- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] mapM is supralinear?
by John Lato 27 Sep '11

27 Sep '11

> From: Daniel Fischer <daniel.is.fischer(a)googlemail.com> > > On Friday 09 September 2011, 00:41:11, Roman Cheplyaka wrote: >> * Ertugrul Soeylemez <es(a)ertes.de> [2011-09-07 16:20:03+0200] >> >> > In general it's a bad idea to use mapM over IO. >> >> Could you explain why? > > Take it with a grain of salt, there's nothing necessarily wrong with using > mapM over IO on short lists. Agreed. Whenever I'd like to use mapM (or any other function for which a *M_ is available), I've found the following rules helpful: 1. If I can guarantee the list is short (~ n<=20), go ahead and use mapM 2. Otherwise use mapM_, foldM_, or foldM if a real reduction is possible (i.e. not "foldM snocM []"). Step 2 sometimes requires changing my design, but it's always been for the better. `mapM_` tends to require more pipeline composition, so it's leveraging the language's strengths. This has served me well, especially in IO, but in other monads as well. John L.

6 10

9th Ghent Functional Programming Group meeting on Tuesday, the 4th of October, 2011
by Jasper Van der Jeugt 27 Sep '11

27 Sep '11

Hello all, We are glad to invite you to our 9th GhentFPG [1] meeting, which takes place on Thursday, the 4th of October, 2011 at 19:30 in the Technicum building of Ghent University. You do not have to be a Functional Programming guru to attend, everyone eager to learn is welcome. As before, the electronic sliding doors will be locked, but a phone number that you can call to get in will be provided at the doors on the far left of the building. The program is as follows: First, we will have a talk by Jurriën Stutterheim, from Utrecht: Snap Framework [2] is a relatively young, but very promising and already quite popular Haskell web framework. Its upcoming 0.6 release has seen a major redesign in the way you write (reusable) components for the framework using so-called snaplets. In this talk, we will look at how you can write snaplets and how you can put them together to create modular web applications. Then, we will have a number of lightning talks in which the following projects are presented: - hCole-server [3], a web interface to a framework for compiler optimizations (Andy Georges); - GA [4], a library to write genetic algorithms in Haskell (Kenneth Hoste); - websockets [5], a Haskell library for writing WebSocket-capable servers (Jasper Van der Jeugt). If you would also like to give a lightning talk (15 mins), please contact us and we will add you to this list. Afterwards, we will have some drinks at a local bar. [1]: http://www.haskell.org/haskellwiki/Ghent_Functional_Programming_Group [2]: http://snapframework.com/ [3]: http://github.com/itkovian/hcole-server [4]: http://github.com/boegel/GA [5]: http://github.com/jaspervdj/websockets Hoping to see you there, The GhentFPG organizing commitee

1 1

A Crypto-API-Tests package
by Thomas DuBuisson 27 Sep '11

27 Sep '11

FYI, since I figure you three are the ones interested in this right now: I'll be releasing crypto-api-tests [1] to hackage someday (this weekend?). If you want to give it a spin or add KATS before the first release then feel free. OTOH, it's not like I'd stop accepting patches after release 0.1. Cheers, Thomas [1] https://github.com/TomMD/crypto-api-tests

