- Haskell-Cafe - Haskell.org

I Need a Better Functional Language!
by Grigory Sarnitskiy 12 Apr '12

12 Apr '12

Hello! I've just realized that Haskell is no good for working with functions! First, what are 'functions' we are interested at? It can't be the usual set-theoretic definition, since it is not constructive. The constructive definition should imply functions that can be constructed, computed. Thus these are computable functions that should be of our concern. But computable functions in essence are just a synonym for programs. One could expect from a language that bears 'functional' as its characteristic to be able to do everything imaginable with functions. However, the only thing Haskell can do with functions is to apply them to arguments and to feed them as arguments, run in parallel (run and concatenate programs). Obviously, that's not all of the imaginable possibilities. One also can rewrite programs. And write programs that rewrite programs. And write programs that rewrite programs that rewrite the first programs and so on. But there is no such possibility in Haskell, except for introducing a DSL. So now I wonder, what are the languages that are functional in the sense above? With a reasonable syntax and semantics, thus no assembler. I guess Lisp might be of this kind, but I'm not sure. In addition, I'm not a fan of parentheses. What else? Pure? Mathematica? Maxima? Note, that the reflectivity is important.

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Is type class 'Addressable' already exists.
by 陈文龙 12 Apr '12

12 Apr '12

To get element in List,Map... in python's way. Python: > strMap["apple"] Haskell: > strMap # "apple" https://gist.github.com/2364395 {-# LANGUAGE TypeFamilies #-} module Addressable where import qualified Data.Map as M import Prelude class Addressable a where type Key a type Value a (#!) :: a -> Key a -> Value a (#) :: a -> Key a -> Maybe (Value a) instance Addressable [a] where type Key [a] = Int type Value [a] = a (#!) = (!!) xs # i | i < 0 = Nothing [] # _ = Nothing (x:_) # 0 = Just x (_:xs) # n = xs # (n-1) instance (Ord k) => Addressable (M.Map k v) where type Key (M.Map k v) = k type Value (M.Map k v) = v a #! i = a M.! i a # i = M.lookup i a main :: IO () main = do let strMap = M.fromList [("one","1"),("two","2"),("three","3")] let strList = ["1","2","3"] print $ strMap # "two" -- Just "2" print $ strMap #! "two" -- "2" print $ strList # 0 -- Just "1" print $ strList #! 0 -- "1" print $ strMap # "no-exist" -- Nothing print $ strList # 100 -- Nothing print $ strMap #! "no-exist" -- error print $ strList #! 100 -- error

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Haskell Weekly News: Issue 222
by Daniel Santa Cruz 12 Apr '12

