :: IO () main = do a ← ask "a" b ← ask "b" c ← ask "c" eval a b c ask v = do putStr (v ++ " = ") readLn eval a b c = do case delta < 0 of True → putStr "neg" False → putStr ("n1" ++ show n1 ++ "n2" ++ show n2) where delta = b*b - 4*c*a n o = (-b `o` sqrt(delta))/(2*a) n1 = n (+) n2 = n (-)

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system in forkIO
by Marco Túlio Gontijo e Silva 15 Sep '08

15 Sep '08

Hello. > import System.Cmd > import GHC.Conc > > main :: IO () > main > = forkIO > ( do > putStrLn "fork" > system "ls" > return ()) > >> getChar > >> return () When I run this code, I get fork and the result of ls only after I press a key. Does getChar blocks the other threads? Greetings. -- marcot Página: http://marcotmarcot.iaaeee.org/ Blog: http://marcotmarcot.blogspot.com/ Correio: marcot(a)riseup.net XMPP: marcot(a)jabber.org IRC: marcot(a)irc.freenode.net Telefone: 25151920 Celular: 98116720 Endereço: Rua Turfa, 639/701 Prado 30410-370 Belo Horizonte/MG Brasil

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message passing style in Monad
by jinjing 15 Sep '08

15 Sep '08

I found that as I can do xs.map(+1).sort by redefine . to be a . f = f a infixl 9 . I can also do readFile "readme.markdown" <.> lines <.> length by making a <.> b = a .liftM b infixl 9 <.> Kinda annoying, but the option is there. - jinjing

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Views
by Stephan Friedrichs 15 Sep '08

15 Sep '08

Hello! List-like data structures should IMHO provide safe versions of 'head' and 'tail' (with safe I mean 'not partial', i. e. functions that don't provoke an 'error' when called with an empty collection). As far as I know, this is usually called 'view' and and has a type signature like view :: SomeDataStructure a -> View a with data View = Empty | Cons a (SomeDataStructure a) A good example for this is Data.Sequence. My question is: Why is this not expressed in terms of Maybe? view :: SomeDataStructure a -> Maybe (a, SomeDataStructure a) This would be easier to use, as there's no need for a new View type and because Maybe already provides instance declarations for a lot of useful classes (Functor, Monad, etc.) and handy helper functions (e. g. 'maybe'). Then you could, for instance, say: head xs = maybe undefined fst (view xs) tail xs = maybe undefined snd (view xs) You can of course argue that you want viewl and viewr to have different types in the case of Data.Sequence, but this is not the case for other, rather one-ended, data types, is it? Long story short, my problem is the following: I want to provide a 'view' function for Data.Heap in my heap [1] package and I don't know whether... a) ... to use Maybe b) ... to provide my own Data.Heap.View type c) ... a Data.View package with a View type should be included in the containers- or even base-package. This would prevent lots of small projects from creating totally equivalent View types. Maybe there even exists a Data.View package that I didn't find? Regards, Stephan [1] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/heap-0.3.1 -- Früher hieß es ja: Ich denke, also bin ich. Heute weiß man: Es geht auch so. - Dieter Nuhr

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random colors, stack space overflow, mersenne and mersenne.pure64
by Cetin Sert 15 Sep '08

15 Sep '08

Hi, The following piece of code runs just fine, if I say: instance Random RGB where random = color randomR = colorR instead of: instance Random RGB where random = color2 randomR = colorR When I use random = color2 I encounter a stack space overflow: cetin@linux-d312:~/lab/test/colors> ./var2 +RTS -K30000000 Stack space overflow: current size 30000000 bytes. Use `+RTS -Ksize' to increase it. I think I'm doing something wrong with the definition of colorR. Can anyone explain me what's wrong? import GHC.Word import Data.Word import System.Random import System.Random.Mersenne.Pure64 type RGB = (Int,Int,Int) instance Bounded RGB where minBound = minRGB maxBound = maxRGB minRGB = (0 ,0 ,0 ) maxRGB = (255,255,255) instance Random RGB where random = color2 randomR = colorR color2 :: RandomGen g ⇒ g → (RGB,g) color2 = colorR (minRGB,maxRGB) color :: RandomGen g ⇒ g → (RGB,g) color s0 = ((r,g,b),s3) where (r,s1) = q s0 (g,s2) = q s1 (b,s3) = q s2 q = randomR (0,255) colorR :: RandomGen g ⇒ (RGB,RGB) → g → (RGB,g) colorR ((a,b,c),(x,y,z)) s0 = ((r,g,b),s3) where (r,s1) = q (a,x) s0 (g,s2) = q (b,y) s1 (b,s3) = q (c,z) s2 q = randomR main :: IO () main = do mt ← newPureMT let cs = randoms mt :: [RGB] print cs ------------------------------------------ This one also just works fine: import Data.Word import System.Random.Mersenne type RGB = (Word8,Word8,Word8) instance MTRandom RGB where random m = do r ← random m :: IO Word8 g ← random m :: IO Word8 b ← random m :: IO Word8 return (r,g,b) main :: IO () main = do g ← newMTGen Nothing cs ← randoms g :: IO [RGB] print cs but I really need the range constraints colorR might provide me and would greatly appreciate any help to understand/solve the issue. Best Regards, Cetin P/s: uname -a Linux linux-d312 2.6.25.16-0.1-default #1 SMP 2008-08-21 00:34:25 +0200 x86_64 x86_64 x86_64 GNU/Linux