1 0

which tags program should I use?
by Henry Laxen 27 Sep '11

27 Sep '11

Dear Group, I have a simple question, that as far as I can tell, has never really been well answered. I would like to generate TAGS files for haskell source. Reading the http://www.haskell.org/haskellwiki/Tags page suggests using :etags in GHCI or hasktags, or gasbag. Of the three, hasktags comes closest to "working" but it has (for me) a major inconvenience, namely it finds both function definitions and type signatures, resulting in two TAGS entries such as: ./Main.hs,63 module Main where6,7 main ::24,25 main =25,26 Now when I do an emacs find-tag (I use icicles) I will always have to choose which tag I want to visit, and the completion buffer contains something like: main :: hs/Main.hs main = hs/Main.hs Granted, this is a minor (and very specialized) complaint, but if hasktags were to select only ONE of either the type signature (my first choice) or the function definition, (if no type signature) this annoyance would disappear. I also tried using etags, which I think would work, but it seems to have one killer bug (feature), namely that it dies if it finds an uninterpreted import: when (not is_interpreted) $ let mName = GHC.moduleNameString (GHC.moduleName m) in ghcError (CmdLineError ("module '" ++ mName ++ "' is not interpreted")) I think it would work much better if it just warned you, instead of dying. This makes it unusable any time you import something precompiled. Now some looking at the README of hasktags leads me to: "In the past this tool was distributed with ghc. I forked and added some features. hasktags itself was moved out of the ghc repository. Then I only verified that my fork finds at least as much tags as the one forked by Igloo." That makes me feel a little queasy. A google search for hasktags igloo turns up http://hackage.haskell.org/trac/ghc/ticket/1508 whose title is "hasktags program needs replacement" which makes me feel even more queasy. So I guess my question is, what are us disciples of "the one true editor" to do? Thanks in advance for you sage advice. Best wishes, Henry Laxen

4 3

Attoparsec: Limiting Parsers to N Bytes, or Combing Parsers?
by Michael Craig 27 Sep '11

27 Sep '11

Suppose we want to parse a 24-bit hex color value: input :: ByteString input = "af093c blah blah blah" type Color = (Word8, Word8, Word8) Attoparsec.Char8 exports a nice hexadecimal parser, but it consumes all available hex-flavored input. I'd like to make it consume exactly two bytes, so I could write my color parser like this: color :: Parser Color color = do r <- hex2 g <- hex2 b <- hex2 return $ Color (r, g, b) hex2 :: Parser Word8 hex2 = ??? So my question is "how do I write hex2?" I could easily rewrite hexadecimal, but it would be nicer to reuse its hex-handling logic. Best, Mike S Craig (908) 328 8030

4 6

Job opportunities at Citrix Systems (Cambridge, UK)
by Mike McClurg 26 Sep '11

26 Sep '11

Hi folks, I just wanted to let everyone know that Citrix is hiring developers to work on the OCaml-based XenAPI toolstack. We are looking to recruit top-class engineers to work on the toolstack; 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 with applications programming in high-level functional languages (such as OCaml) * an aptitude for implementing (and reasoning about) complex concurrent and 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 clean and reliable solutions that 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. Thanks, Mike McClurg PS: Please note that you must have UK right to work. Here are the official job postings, but if you apply please send me your CV directly. * Req #11673 - Software Development Engineer: http://careers.peopleclick.com/careerscp/client_citrix/emea_region/jobDetai… * Req #11897 - Senior Software Developer: http://careers.peopleclick.com/careerscp/client_citrix/emea_region/jobDetai… And here are some links to our company, products and open source projects and code. * The Xen Hypervisor - http://xen.org/ * Citrix XenServer - http://www.citrix.com/English/ps2/products/product.asp?contentID=683148 * Our GitHub account - https://github.com/xen-org

1 0

SPECIALIZE in the presence of constraints
by Nicu Ionita 26 Sep '11

26 Sep '11

Hallo List, I can't understand how pragma SPECIALIZE works in the presence of constraints. I have 2 modules, one which defines a general search framework, and one which implements it in a concrete context. The general module defines functions like: {-# SPECIALIZE pvQSearch :: Node (Game m) Move Int => Int -> Int -> Int -> Search (Game m) Move Int Int #-} pvQSearch :: Node m e s => s -> s -> Int -> Search m e s s pvQSearch a b c = do ... while the implementation uses concrete data types Move and Int (for e and s respectively) and defines instances and other types (like the polimorphic type Game m, where m is a monad). Node m e s is a class and the instance Node (Game m) Move Int is defined in the implementation module. From outside this construct is used by calling a generic search function, but giving parameters which match the implementation (i.e. with Move and Int for e and s, for example). Now what I don't understand: 1. how can the compiler (here ghc) know which function to expose as the correct generic search function? There must be two search functions generated, one generic and its specialization. Does the module export both and later the linker chooses the correct one, when the client module is known? 2. how can I know that the specialization is really used? When I have constraints, will the specializations be generated in the assumption that the constraint is matched? When will be this match checked? My problem is that, with or without specializations, the performance of the code is the same - so I doubt the specializations are used. Thanks, Nicu