12 Apr '12

Welcome to issue 222 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of April 1 to 7, 2012. Quotes of the Week * tgeeky_: "What I cannot create, I do not understand." -- Richard Feynman, on his blackboard when he died in 1988. * elliott: ... [a] is more of a control structure than a data structure. * Cale: Basically, we've known how to implement first class functions efficiently for 20 or 30 years now, and we've known about their importance to abstraction in programming since before the advent of electronic computers. There's no excuse to still be writing new programs in languages without them. * edwardk: When someone throws an exception at you, duck. * luite: I think it uses levenshtein distance (he invented this distance because nobody could spell his name correctly) * edwardk: 'hey ghc devs can you fix this thing that might or might not be a bug so we can break a huge pile of invariants in your compiler to put go faster stripes on something nobody uses? k thx' Top Reddit Stories * Tfoo, my simple Five in a Row game. Online, with server-sent events, deployed to Heroku, open source. Domain: tfoo.herokuapp.com, Score: 49, Comments: 18 On Reddit: [1] http://goo.gl/Klevh Original: [2] http://goo.gl/PtjKk * Interacting with inner-functions in GHCi [StackOverflow] Domain: stackoverflow.com, Score: 48, Comments: 12 On Reddit: [3] http://goo.gl/23l0G Original: [4] http://goo.gl/2H5kI * ANN: acme-http. 221,693.0 req/s on the PONG benchmark. Domain: groups.google.com, Score: 42, Comments: 4 On Reddit: [5] http://goo.gl/XROlu Original: [6] http://goo.gl/PuOt3 * Recent haskell-mode changes screencast Domain: youtube.com, Score: 39, Comments: 15 On Reddit: [7] http://goo.gl/JyX0s Original: [8] http://goo.gl/VWLIF * An Haskell tutorial. What do you think? Domain: yannesposito.com, Score: 39, Comments: 47 On Reddit: [9] http://goo.gl/eLiQT Original: [10] http://goo.gl/yBDnQ * Galois' Open-Source code available on github Domain: corp.galois.com, Score: 38, Comments: 3 On Reddit: [11] http://goo.gl/xoz4K Original: [12] http://goo.gl/OhlaA * Wadler’s Law Revisited Domain: comonad.com, Score: 27, Comments: 9 On Reddit: [13] http://goo.gl/8p9As Original: [14] http://goo.gl/7iB53 * With Linux supporting x32 ABI, should GHC follow suit? Domain: self.haskell, Score: 23, Comments: 6 On Reddit: [15] http://goo.gl/Nkuob Original: [16] http://goo.gl/Nkuob * cabal-meta: transcending to dependency heaven Domain: yesodweb.com, Score: 22, Comments: 27 On Reddit: [17] http://goo.gl/nsVhn Original: [18] http://goo.gl/U2cDK * [Haskell-cafe] A Modest Records Proposal Domain: haskell.org, Score: 21, Comments: 7 On Reddit: [19] http://goo.gl/MvbRq Original: [20] http://goo.gl/RgaRU Top StackOverflow Questions * What is the best way to test and interact with inner functions defined inside a toplevel function? votes: 44, answers: 1 Read on SO: [21] http://goo.gl/2H5kI * How can a Windows service application be written in Haskell? votes: 15, answers: 1 Read on SO: [22] http://goo.gl/kz8TA * What does ':..' mean in Haskell? votes: 12, answers: 2 Read on SO: [23] http://goo.gl/Whi8a * Is the whole Map copied when a new binding is inserted? votes: 10, answers: 3 Read on SO: [24] http://goo.gl/OabLu * Haskell functions left-to-right votes: 10, answers: 1 Read on SO: [25] http://goo.gl/rhCm3 * How can I avoid writing boilerplate code for functions performing pattern matching? votes: 9, answers: 3 Read on SO: [26] http://goo.gl/6cvXJ * Controlling how test data is generated in QuickCheck votes: 8, answers: 1 Read on SO: [27] http://goo.gl/BNSug * Can I get warnings about overly-restrictive type signatures? votes: 8, answers: 1 Read on SO: [28] http://goo.gl/X7PkN * Slowdown when using parallel strategies in Haskell votes: 8, answers: 1 Read on SO: [29] http://goo.gl/ghV3C * Dealing with boilerplate in Haskell votes: 7, answers: 2 Read on SO: [30] http://goo.gl/Um4Fm Until next time, Daniel Santa Cruz References 1. http://tfoo.herokuapp.com/ 2. http://www.reddit.com/r/haskell/comments/rx20z/tfoo_my_simple_five_in_a_row… 3. http://stackoverflow.com/questions/9987739/what-is-the-best-way-to-test-and… 4. http://www.reddit.com/r/haskell/comments/rr7se/interacting_with_innerfuncti… 5. https://groups.google.com/group/happs/browse_thread/thread/9b078d23b59a7374 6. http://www.reddit.com/r/haskell/comments/ro77b/ann_acmehttp_2216930_reqs_on… 7. http://www.youtube.com/watch?v=E6xIjl06Lr4 8. http://www.reddit.com/r/haskell/comments/rtctu/recent_haskellmode_changes_s… 9. http://yannesposito.com/Scratch/en/blog/Haskell-the-Hard-Way/ 10. http://www.reddit.com/r/haskell/comments/rw61r/an_haskell_tutorial_what_do_… 11. http://corp.galois.com/blog/2012/4/4/galois-open-source-projects-on-github.… 12. http://www.reddit.com/r/haskell/comments/rt8m3/galois_opensource_code_avail… 13. http://comonad.com/reader/2012/wadlers-law-revisited/ 14. http://www.reddit.com/r/haskell/comments/rpyhr/wadlers_law_revisited/ 15. http://www.reddit.com/r/haskell/comments/rpbcx/with_linux_supporting_x32_ab… 16. http://www.reddit.com/r/haskell/comments/rpbcx/with_linux_supporting_x32_ab… 17. http://www.yesodweb.com/blog/2012/04/cabal-meta 18. http://www.reddit.com/r/haskell/comments/rurn4/cabalmeta_transcending_to_de… 19. http://www.haskell.org/pipermail/haskell-cafe/2012-April/100527.html 20. http://www.reddit.com/r/haskell/comments/rnwgn/haskellcafe_a_modest_records… 21. http://stackoverflow.com/questions/9987739/what-is-the-best-way-to-test-and… 22. http://stackoverflow.com/questions/10037654/how-can-a-windows-service-appli… 23. http://stackoverflow.com/questions/9996740/what-does-mean-in-haskell 24. http://stackoverflow.com/questions/10002956/is-the-whole-map-copied-when-a-… 25. http://stackoverflow.com/questions/10003796/haskell-functions-left-to-right 26. http://stackoverflow.com/questions/10024455/how-can-i-avoid-writing-boilerp… 27. http://stackoverflow.com/questions/9977734/controlling-how-test-data-is-gen… 28. http://stackoverflow.com/questions/9986508/can-i-get-warnings-about-overly-… 29. http://stackoverflow.com/questions/10009361/slowdown-when-using-parallel-st… 30. http://stackoverflow.com/questions/9969638/dealing-with-boilerplate-in-hask…