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Heads Up: code.haskell.org is upgrading to darcs 2
by Duncan Coutts 14 Sep '08

14 Sep '08

This email is for darcs users in general and in particular for people who host a project on code.haskell.org. What we are doing ================= We are upgrading /usr/bin/darcs to version 2 on the machine that hosts code.haskell.org. That means it will be used by everyone who uses ssh to push or pull from a darcs repository on code.haskell.org. Pulling via http is completely unaffected. Do I have to do anything? ========================= No. You do not have to change anything. darcs v2 can talk to darcs v1 clients perfectly well so you do not need to upgrade your local version of darcs. You do not need to change the format of your local or server-side repositories. Darcs v2 works fine with darcs v1 format repositories. Should I change anything? ========================= We recommend that you upgrade your local darcs to version 2. This will allow you to take advantage of substantially faster push/pull operations over ssh (darcs 2 makes far fewer ssh connections). As noted above however it is not necessary for you to upgrade or for you to synchronise your upgrading with anyone else who uses the same repository. It is also possible to use a darcs "v1.5" hashed format locally and continue to use darcs v1 format on the server side. This has some performance and reliability advantages. Again this is something you can decide yourself without coordinating with other users of the repository. If your development style means that you bump into the infamous merging problems with darcs v1 format then you may consider converting your repository to the darcs v2 format. Remember that using darcs 2 with darcs v1 format repositories does not eliminate the merging issue. To banish the merging problems you have to use the darcs v2 format. This is however a more substantial change and has to be synchronised between users of the repository because it involves converting the server side repository to v2 format and all users getting the repository again. So if merging is an issue for you then you should discuss it with other users of your repository. Is this change safe? ==================== Yes. Darcs 2 has a substantial test suite. It has unit tests covering the core patch operations and also over 100 scripts doing functional testing, including network tests. The darcs build-bots run all these tests on all the popular platforms. Additionally, the code in darcs 2 for handling the v1 format is mostly the same as in darcs 1, so there are no big risks in using darcs 2 while continuing to use darcs v1 format repositories. The main change in the v1 format code is bug fixes, instrumentation code for debugging/introspection and more cunning types in the patch handling code that enforces some of the patch invariants. Furthermore, we have done real world tests using copies of all the 153 repositories on code.haskell.org which comes to around 2GB. We wanted to verify a couple things: 1. that using darcs 1 on the client and darcs 2 on the server works fine to pull and push patches. This corresponds to a project where all the users are still using darcs 1. 2. that using a mixture of darcs 1 and darcs 2 clients works when pushing and pulling patches between the clients via the server. This corresponds to a project where some users have upgraded but others have not yet. We tested with darcs 1.0.9 and 2.0.2 in three combinations: client darcs 1, server darcs 1 client darcs 1, server darcs 2 client darcs 2, server darcs 2 The test consisted of obliterating a significant number of patches from each repository and pushing them back. In a separate experiment each repository was converted to darcs v2 format which worked without problem in every case. What if I have problems? ======================== Contact support(a)community.haskell.org. We have made backups of all the repositories in case there are any critical problems. Thanks to the darcs hackers Eric Kow and Jason Dagit for doing all the hard testing work. Duncan, Ian and Malcolm (part of the community server admin team)

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static linking failure -- can't find crt0
by Jason Dusek 14 Sep '08

14 Sep '08

I'm building a "really static" executable on OS X with options `-optl-static -static` and, while the libraries seem to link fine, the executable itself does not -- it can't find the C runtime. This is actually by design -- Apple "does not support" static binaries. There is a work around -- just symlink stuff till the errors go away -- but it might be better if I could statically link only those C libraries that I knew were not on stock Macs. Are there GHC options for that? I assume this all works fine on Linux and Windows, but I'll know soon enough... -- _jsn |...can't find the C runtime.| ld_classic: can't locate file for: -lcrt0.o collect2: ld returned 1 exit status |"does not support"| http://developer.apple.com/qa/qa2001/qa1118.html

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