January 2012 - Haskell-Cafe

ANN: pointless fun
by wren ng thornton 29 Jan '12

29 Jan '12

-------------------------------------------- -- pointless-fun 1.1.0 -------------------------------------------- The pointless-fun package offers some common point-free combinators (common for me at least). -------------------------------------------- -- Long Description -------------------------------------------- * Perhaps the most useful is that it packages up Matt Helige's classic multicomposition trick[1]. These combinators allow you to easily modify the types of a many-argument function with syntax that looks like giving type signatures. For example, > foo :: A -> B -> C > > albert :: A -> X > beth :: B -> Y > carol :: C -> Z > > bar :: X -> Y -> Z > bar = foo $:: albert ~> beth ~> carol I've found this to be especially helpful for defining non-derivable type class instances for newtypes since it both abstracts away the plumbing and also makes explicit what you mean. * Other prevalent combinators include, ** (.:) for binary composition: > (f .: g) x y = f (g x y) > -- or, > f .: g = curry (f . uncurry g) This is the same as the common idiom @(f .) . g@ but more easily extended to multiple uses, due to the fixity declaration. ** (.!) for function composition which calls the right-hand function eagerly; i.e., making the left-hand function strict in its first argument. > (f .! g) x = f $! g x This defines the composition for the sub-category of strict Haskell functions. If the 'Functor' class were parameterized by the domain and codomain categories (e.g., a regular @Functor f@ would be @CFunctor (->) (->) f@ instead) then this would allow us to define functors @CFunctor (->) (!->) f@ where @fmap f . fmap g = fmap (f .! g)@. [1] http://matt.immute.net/content/pointless-fun -------------------------------------------- -- Links -------------------------------------------- Homepage: http://code.haskell.org/~wren/ Hackage: http://hackage.haskell.org/package/pointless-fun Darcs: http://community.haskell.org/~wren/pointless-fun Haddock (Darcs version): http://community.haskell.org/~wren/pointless-fun/dist/doc/html/pointless-fun -- Live well, ~wren

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Reminder Munich Haskell Meeting
by Heinrich Hördegen 29 Jan '12

29 Jan '12

Dear all, just to remind you: Tomorrow, Monday 30. Jan 2012, Haskellers meet in Munich. Check out the details and click the button, if you plan to join: http://www.haskell-munich.de/dates See you, Heinrich -- -- hoerdegen(a)funktional.info www.funktional.info --

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ANN: data-or
by wren ng thornton 29 Jan '12

29 Jan '12

-------------------------------------------- -- data-or 1.0.0 -------------------------------------------- The data-or package offers a data type for non-exclusive disjunction. This is helpful for things like a generic merge function on sets/maps which could be union, mutual difference, etc. based on which 'Or' value a function argument returns. Also useful for non-truncating zips (cf. 'zipOr') and other cases where you sometimes want an 'Either' and sometimes want a pair. -------------------------------------------- -- Links -------------------------------------------- Homepage: http://code.haskell.org/~wren/ Hackage: http://hackage.haskell.org/package/data-or Darcs: http://community.haskell.org/~wren/data-or Haddock (Darcs version): http://community.haskell.org/~wren/data-or/dist/doc/html/data-or -- Live well, ~wren

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Named captures in regex-pcre?
by Ilya Portnov 28 Jan '12

28 Jan '12

Hi haskell-cafe. Is there a way to get named captures from regex using regex-pcre (or maybe other PCRE-package)? For example, I want to write something like let result = "ab 12 cd" =~ "ab (?P<number>\d+) cd" :: SomeCrypticType and then have namedCaptures result == [("number", "12")]. I do not see somewhat similar in regex-pcre documentation. It parses such regexs fine, and captures work, but i do not see way to get _named_ captures. WBR, Ilya Portnov.

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Re: [Haskell-cafe] Natural Transformations and fmap
by Janis Voigtländer 28 Jan '12

28 Jan '12

Ryan Ingram wrote: > However, the type of natural transformations comes with a free theorem, for > example > > concat :: [[a]] -> [a] > > has the free theorem > > forall f :: a -> b, f strict and total, fmap f . concat = concat . fmap > (fmap f) > > The strictness condition is needed; consider > > broken_concat :: [[a]] -> [a] > broken_concat _ = [undefined] > f = const () > > fmap f (broken_concat []) = fmap f [undefined] = [()] > broken_concat (fmap (fmap f) []) = broken_concat [] = [undefined] > > The 'taming selective strictness' version of the free theorem generator[1] > allows removing the totality condition on f, but not the strictness > condition. > > But in the case of concat, you can prove a stronger theorem: > > forall f :: a -> b, fmap f . concat = concat . fmap (fmap f) > > My suspicion is that this stronger theorem holds for all strict and total > natural transformations, but I don't know how to go about proving that > suspicion. I'm a hobbyist mathematician and a professional programmer, not > the other way around:) > > ... > > [1]http://www-ps.iai.uni-bonn.de/cgi-bin/polyseq.cgi There is an analogous approach to the "taming selective strictness" version of the free theorem generator where it is general recursion that is tamed. In that setting, you then get free theorems like that for concat without either strictness or totality side conditions. It is really very similar, indeed the "taming selective strictness" work takes over and develops further the much earlier "taming general recursion" ideas. The original source for the latter is: http://dblp.uni-trier.de/rec/bibtex/conf/esop/LaunchburyP96 Just nobody ever bothered to implement it in a tool. (Well, actually, such an implementation is essentially hidden inside the counterexample generator http://www-ps.iai.uni-bonn.de/cgi-bin/exfind.cgi) Best, Janis. -- Jun.-Prof. Dr. Janis Voigtländer http://www.iai.uni-bonn.de/~jv/ mailto:jv@iai.uni-bonn.de