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Nice article on FP and concurrency from a born-again C++ programmer
by Obscaenvs 11 Apr '12

11 Apr '12

http://fpcomplete.com/the-downfall-of-imperative-programming/ /F

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Memory leak from close fields in conduit
by Michael Snoyman 11 Apr '12

11 Apr '12

Hi all, Hideyuki Tanaka alerted me[1] to a memory leak in conduit. Long story short: it appears that Pipe composition leads to collection of a large number of `return ()` actions for unnecessary memory cleanup. We came up with a possible solution: a Finalize type[2]. In both of our testing, this eliminates the memory leak in question. So now I have a few questions: 1. Does anyone have a different idea on how to approach this problem? 2. Are there any refinements to be applied to the solution? 3. Technically, this is an API breakage, but (a) conduit 0.4 was released very recently, (b) this is a fairly important change, and (c) the real-world breakage from this is very minimal. Will anyone call foul if I release such a change as 0.4.1? Just to stress 3c: I compiled all of Yesod against this modified version of conduit, and only http-conduit had to be tweaked at all. Michael [1] https://github.com/snoyberg/conduit/issues/40 [2] https://github.com/snoyberg/conduit/commit/ebc7a68e0f1f0c1b80c0105916b00060…

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Installing REPA
by Dominic Steinitz 11 Apr '12

11 Apr '12

Hi, I'm trying to install REPA but getting the following. Do I just install base? Or is it more complicated than that? Thanks, Dominic. Tests-MacBook-Pro:PDE Test$ cabal install repa Resolving dependencies... cabal: cannot configure repa-2.2.0.1. It requires base ==4.4.* For the dependency on base ==4.4.* there are these packages: base-4.4.0.0 and base-4.4.1.0. However none of them are available. base-4.4.0.0 was excluded because of the top level dependency base -any base-4.4.1.0 was excluded because of the top level dependency base -any Tests-MacBook-Pro:PDE Test$ cabal --version cabal-install version 0.10.2 using version 1.10.1.0 of the Cabal library Tests-MacBook-Pro:PDE Test$ Tests-MacBook-Pro:PDE Test$ ghc --version The Glorious Glasgow Haskell Compilation System, version 7.0.3

