- Haskell-Cafe - Haskell.org

Most popular haskell applications
by Bulat Ziganshin 07 Nov '10

07 Nov '10

Hello haskell, people, are you know haskell apps that has more than 50k downloads per month (or more than 25k users) ? -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

4 6

What do you call Applicative Functor Morphism?
by roconnor＠theorem.ca 06 Nov '10

06 Nov '10

An applicative functor morphism is a polymorphic function, eta : forall a. A1 a -> A2 a between two applicative functors A1 and A2 that preserve pure and <*>: eta (pure c) = pure c eta (f <*> x) = eta f <*> eta x What do you guys call such a thing? My leading candidate is "idomatic transformation". -- Russell O'Connor <http://r6.ca/> ``All talk about `theft,''' the general counsel of the American Graphophone Company wrote, ``is the merest claptrap, for there exists no property in ideas musical, literary or artistic, except as defined by statute.''

7 7

Flagstone problem
by michael rice 06 Nov '10

06 Nov '10

Hi, I've been looking at a "flagstone problem," where no two adjacent n-tuples can be identical. I solved the problem with Icon using an array of stacks and was going to explore how to do it in Haskell when I saw another way to do it explained in the same text. Just count the ones between the zeros in a(n) to get b(i), a non-repeating sequence of zeros, ones, and twos. An a(n) is 0 if the number of one bits in the binary representation of n is even, otherwise odd, and the initial a(n) must be even. n a(n) b(i) 20 = 010100 0 21 = 010101 1 2 22 = 010110 1 23 = 010111 0 0 24 = 011000 0 25 = 011001 1 2 26 = 011010 1 27 = 011011 0 1 28 = 011100 1 29 = 011101 0 0 30 = 011110 0 31 = 011111 1 2 32 = 100000 1 33 = 100001 0 0 34 = 100010 0 1 35 = 100011 1 36 = 100100 0 ========= Here's my Haskell code ======== import Data.Bits import Data.List flagstone n = foldl (++) "" (take n (map show (f $ group [if even y then 0 else 1 | y <- [bitcount x | x <- [20..]]]))) bitcount :: (Integral t) => t -> t bitcount 0 = 0 bitcount n = let (q,r) = quotRem n 2 in bitcount q + r f [] = [] f ([0]:xs) = f xs f ([0,0]:xs) = 0 : f xs f ([1]:xs) = 1 : f xs f ([1,1]:xs) = 2 : f xs ========= My question ======== A further exercise in the text: "Exercise 5.-(a) Define a(n) as the sum of the binary digits in the binary representation of n. Define b(i) as the number of a's between successive zeros as before. Then T = b(1) b(2) b(3) b(4) ... gives an infinite sequence of *seven* symbols with no repeats. (b) Write a routine to generate a sequence for seven colors of beads on a string with no repeats." I may be misreading, but does this make any sense? There's a reference to The American Mathematical Monthly, June-July 1963, p. 675, by C. H. Braunholtz. Michael

3 4

http://functionalley.eu
by Alistair Ward 06 Nov '10

06 Nov '10

I've just written a few packages which I think may be useful, and have made them available as free opensource on a personal website http://functionalley.eu. I opted to host them there rather than uploading them to Hackage, because they're part of a wider project. The main offerings are; a regex-engine which is polymorphic in terms of the type of the input-data, a traditional character-based regex-engine derived from it, and an application to determine the most efficient order in which to pack files into a given storage-space. I understand this isn't the ideal venue in which announce new packages, but having looked briefly at the "haskell(a)haskell.org" mail-group and the cerebral offerings there, I'm rather concerned that my meagre offerings may resemble germs in a Dettol factory. Is there a more appropriate forum ? If anyone has the time, I'd also greatly appreciate any feedback on this s/w. Regards, Alistair Ward P.S. This is my first posting.