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Something's weird about System.Directory.removeDirectoryRecursive
by John Millikin 28 Jan '12

28 Jan '12

http://hackage.haskell.org/packages/archive/directory/1.1.0.1/doc/html/Syst… The documentation says that removeDirectoryRecursive follows symlinks. However, the implementation doesn't (in most cases, see below). ------------------------------------------------------------------------------------------------------------ $ mkdir test-removeDirectoryRecursive $ cd test-removeDirectoryRecursive $ mkdir a b $ touch a/a.txt b/b.txt $ ln -s $PWD/b a/ $ ln -s $PWD/b/b.txt a/ $ ls -l a total 8.2k -rw-rw-r-- 1 john john 0 2012-01-27 23:33 a.txt lrwxrwxrwx 1 john john 65 2012-01-27 23:33 b -> /home/john/test-removeDirectoryRecursive/b lrwxrwxrwx 1 john john 71 2012-01-27 23:34 b.txt -> /home/john/test-removeDirectoryRecursive/b/b.txt # OK, a/ has a normal file and two symlinks in it. Let's recursively remove a/ and see what happens. $ ghci Prelude> import System.Directory Prelude System.Directory> removeDirectoryRecursive "a" Prelude System.Directory> Leaving GHCi. $ ls -l a b ls: cannot access a: No such file or directory b: total 0 -rw-rw-r-- 1 john john 0 2012-01-27 23:33 b.txt # a/ was removed -- good! # # b/ and its contents are untouched, good, but goes against the docs ------------------------------------------------------------------------------------------------------------ Now, there is one case where this function *will* follow symlinks. However, I believe it is a bug because it produces odd behavior: ------------------------------------------------------------------------------------------------------------ $ sudo mkdir a [sudo] password for john: $ sudo ln -s $PWD/b a/ $ ls -l a b a: total 4.1k lrwxrwxrwx 1 root root 65 2012-01-27 23:38 b -> /home/john/test-removeDirectoryRecursive/b b: total 0 -rw-rw-r-- 1 john john 0 2012-01-27 23:33 b.txt # Now a/ has a symlink, which cannot be deleted, because its containing directory is read-only to the current user. $ rm a/b rm: remove symbolic link `a/b'? y rm: cannot remove `a/b': Permission denied # What happens if removeDirectoryRecursive is called now? $ ghci Prelude> import System.Directory Prelude System.Directory> removeDirectoryRecursive "a" *** Exception: a/b: removeDirectory: permission denied (Permission denied) Prelude System.Directory> Leaving GHCi. $ ls -l a b a: total 4.1k lrwxrwxrwx 1 root root 65 2012-01-27 23:38 b -> /home/john/test-removeDirectoryRecursive/b b: total 0 # a/ is untouched, but b/ has been emptied! ------------------------------------------------------------------------------------------------------------ So what is the expected behavior of this function? What should it do in the presence of symlinks? IMO, the function should be documented as *not* following symlinks, and the directory check should be changed so that it returns False for symlink-to-directory.

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Natural Transformations and fmap
by Ryan Ingram 28 Jan '12

28 Jan '12

I've been playing around with the relationship between monoids and monads (see http://www.jonmsterling.com/posts/2012-01-12-unifying-monoids-and-monads-wi… http://blog.sigfpe.com/2008/11/from-monoids-to-monads.html) and I put together my own implementation which I'm quite happy with, that you can see at http://hpaste.org/56903 ; relying only on the extensions RankNTypes, TypeOperators, NoImplicitPrelude, ScopedTypeVariables; At the end of that paste, I prove the three Haskell monad laws from the functor laws and "monoid"-ish versions of the monad laws, but my proofs all rely on a property of natural transformations that I'm not sure how to prove; given type m :-> n = (forall x. m x -> n x) class Functor f where fmap :: forall a b. (a -> b) -> f a -> f b -- Functor identity law: fmap id = id -- Functor composition law fmap (f . g) = fmap f . fmap g Given Functors m and n, natural transformation f :: m :-> n, and g :: a -> b, how can I prove (f . fmap_m g) = (fmap_n g . f)? Is there some more fundamental law of natural transformations that I'm not aware of that I need to use? Is it possible to write a natural transformation in Haskell that violates this law? -- ryan

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Re: [Haskell-cafe] Some thoughts on Type-Directed Name
by Greg Weber 27 Jan '12

