September 2010 - Haskell-Cafe

Re: ANNOUNCE: countable-0.1
by Maciej Piechotka 06 Sep '10

06 Sep '10

On Mon, 2010-09-06 at 03:54 -0700, Ashley Yakeley wrote: > countable: Countable, Searchable, Finite, Empty classes. > > class Countable, for countable types > class AtLeastOneCountable, for countable types that have at least one > value > class InfiniteCountable, for infinite countable types > class Searchable, for types that can be searched over > class Finite, for finite types > class Empty, for empty types > data Nothing, an empty type > > Also includes these orphan instances: > > instance (Searchable a,Eq b) => Eq (a -> b) > instance (Finite a) => Foldable ((->) a) > instance (Finite a) => Traversable ((->) a) > instance (Show a,Finite a,Show b) => Show (a -> b) > > It turns out Searchable includes some infinite types. Specifically, > countable implements this: > > instance (Countable c,Searchable s) => Searchable (c -> s) > > using the algorithm described here: > > http://math.andrej.com/2007/09/28/seemingly-impossible-functional-programs/ > > I would welcome improvements. > > http://hackage.haskell.org/package/countable-0.1 > cabal install countable > darcs get http://code.haskell.org/countable/ > > See also this thread: > http://thread.gmane.org/gmane.comp.lang.haskell.cafe/73275 > Hmm. 1. Code formatting is very C#-like. At least for me it is hard to read the code that way (it IS matter of preference however) 2. countMaybeNext seems to need documentation. If I understand it correctly: countMaybeNext Nothing = 'minimalValue' countMaybeNext (Just x) = x + 1 Wouldn't be split it to: countNext :: a -> Maybe a initial :: Maybe a 3. Regards

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ANNOUNCE: Haddock version 2.8.0
by David Waern 06 Sep '10

06 Sep '10

-------------------------------------------- -- Haddock 2.8.0 -------------------------------------------- A new version of Haddock, the Haskell documentation tool, is out! The biggest news this time is that we have a shiny new XHTML backend, created by Mark Lentczner, which outputs semantically correct XHTML, making it much easier to create new themes for Haddock. Mark has made an extraordinary effort by going through and rewriting the old table based HTML backend, fixing non-standards-compliant quirks, and adding a bunch of new useful features on top of that. See the changelog for more details. Included is a new default CSS theme created by Thomas Schilling, Mark and Johan Tibell, as well as the classic theme converted to work with the new backend. Another great new feature is markup support for examples, contributed by Simon Hengel. The idea is to be able to write examples that function both as documentation and unit tests. Last but not least, we now also have a LaTeX backend. It was written by Simon Marlow and it was used to generate the libraries section of the Haskell 2010 report. This version is compatible with .haddock files produced by Haddock 2.6.1 and above, provided that the version of GHC used to build Haddock stays the same. -------------------------------------------- -- Changes in version 2.8.0 -------------------------------------------- * HTML backend completely rewritten to generate semantically rich XHTML using the xhtml package. * New default CSS based on the color scheme chosen for the new Haskell wiki, with a pull-out tab for the synopsis. * Theme engine based on CSS files. Themes can be switched from the header menu. (New flags --built-in-themes and --theme. The latter is an alias for --css which now has extended semantics). * Markup support for executable examples/unit-tests. To be used with an upcoming version of the DocTest program. * Addition of a LaTeX backend. * Frames-mode can be enabled from the header menu. * Path to source entities can be specified per package, so that source links work for cross-package documentation. * Support for a second form of enumerated lists (1. 2. etc). * Additions and changes to the Haddock API. * New flag --no-tmp-comp-dir to tell Haddock to read/write compilation files (.o, .hi, etc) to/from GHC's output directory instead of to/from a temporary directory. * Various bug fixes. -------------------------------------------- -- Links -------------------------------------------- Homepage: http://www.haskell.org/haddock Hackage page: http://hackage.haskell.org/package/haddock-2.8.0 Bugtracker and wiki: http://trac.haskell.org/haddock Mailing list: haddock(a)projects.haskell.org Code repository: http://code.haskell.org/haddock -------------------------------------------- -- Contributors -------------------------------------------- The following people contributed patches to this release: Simon Hengel Mark Lentczner Ian Lynagh Simon Marlow Simon Peyton-Jones Thomas Schilling David Waern -------------------------------------------- -- Get Involved -------------------------------------------- We would be very happy to get more contributors. To get involved, start by grabbing the code: http://code.haskell.org/haddock Then take a look at the bug and feature tracker for things to work on: http://trac.haskell.org/haddock

