January 2010 - Haskell-Cafe

Re: classes with types which are wrapped in
by Andrew U. Frank 27 Jan '10

27 Jan '10

i thank for the two suggestions (newtype as a wrapper and type families) and i have explored them. haskell-cafe was very helpful! i use often a piece of code to update a part of a record with field names which codes as follows: data X a b c = X {afield :: a, bfield :: b, cfield :: c} dotoBfield :: (b -> b) -> X a b c -> X a b c dotoBfield op x = x { bfield = op (bfield x)} and similar for A and C. is there a better idiom to achieve the same effect? can this be automated (for example, using generics)?

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Maybe, maybe not.
by michael rice 27 Jan '10

27 Jan '10

Just noticed this difference in the definition of fromMaybe in two different places: http://haskell.org/ghc/docs/latest/html/libraries/base-4.2.0.0/src/Data-May… -- | The 'fromMaybe' function takes a default value and and 'Maybe' -- value. If the 'Maybe' is 'Nothing', it returns the default values; -- otherwise, it returns the value contained in the 'Maybe'. fromMaybe :: a -> Maybe a -> a fromMaybe d x = case x of {Nothing -> d;Just v -> v} and http://en.wikibooks.org/wiki/Haskell/Hierarchical_libraries/Maybe fromMaybe :: a -> Maybe a -> a fromMaybe z = maybe z id Michael

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ghc: unrecognised flags: -I
by Lyndon Maydwell 26 Jan '10

26 Jan '10

Hi Cafe! That's a capital "i" for anyone with font issues. I posted this question to beginners a while ago, but received no meaningful response so I'm trying cafe instead :-) Most packages I try to install off Hackage with cabal are giving me the error "ghc: unrecognised flags: -I". For example, when I "cabal install -v storable-complex" I get the following: /usr/bin/ghc -package-name storable-complex-0.2.1 --make -hide-all-packages -i -idist/build -i. -idist/build/autogen -Idist/build/autogen -Idist/build -I -optP-include -optPdist/build/autogen/cabal_macros.h -odir dist/build -hidir dist/build -stubdir dist/build -package base-3.0.3.1 -O Foreign.Storable.Complex ghc: unrecognised flags: -I I've updated everything several times since this error started occurring, but it persists. I've tried digging to find out where the lone -I is being injected but can't seem to track it down. Has anyone encountered this before, or more realistically, can anyone give me some advice on how to narrow this problem down further? I'm running cabal version 1.6.0.1, ghc 6.10.4 on OS X 10.5. Thanks guys!

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foldl in terms of foldr
by Xingzhi Pan 26 Jan '10

26 Jan '10

On Tue, Jan 26, 2010 at 11:24 PM, Eduard Sergeev <Eduard.Sergeev(a)gmail.com> wrote: > > > Xingzhi Pan wrote: >> >> The first argument to foldr is of type (a -> b -> a), which takes 2 >> arguments. But 'step' here is defined as a function taking 3 >> arguments. What am I missing here? > > You can think of step as a function of two arguments which returns a > function with one argument (although in reality, as any curried function, > 'step' is _one_ argument function anyway): > step :: b -> (a -> c) -> (b -> c) > > e.g. 'step' could have been defined as such: > step x g = \a -> g (f a x) > > to save on lambda 'a' was moved to argument list. Right. But then step is of the type "b -> (a -> c) -> (b -> c)". But as the first argument to foldr, does it agree with (a -> b -> a), which was what I saw when I type ":t foldr" in ghci? > -- > View this message in context: http://old.nabble.com/foldl-in-terms-of-foldr-tp27322307p27324376.html > Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -- Pan, Xingzhi http://www.panxingzhi.net

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Darcs fundraising 2010 - send us to ZuriHac! ($2000 by 15 Feb)
by Eric Y. Kow 26 Jan '10