5 6

How to put a string into Data.Binary.Put
by C K Kashyap 06 Nov '10

06 Nov '10

Hi, I was trying to put a String in a ByteString import qualified Data.ByteString.Lazy as BS message :: BS.ByteString message = runPut $ do let string="SOME STRING" map (putWord8.fromIntegral.ord) string -- this ofcourse generates [Put] How can I convert the list of Put's such that it could be used in the Put monad? For now I used the workaround of first converting the string to ByteString like this - stringToByteString :: String -> BS.ByteString stringToByteString str = BS.pack (map (fromIntegral.ord) str) and then using putLazyByteString inside the Put monad. -- Regards, Kashyap

4 4

Packagse for a simple database application
by jean-christophe mincke 06 Nov '10

06 Nov '10

Hello Cafe, More as an exercice than anything else I would like to write a small "database application" which would allow a user to read/write data from/to a DB. The user would interact with the application via a web browser. Which (reliable/stable/workable) packages are the best for this task? I was thinking of Happstack and HaskellDB. Any others? Thank you Regards J-C

1 0

vector-space and standard API for vectors
by Alexey Khudyakov 06 Nov '10

06 Nov '10

Hello everyone! It's well known that Num & Co type classes are not adequate for vectors (I don't mean arrays). I have an idea how to address this problem. Conal Elliott wrote very nice set of type classes for vectors. (Definition below). I used them for some time and quite pleased. Code is concise and readable. > class AdditiveGroup v where > zeroV :: v > (^+^) :: v -> v -> v > negateV :: v -> v > > class AdditiveGroup v => VectorSpace v where > type Scalar v :: * > (*^) :: Scalar v -> v -> v > > class VectorSpace v => InnerSpace v where > (<.>) :: v -> v -> Scalar v There are few problems though. Fisrt one is inlining. Functions are not inlined so if client code uses uses stream fusion it breaks. Second is dependecies. Vector-space doesn't have big chain of dependencies. People may be reluctant to use vector-spaces just for API for small packages. My proposal is to split type classes above into separate package which provide only type classes, doesn't have any dependencies. Second point is to lean more on performance side in performance vs elegance. Add INLINE pragmas everywhere (unfortunate feature of stream fusion), move ^-^ to AdditiveGroup with default implementation. I'd like to know opinion of haskellers on this and specifically opinion of Conal Eliott as author and maintainer (I CC'ed him)

11 24

Rigid types fun
by Mitar 06 Nov '10

06 Nov '10

Hi! I have much fun with rigid types, type signatures and GADTs. And I would like to invite also others in and share my joy. ;-) Please see the attached file and chase a solution to how to make it compile. I would like to have a function which I would call like: createNerve (Axon undefined) (AxonAny undefined) and it would return proper Nerve. Similar to how asTypeOf works. I would like to do that to remove repeating code like: from <- newChan for <- newChan let nerve = Nerve (Axon from) (AxonAny for) which I have to write again and again just to make types work out. Why I cannot move that into the function? I am using GHC 6.12.3. Is this something which will work in 7.0? Mitar

4 8

What is simplest extension language to implement?
by Permjacov Evgeniy 06 Nov '10

06 Nov '10

Let us think, that we need some scripting language for our pure haskell project and configure-compile-run is not a way. In such a case a reasonably simple, yet standartized and wide known language should be implemented. What such language may be? R(4/5/6)RS ? EcmaScript ? Some other ?

16 17

Cabal and using a throw-away package database during distro package building
by Magnus Therning 06 Nov '10

06 Nov '10

Recently I experimented a little with putting several Hackage packages into a single Linux distro package, but I ran into a slight problem with registering Haskell packages temporarily. These are the basic steps taken to configure, compile, and stage a Haskell package in ArchLinux: runhaskell Setup configure --prefix=/usr --docdir=/usr/share/doc/${pkgname} runhaskell Setup build runhaskell Setup haddock runhaskell Setup register --gen-script runhaskell Setup unregister --gen-script runhaskell Setup copy --destdir=${pkgdir} This works fine for a single Hackage package, but if I want to compile two or more of them into a single distro package, and there are dependencies between them, then it won't do. I also don't want to fully install and/or register the Hackage packages during the build for obvious reasons of containment and system contamination. So, looking at the Cabal help I found the --package-db flag for configure. The steps now become: runhaskell Setup configure --prefix=/usr --docdir=/usr/share/doc/${pkgname} --package-db=my-temp-db runhaskell Setup build runhaskell Setup haddock runhaskell Setup register --inplace runhaskell Setup register --gen-script runhaskell Setup unregister --gen-script runhaskell Setup copy --destdir=${pkgdir} That works and I can satisfy dependencies between the Hackage packages, without affecting the system-wide database. Great. Except that the generated register/unregister scripts now also point to my-temp-db, and there seems to be no way to prevent this. I solved it for now by using sed, but I'd love to leave sed out, if at all possible. So, have I missed something, or is this a use case that wasn't considered when developing Cabal? Would it be worth raising a bug for this at all? /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus identi.ca|twitter: magthe