2 1

Is there still at list of active Haskell projects
by Vasili I. Galchin 25 Sep '11

25 Sep '11

Hello, On http://www.haskell.org I didn't see a list of active Haskell projects. ?? Thanks, Bill

3 2

A Missing Issue on Second Generation Strategies
by Burak Ekici 25 Sep '11

25 Sep '11

Dear List, I am trying to parallelize RSA encryption and decryption by using below manner, but when I run executable output file with "+RTS -s -N2" command on Windows 7, output stats say 4 sparks are being created however none of them converted into real OS threads. -- SPARKS :4 (0 converted, 4 pruned) -- I was thinking that the problem could occur due to lack of forcing parallelization but, as far as I know 'rdeepseq' works for that aim. Briefly, I could not solve the issue why parallelization was not being implemented. I would be appreciated if any of you shed a light on the issue that I missed. Here is the mentioned part of code: split4ToEnc :: RSAPublicKey -> [Integer] -> [Integer] split4ToEnc (PUB n e) [] = [] split4ToEnc (PUB n e) (x:xs) = ((ersa (PUB n e) secondPart2) ++ (ersa (PUB n e) secondPart1) ++ (ersa (PUB n e) firstPart2) ++ (ersa (PUB n e) firstPart1)) `using` strategy where firstPart1 = fst (Main.splitAt((length (x:xs)) `div` 4) (fst(Main.splitAt ((length (x:xs)) `div` 2) (x:xs)))) firstPart2 = snd (Main.splitAt((length (x:xs)) `div` 4) (fst(Main.splitAt ((length (x:xs)) `div` 2) (x:xs)))) secondPart1 = fst (Main.splitAt((length (x:xs)) `div` 4) (snd(Main.splitAt ((length (x:xs)) `div` 2) (x:xs)))) secondPart2 = snd (Main.splitAt((length (x:xs)) `div` 4) (snd(Main.splitAt ((length (x:xs)) `div` 2) (x:xs)))) strategy res = do a <- rpar (ersa (PUB n e) (firstPart1) `using` rdeepseq) b <- rpar (ersa (PUB n e) (firstPart2) `using` rdeepseq) c <- rpar (ersa (PUB n e) (secondPart1) `using` rdeepseq) d <- rpar (ersa (PUB n e) (secondPart2) `using` rdeepseq) rdeepseq res Thanks a lot, Burak.

3 4

Fwd: problem with cabal install MissingH-1.1.1.0
by Ozgur Akgun 24 Sep '11

24 Sep '11

Ooops, apparently I forgot to reply to the list. Sorry. ---------- Forwarded message ---------- From: Ozgur Akgun <ozgurakgun(a)gmail.com> Date: 22 September 2011 16:49 Subject: Re: [Haskell-cafe] problem with cabal install MissingH-1.1.1.0 To: Mariano Cortesi <mcortesi(a)gmail.com> Hi Mariano. On 22 September 2011 12:48, Mariano Cortesi <mcortesi(a)gmail.com> wrote: > I'm with mac OS X lion, ghc version 7.2.1 > and when a i try to install MissingH version 1.1.1.0 it fails with > > [24 of 47] Compiling Data.BinPacking ( src/Data/BinPacking.hs, > dist/build/Data/BinPacking.o ) > > src/Data/BinPacking.hs:45:6: > Illegal datatype context (use -XDatatypeContexts): (Num size, > This is weird, because on OS X Lion, and with the following versions, it doesn't fail for me: The Glorious Glasgow Haskell Compilation System, version 7.2.1 cabal-install version 0.10.2 using version 1.12.0 of the Cabal library Nevertheless, you can try the following to do what cabal asks you to do: cabal install --ghc-option=-XDatatypeContexts Which will give a warning, but should succeed anyway. Hope this helps, Ozgur -- Ozgur Akgun

5 4