26 Jan '10

Dear Haskellers, Since 2008, the Darcs Team has made a big push to find hacking time. We've been streamlining our patch approval processes, running Google Summer of Code projects and holding bi-annual hacking sprints. These efforts are starting to pay off, but we're not quite there yet and we still need your help! If you love Darcs and you want to see it succeed, if you want to see it get faster and easier to use, the best thing you can do is to send us your patches (how about attending a sprint?). But if you cannot do this, you can still support the cause. The 4th Darcs Hacking Sprint will be taking place in March as part of the Zurich Haskell Hackathon. Zurich presents a opportunity for us pull a lot of the Darcs team together, to recruit new developers to the project, and also to mingle with the wider Haskell community. We would love to subsidise travel costs to make it easier for developers to attend this sprint and the one after. Do you think you can help? How about sending just $20 our way? Donating to Darcs is now easier than ever, as we are now proud members of the Software Freedom Conservancy. You can either send us a cheque or use Paypal or Google Checkout. Furthermore, donations from US tax-payers will now be tax-deductible. More details below, or at <http://darcs.net/donations.html> Many thanks! Eric PS. If you could send me an email when you make a donation, I can provide a daily update on our fundraising status! Last year, we managed to raise a thousand dollars to send Darcs hackers to the Utrecht and Vienna hackathons. This year, I want to see if we can raise $2000 for Darcs hacking by the 14th of February. Stay tuned at http://darcs.net/donations.html for our progress. Only $1710 to go... :-) Donating by check (USA) ---------------------------------------------------------------------- If you have a US bank account, we prefer donations by check as they will go the longest way. Checks should be made payable to "Software Freedom Conservancy, Inc.", with "Directed donation: Darcs" in the memo. They can be mailed to Software Freedom Conservancy 1995 BROADWAY FL 17 NEW YORK NY 10023-5882 USA Donating by Paypal or Google Checkout ---------------------------------------------------------------------- Please visit http://darcs.net/donations.html for more details. -- Eric Kow <http://www.nltg.brighton.ac.uk/home/Eric.Kow> PGP Key ID: 08AC04F9

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foldl in terms of foldr
by Xingzhi Pan 26 Jan '10

26 Jan '10

Hi, I am reading Real World Haskell and have some questions about the piece of code implementing foldl in terms of foldr: -- file: ch04/Fold.hs myFoldl :: (a -> b -> a) -> a -> [b] -> a myFoldl f z xs = foldr step id xs z where step x g a = g (f a x) The first argument to foldr is of type (a -> b -> a), which takes 2 arguments. But 'step' here is defined as a function taking 3 arguments. What am I missing here? (Partial application? The order of execution?) Btw, is there a way I can observe the type signature of 'step' in this code? Thanks in advance! -- Pan, Xingzhi http://www.panxingzhi.net

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darcs 2.4 beta 3 release
by Reinier Lamers 26 Jan '10