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Re: [Haskell-cafe] Unnecessarily strict implementations
by Daniel Fischer 06 Sep '10

06 Sep '10

On Friday 03 September 2010 12:28:43, Jan Christiansen wrote: > Hi, > > On 03.09.2010, at 01:32, Daniel Fischer wrote: > > No surprise, there aren't many 'ä's in Shakespeare's works, are there? > > nope > > > On the other hand, the current implementation of lines does not seem > > to > > suffer from Wadler's tuple space leak (according to one test I > > made), so > > I'd stick with the current implementation for the time being. > > Well, I think it does. Indeed. I botched my test, allowing it to drop the reference to the first line too early by not using enough of it, so although lines shows the leak, let { ds1_s1ts :: ([Char], [Char]) LclId ds1_s1ts = case $wbreak @ Char lvl_r1uj wild_B1 of _ { (# ww1_anp, ww2_anq #) -> (ww1_anp, ww2_anq) } } in : @ String (case ds1_s1ts of _ { (l_aij, _) -> l_aij }) (case ds1_s1ts of _ { (_, s'_ail) -> case s'_ail of _ { , I managed to conceal it. > But obviously one can argue that this is a rare application. Yes. Ordinarily, lines in text files aren't longer than a few hundred characters, leaking those, who cares? But. Occasionally, long lines occur, and avoiding the space leak seems more important to me than having lines (_|_ : _|_) = _|_ : _|_ instead of lines (_|_ : _|_) = _|_ > Hopefully I haven't made a mistake here? No. > > By accident I stumbled across an odd behaviour. If I use the following > definition and replace all occurrences of break by break' in the > program above the memory behaviour is bad if I compile it without > profiling. But if I compile the program with -prof -auto-all the > program runs in constant space. Is this a known behaviour? I can't reproduce that. For me, it leaks also with profiling. > > break' :: (a -> Bool) -> [a] -> ([a],[a]) > break' p xs = (ys,zs) > where > (ys,zs) = break p xs > > Cheers, Jan

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AngloHaskell 2010 - Final Reminder + Wi-Fi Sign-Up
by Derek Wright 06 Sep '10

06 Sep '10

AngloHaskell 2010 is this week! AngloHaskell 2010 - 5th Annual Haskell Meeting in England Friday September 10th and Saturday September 11th, 2010 Microsoft Research, Cambridge, UK http://www.haskell.org/haskellwiki/AngloHaskell/2010 AngloHaskell is a free gathering of all people Haskell-related from beginners to seasoned hackers, both academic and industrial. AngloHaskell 2010 is generously hosted by Microsoft Research. As a result no charge is made for attending or speaking. Friday will be a day of talks and discussion at MSR followed by a more informal day of activities on Saturday somewhere else in Cambridge. AngloHaskell welcomes talks on all Haskell related subjects. From short comments (Functional Grit) to full presentations, all are welcome. Please add your name and the title of your talk to the wiki page (or email me and I will add it for you). If you want to attend please add your name to the wiki page, or let me know by email. Even if you can't attend you can add you name to the "Not this year, but hopefully next!" section. Wi-Fi accounts (at Microsoft Research on Friday) are available on request. The sign-up deadline is Wednesday 8th September so please sign-up on the wiki page or email me (with Full Name, Institution, Country of Residence and Email Address). Organisational discussions are on held on the #anglohaskell channel on the Freenode IRC network, feel free to drop in with questions or comments.

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Re: [Haskell-cafe] help me evangelize haskell.
by Mathew de Detrich 06 Sep '10

06 Sep '10

I Think you misinterpreted what I said. I didn't say you should tell the programmers how to code, I said you should show the perl coders how Haskell has advantages over pearls without much cost On 06/09/2010 5:21 PM, "Stephen Tetley" <stephen.tetley(a)gmail.com> wrote: On 6 September 2010 03:46, Mathew de Detrich <deteego(a)gmail.com> wrote: > If they are perl programme... Now that's going to go down well... Maybe the original poster might want to soak up the company culture for a while before than telling co-workers how to do things. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org h...