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building ghc on arch linux ARM?
by . 11 Apr '12

11 Apr '12

Hi Cafe, I hope this is the right place to ask this kind of stuff. I would like to try ghc on a panda board (armv7l) with arch linux. There is apparently no pre-built package, so I was trying the instructions to build, from here: <http://hackage.haskell.org/trac/ghc/wiki/Building/Porting>. However, I still seem to need a ghc and ghc-pkg installed: I am getting this error message: .... checking for tar... /bin/tar checking for gpatch... no checking for patch... /usr/bin/patch checking for dtrace... no checking for HsColour... no checking for xmllint... no configure: WARNING: cannot find xmllint in your PATH, you will not be able to validate your documentation checking for xsltproc... no configure: WARNING: cannot find xsltproc in your PATH, you will not be able to build the HTML documentation checking for dblatex... no configure: WARNING: cannot find dblatex in your PATH, you will not be able to build the PDF and PS documentation checking for ghc-pkg matching ... configure: error: Cannot find matching ghc-pkg --------- Does anyone know if it is possible to build ghc without ghc on this platform? I was using the tarball sources for ghc 7.4.1. Thanks, Christian

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Reconstructing a tree from a list of its paths (to leaves)
by Arnaud Bailly 11 Apr '12

11 Apr '12

Hello, I am manipulating labeled multiway trees, some kind of "lightweight" XML notation. One thing I would like to be able to do is manipulating a tree as a list of (Path, Value). Generating such a list is easy but I am a little bit surprised to find it harder to reconstruct a tree, given such a list assuming some sensible properties (list is ordered, for example). I got the intuition this has already been tackled in one way or another in a functional setting in Haskell (I code in Java but using mostly functional constructs), but don't know where to look. Thanks for your pointers and help, Regards, Arnaud

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XMPP library recommendations
by John Goerzen 11 Apr '12

11 Apr '12

Hi folks, I'm looking for suggestions for XMPP libraries. So far I have found three on Hackage. Thanks to the authors of these - I think a lot of good can come from XMPP in Haskell. However, all of them appear to be minimally maintained, if at all: * XMPP - only one upload, over 2 years ago. Does not support TLS (required by many modern servers). Does not support SASL (also required by many modern servers). After finding the options on a test server to disable the requirements for TLS and SASL, it worked. * network-protocol-XMPP - uploaded recently to fix 7.4 build issue, not much other recent activity. Refuses to connect to a test account with "Received empty challenge" (this does not appear to be an accurate error message, but rather a bug in XML parsing). A bit more low-level than I'm looking for. * haskell-xmpp - only one upload, over 1 year ago. Connects and works. I like the design. Is hard-coded to write debug messages to stdout, making it a bit of a problem for programs that use stdout (though a one-liner mod to the code would fix that). Says it's "alpha" and "work in progress". What experience do people have with these? Any recommendations? Thanks, -- John

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ANN: Selenium WebDriver client for Haskell
by Adam Curtis 10 Apr '12

10 Apr '12

The first release of the webdriver package has been uploaded to Hackage. Selenium is a test suite that allows you to automate web browsers on a variety of platforms. The webdriver package acts as a client library that speaks Selenium's WebDriver protocol, using a simple monadic interface. This alpha release has received a month of testing through my own usage, and has most of the capabilities of the official Selenium clients, with more to come. Future plans include utilizing the http-conduit and attoparsec-conduit packages for memory efficient JSON parsing, and using the reflection package to safely handle multiple versions of the WebDriver protocol through implicit configurations. For more information, see http://hackage.haskell.org/package/webdriver

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