26 Jan '10

Hi all, The darcs team would like to announce the immediate availability of darcs 2.4 beta 3. darcs 2.4 will contain many improvements and bugfixes compared to darcs 2.3.1. Highlights are the faster operation of record, revert and related commands, and the experimental interactive hunk editing. This beta is your chance to test-drive these improvements and make darcs even better. Compared to darcs 2.4 beta 2, the interface of interactive hunk editing has become more user-friendly. The easiest way to install darcs is using the Haskell Platform [1]. If you have installed the Haskell Platform or cabal-install, you can install this beta release by doing: $ cabal update $ cabal install --reinstall darcs-beta Alternatively, you can download the tarball from http://darcs.net/releases/darcs-beta-2.3.98.3.tar.gz , and build it by hand as explained in the README file. Interactive hunk editing ------------------------ To try out interactive hunk editing, press 'e' when you are prompted with a hunk patch by 'darcs record'. You will then be shown an editor screen in which you can edit the state you want to record between the last two ruler lines. You can find more information about the hunk editing feature on http://wiki.darcs.net/HunkEditor . Reporting bugs -------------- If you have an issue with darcs 2.4 beta 3, you can report it via the web on http://bugs.darcs.net/ . You can also report bugs by email to bugs(a)darcs.net. What's New ---------- A list of important changes since 2.3.1 is as follows (please let me know if there's something you miss!): * Use fast index-based diffing everywhere (Petr) * Interactive patch splitting (Ganesh) * An 'optimize --upgrade' option to convert to hashed format in-place (Eric) * Hunk matching (Kamil Dworakowski, tat.wright) * Progress reporting is no longer deceptive (Roman) * A 'remove --recursive' option to remove a directory tree from revision control (Roman) * 'show files' accepts arguments to show a subset of tracked files (Luca) * A '--remote-darcs' flag for pushing to a host where darcs isn't called darcs * Many miscellaneous Windows improvements (Salvatore, Petr and others) * 'darcs send' now mentions the repository name in the email body (Joachim) * Handle files with boring names in the repository correctly (Petr) * Fix parsing of .authorspellings file (Tomáš) * Various sane new command-line option names (Florent) * Remove the '--checkpoint' option (Petr) * Use external libraries for all UTF-8 handling (Eric, Reinier) * Use the Haskell zlib package exclusively for compression (Petr) A list of issues resolved since 2.3.1: * 183: do not sort changes --summary output * 223: add --remote-darcs flag to specify name of remote darcs executable * 291: provide (basic) interactive patch splitting * 540: darcs remove --recursive * 835: 'show files' with arguments * 1122: get --complete should not offer to create a lazy repository * 1216: list Match section in ToC * 1224: refuse to convert a repo that's already in darcs-2 format * 1300: logfile deleted on unsucessful record * 1308: push should warn about unpulled patches before patch-selection * 1336: sane error message on --last "" (empty string to numbers parser) * 1362: mention repo name in mail send body * 1377: getProgname for local darcs instances * 1392: use parsec to parse .authorspelling * 1424: darcs get wrongly reports "using lazy repository" if you ctrl-c old-fashioned get * 1447: different online help for send/apply --cc * 1488: fix crash in whatsnew when invoked in non-tracked directory * 1548: show contents requires at least one argument * 1554: allow opt-out of -threaded (fix ARM builds) * 1563: official thank-you page * 1578: don't put newlines in the Haskeline prompts * 1583: on darcs get, suggest upgrading source repo to hashed * 1584: provide optimize --upgrade command * 1588: add --skip-conflicts option * 1594: define PREPROCHTML in makefile * 1620: make amend leave a log file when it should * 1636: hunk matching * 1643: optimize --upgrade should do optimize * 1652: suggest cabal update before cabal install * 1659: make restrictBoring take recorded state into account * 1677: create correct hashes for empty directories in index * 1681: preserve log on amend failure * 1709: fix short version of progress reporting * 1712: correctly report number of patches to pull * 1720: fix cabal haddock problem * 1731: fix performance regression in check and repair Kind Regards, the darcs release manager, Reinier Lamers [1]: You can download the Haskell platform from http://hackage.haskell.org/platform/

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Job opportunities at Citrix Systems (Cambridge, UK)
by Matthias Görgens 26 Jan '10