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Re: ANNOUNCE: countable-1.0
by Nicolas Pouillard 06 Sep '10

06 Sep '10

On Mon, 06 Sep 2010 03:46:18 -0700, Ashley Yakeley <ashley(a)semantic.org> wrote: > countable: Countable, Searchable, Finite, Empty classes. > > class Countable, for countable types > class AtLeastOneCountable, for countable types that have at least one > value > class InfiniteCountable, for infinite countable types > class Searchable, for types that can be searched over > class Finite, for finite types > class Empty, for empty types > data Nothing, an empty type > > Also includes these orphan instances: > > instance (Searchable a,Eq b) => Eq (a -> b) > instance (Finite a) => Foldable ((->) a) > instance (Finite a) => Traversable ((->) a) > instance (Show a,Finite a,Show b) => Show (a -> b) Could you put these instances in a dedicated module? In the same vein I would like to have a newtype wrapper over functions were intention is to be used extensionally. Nice package BTW! -- Nicolas Pouillard http://nicolaspouillard.fr

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PhD position at the Software Technology group at Utrecht University
by Atze Dijkstra 06 Sep '10

06 Sep '10

There is currently a PhD position available at he Software Technology group, Department of Information and Computing Sciences, Utrecht University, Netherlands. The ST group focusses its research on programming methodologies, compiler construction, and program analysis. In this area the research will take place, in particular the topic is semi-automatic incrementalization of programs narrowed down to our Utrecht Haskell Compiler (UHC), using our Attribute Grammar system (UUAG). For more details and application see http://www.academictransfer.com/employer/UU/vacancy/6114/lang/en/ The application deadline is October 5, 2010. Please contact Prof. Dr. S.D. Swierstra at doaitse(a)cs.uu.nl for more information. The project is funded by a research grant from the Netherlands 'Organization for Scientific Research’ (NWO). Best regards, - Atze - Atze Dijkstra, Department of Information and Computing Sciences. /|\ Utrecht University, PO Box 80089, 3508 TB Utrecht, Netherlands. / | \ Tel.: +31-30-2534118/1454 | WWW : http://www.cs.uu.nl/~atze . /--| \ Fax : +31-30-2513971 .... | Email: atze(a)cs.uu.nl ............ / |___\

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help me evangelize haskell.
by Michael Litchard 06 Sep '10

06 Sep '10

I'll be starting a new job soon as systems tool guy. The shop is a perl shop as far as internal automation tasks go. But I am fortunate to not be working with bigots. If they see a better way, they'll take to it. So please give me your best arguments in favor of using haskell for task automation instead of perl, or awk or any of those scripting lanugages.

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Re: [Haskell-cafe] Unnecessarily strict implementations
by Henning Thielemann 05 Sep '10

05 Sep '10

Ivan Lazar Miljenovic schrieb: > On 3 September 2010 04:57, Arie Peterson <ariep(a)xs4all.nl> wrote: >> On Thu, 2 Sep 2010 19:30:17 +0200, Daniel Fischer >> <daniel.is.fischer(a)web.de> wrote: >>> Why would one consider using Ord for Map an abuse? >>> A kludge, for performance reasons, but an abuse? >> Because it forces one to declare Ord instances for types which have no >> natural ordering. It is useful to *not* have such instances, in order to >> catch programming errors. > > What precisely do you mean by natural ordering? E.g. I wanted to have a Set of Gaussian (complex) integers, but I did not want to define an Ord instance for them, because writing a < (b :: Gaussian) is a bug with high probability. >> A separate type class for types which can be ordered in some (possibly >> arbitrary) way, for use in Data.Map, would remedy this. > > Sure... except that the way Data.Map and Data.Set are implemented is > by a binary tree, and you typically want some kind of ordering for > those. > > How is a type class that represents arbitrary ordering any different > from what we already have? The notation might not be the best if you > consider the ordering to be arbitrary, but what else would you use? > "isArbitrarilyBefore :: (ArbitraryOrdering a) => a -> a -> Bool" ? Yes, something this way. (<) suggests a notion of magnitude for me, which some orderings do not have.

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base-3 -> base-4
by Johannes Waldmann 05 Sep '10

05 Sep '10

Is there a "conversion guide" for switching from base-3 to base-4? I noticed that ghc-HEAD (6.13) comes with base-4.3 (and no base-3). This will break an awful lot of packages (?), in fact the breakage starts with cabal-install (since it depends on HTTP which depends on base-3). PS: I use gmane.org for posting, and they have these seemingly random "text captchas", and for this post, it reads "continuity"...

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