27 Jan '12

Hi guys, There is an effort underway to make Haskell's Records better. The discussion is ongoing on the ghc-users mail list, feel free to join, or at least read on gmane. The wiki pages are up to date. Please note that we are moving in the direction of making the most minimal changes possible to achieve some simple record name-spacing. Thanks, Greg Weber

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Terminology: different levels of strictness
by Yves Parès 27 Jan '12

27 Jan '12

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Call for Papers - Haskell Symposium 2012
by Janis Voigtländer 27 Jan '12

27 Jan '12

======================================================================== "Haskell 2012" ACM SIGPLAN Haskell Symposium 2012 Copenhagen, Denmark 13th September, 2012 CALL FOR PAPERS http://www.haskell.org/haskell-symposium/2012/ ======================================================================== The ACM SIGPLAN Haskell Symposium 2012 will be co-located with the 2012 International Conference on Functional Programming (ICFP), in Copenhagen, Denmark. The purpose of the Haskell Symposium is to discuss experiences with Haskell and future developments for the language. Topics of interest include, but are not limited to: * Language Design, with a focus on possible extensions and modifications of Haskell as well as critical discussions of the status quo; * Theory, such as formal treatments of the semantics of the present language or future extensions, type systems, and foundations for program analysis and transformation; * Implementations, including program analysis and transformation, static and dynamic compilation for sequential, parallel, and distributed architectures, memory management as well as foreign function and component interfaces; * Tools, in the form of profilers, tracers, debuggers, pre-processors, testing tools, and suchlike; * Applications, using Haskell for scientific and symbolic computing, database, multimedia, telecom and web applications, and so forth; * Functional Pearls, being elegant, instructive examples of using Haskell; * Experience Reports, general practice and experience with Haskell, e.g., in an education or industry context. Papers in the latter three categories need not necessarily report original research results; they may instead, for example, report practical experience that will be useful to others, reusable programming idioms, or elegant new ways of approaching a problem. (Links with more advice appear on the symposium web page.) The key criterion for such a paper is that it makes a contribution from which other Haskellers can benefit. It is not enough simply to describe a program! Regular papers should explain their research contributions in both general and technical terms, identifying what has been accomplished, explaining why it is significant, and relating it to previous work (also for other languages where appropriate). In addition, we solicit proposals for system demonstrations, based on running (perhaps prototype) software rather than necessarily on novel research results. Such short demo proposals should explain why a demonstration would be of interest to the Haskell community. Travel Support: =============== Student attendees with accepted papers can apply for a SIGPLAN PAC grant to help cover travel expenses. PAC also offers other support, such as for child-care expenses during the meeting or for travel costs for companions of SIGPLAN members with physical disabilities, as well as for travel from locations outside of North America and Europe. For details on the PAC programme, see its web page (http://www.sigplan.org/PAC.htm) Proceedings: ============ There will be formal proceedings published by ACM Press. In addition to printed proceedings, accepted papers will be included in the ACM Digital Library. Authors must transfer copyright to ACM upon acceptance (for government work, to the extent transferable), but retain various rights (http://www.acm.org/publications/policies/copyright_policy) Authors are encouraged to publish auxiliary material with their paper (source code, test data, etc.); they retain copyright of auxiliary material. Accepted demo proposals, assessed for relevance by the PC, will be published on the symposium web page, but not formally published in the proceedings. Submission Details: =================== * Abstract Submission: 31st May 2012 * Submission Deadline: 3rd June 2012, 11:00 am, UTC * Author Notification: 27th June 2012 * Final Papers Due : 10th July 2012 Submitted papers should be in portable document format (PDF), formatted using the ACM SIGPLAN style guidelines (9pt format, more details appear on the symposium web page). The length is restricted to 12 pages, except for "Experience Report" papers, which are restricted to 6 pages. Each paper submission must adhere to SIGPLAN's republication policy, as explained on the web. Demo proposals are limited to 2-page abstracts, in the same format. "Functional Pearls", "Experience Reports", and "Demo Proposals" should be marked as such with those words in the title at time of submission. The paper submission deadline and length limitations are firm. There will be no extensions, and papers violating the length limitations will be summarily rejected. Programme Committee: ==================== * Amal Ahmed, Northeastern University * Jost Berthold, University of Copenhagen * Nils Anders Danielsson, University of Gothenburg * Iavor Diatchki, Galois Inc. * Jeremy Gibbons, University of Oxford * Jurriaan Hage, Utrecht University * Zhenjiang Hu, National Institute of Informatics Tokyo * Daan Leijen, Microsoft Research * Ben Lippmeier, University of New South Wales * Simon Peyton Jones, Microsoft Research * Colin Runciman, University of York * Eijiro Sumii, Tohoku University * Janis Voigtländer (chair), University of Bonn * Brent Yorgey, University of Pennsylvania Links: ====== * http://www.haskell.org/haskell-symposium (the permanent web page of the Haskell Symposium) * http://www.haskell.org/haskell-symposium/2012 (the 2012 Haskell Symposium web page) * http://www.icfpconference.org/icfp2012 (the ICFP 2012 web page) -- Jun.-Prof. Dr. Janis Voigtländer http://www.iai.uni-bonn.de/~jv/ mailto:jv@iai.uni-bonn.de

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