26 Jan '10

Hi folks, I have recently started working for Citrix in Cambridge. We are working on the open source software in Ocaml. Admittedly Ocaml is only a second best compared to Haskell ;o) and I hope my post does not count as too off-topic here. But Ocaml is still a decent language. My experience was primarily with Haskell and I only started working with Ocaml in earnest at Citrix. (So I hope that showing a way to transplant Haskell skills to the real world --- via the roundabout route of Ocaml --- justifies my posting.) So --- if you are looking for a job in Cambridge that involves functional programming, free software [1] and free beer [2], please feel free to drop me a line. I have included our original advert to the Ocaml mailing list for those who want more information first. Cheers, Matthias. [1] As in free speech. LGPL [2] As in free beer. We also get other perks, like snacks and a pool table. ---------- Forwarded message ---------- From: Dave Scott <Dave.Scott(a)eu.citrix.com> Date: 2010/1/18 Subject: [Caml-list] [jobs] Citrix Systems (Cambridge, UK) To: "caml-list(a)yquem.inria.fr" <caml-list(a)yquem.inria.fr>, "ocaml-jobs(a)inria.fr" <ocaml-jobs(a)inria.fr> Dear fellow OCaml users, Summary: We (Citrix Systems) are looking for OCaml programmers to join our team in Cambridge, UK. We use OCaml extensively in the "xapi tool-stack": the control-plane used in the Xen Cloud Platform[1] on which the widely used XenServer server virtualisation product[2] is based. XCP aims to provide a complete open-source cloud infrastructure platform with a powerful management stack based on standards-based APIs, support for multi-tenancy, SLA guarantees and detailed metrics for consumption based charging. We are looking to recruit top-class engineers to work on the tool-stack; applicants must have a good knowledge of data structures and algorithms, experience of programming in the context of large systems and general aesthetic good taste when it comes to code and architecture. Our code base is significant and varied: over 130,000 lines of OCaml, solving problems ranging from the low-level (Xen hypercalls) to the high-level (resource pool management), to the compiler-driven (generating language bindings for our Xen datamodel). Our ideal candidate will have: * significant experience of applications programming in high-level languages (such as OCaml :-) * an aptitude for implementing (and reasoning about) complex concurrent, distributed systems * the skills required to contribute to both the architectural design and day-to-day development of a large code-base * strong communication skills and problem solving ability * a determination to deliver great products that perform brilliantly and meet our customers' needs So if you want to tackle interesting and challenging programming problems and contribute to an innovative, fast-growing product that is already used by tens of thousands of customers across the world, please don't hesitate to send me your CV. [1] http://www.xen.org/products/cloudxen.html [2] http://www.citrix.com/English/ps2/products/feature.asp?contentID=1686939 Thanks, -- Dave Scott <dave.scott(a)eu.citrix.com>

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Re: Codensity improvement of free monads
by Janis Voigtländer 26 Jan '10

26 Jan '10

> On Mon, Jan 25, 2010 at 7:37 PM, Luke Palmer <lrpalmer(a)gmail.com> wrote: >> Hello haskell-cafe, >> >> I have just read "Asymptotic Improvement of Computations over Free >> Monads" by Janis Voigtlander, since I have been working with free >> monads a lot recently and don't want to get hit by their quadratic >> performance when I start to care about that. >> >> But after reading the paper, I still don't really understand how >> "improve" actually improves anything. Can anyone provide a good >> explanation for where the work is saved? Edward gives the perfect explanation below. Thanks, Janis. > With a free monad, the structure keeps growing via substitution. This > requires you on each bind to re-traverse the common root of the free monad, > which isn't changing in each successive version. > > Lets say the size of this accumulated detritus increases by one each time. > Then you will be traversing over 1, 2, 3, 4, ... items to get to the values > that you are substituting as you keep binding your free monadic > computation. > The area near the root of your structure keeps getting walked over and > over, > but there isn't any work do to there, the only thing bind can do is > substitution, and all the substitution is down by the leaves. > > On the other hand, when you have CPS transformed it, at each layer you only > have to climb over the 1 item you just added to get to where you apply the > next computation. > > This only gets better when you are dealing with free monads that provide > multiple points for extension: i.e. that grow in a treelike fashion like: > > data Bin a = Bin a a > data Free f a = Return a | Free (f (Free f a)) > data Codensity f a = Codensity (forall r. (a -> f r) -> f r) > > If you build the free monad Free Bin, then you will wind up having to walk > all the way down to the leaves on each bind, well, modulo demand, that is. > But since it is a free monad, the 'body' of the tree will never change. > Just > the data you are substituting at the leaves. Codensity (Free Bin) lets you > only generate the body of the tree once, while your substitutions just get > pushed down to the leaves in one pass. > > An interesting exercise is to work out why Codensity can change the > asymptotics of certain operations over Free Bin, but Density doesn't change > the asymptotics of anything non-trivial over Cofree Bin for the better. > > -Edward Kmett > -- Jun.-Prof. Dr. Janis Voigtländer http://www.iai.uni-bonn.de/~jv/ mailto:jv@iai.uni-bonn.de

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OT: How a Common Lisp user views other programming languages
by Scott Michel 26 Jan '10

26 Jan '10

Off topic, but funny: http://kvardek-du.kerno.org/2010/01/how-common-lisp-programmer-views-users.… The Haskell icon is very apt! -scooter

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