2 1

Cabal and using a throw-away package database during distro package building
by Dimitry Golubovsky 05 Nov '10

05 Nov '10

Magnus, You might try Capri which operates Cabal-related stuff privately on a local-to-project package database not touching global or user databases. http://hackage.haskell.org/package/capri http://www.haskell.org/haskellwiki/Capri -- Dimitry Golubovsky Anywhere on the Web

1 0

Re: Haskell on ancient machines (Was: "Haskell is a scripting language inspired by Python.")
by Malcolm Wallace 05 Nov '10

05 Nov '10

>> I haven't checked how much RAM nhc98 needs for bootstrapping >> recently, but the >> Makefile suggests 16Mb of heap + 2Mb of stack is more than >> sufficient - >> it could probably manage with less. > > If it was possible to save resources in these days - why isn't it > possible today anymore? > Too many Haskell extensions? Too much auxiliary code for interfacing > with OS? Increased safety? Simply that more people care about speed than about space. Regards, Malcolm

1 0

Who manages <http://hackage.haskell.org/trac/>?
by Peter Simons 05 Nov '10

05 Nov '10

Hi guys, a while ago, I created an account on Trac. Now, it seems that I've forgotten both the password and the e-mail address that I used at the time. I cannot log in, and I cannot make Trac send me the password either. Clearly, I need the help of a human being with administrator privileges to figure that out. Can someone give me a pointer about who I'd want to contact regarding that issue? Take care, Peter

2 1

Reference for technique wanted
by Richard O'Keefe 05 Nov '10

05 Nov '10

There's a long-known technique in functional languages where [x1,...,xn] => \tail -> x1:...xn:tail xs ++ ys => f . g xs => f [] A correspondent mentioned to me that he couldn't find a reference to the idea (which I gather he had independently rediscovered). I know I've read about it somewhere. Can anyone provide a reference?

6 22

Are newtypes optimised and how much?
by Christopher Done 05 Nov '10

05 Nov '10

So I have the following nice things: {-# LANGUAGE GeneralizedNewtypeDeriving, OverloadedStrings #-} import Data.String newtype Foo = Foo { unFoo :: String } deriving (IsString) x :: Foo x = "Hello, World!" newtype Bar = Bar { unBar :: Integer } deriving (Eq,Show,Num,Integral,Real,Enum,Ord) y :: Bar y = 2 I can write literals and they will be converted to the type I wanted with no extra verbiage needed. Questions (I'm talking about GHC when I refer to compilation): (1) Are fromString and fromIntegral ran at compile time? I don't think that this is the case. I think they are just translated to fromString "Hello, World!" and fromIntegral 2 verbatim. (2) Regardless of this, the implementation of fromString and fromIntegral is essentially a no-op, it's just fromString = Foo, fromIntegral = Bar, which is in turn essentially fromString = id, fromIntegral = id, as far as I understand it. It's purely compile time. But supposing I write: fromIntegral (fromIntegral (2::Integer) :: Bar) :: Integer Is this at the end of the day equal to just (2::Integer)? Thinking simple-mindedly, I would say, yes. The compiler knows that fromIntegral :: Integer -> Bar == id, and that fromIntegral :: Bar -> Integer == id (right?). But is that the case? Perhaps the type class methods have some dictionary and thus cannot be inlined, or maybe that doesn't matter? At the end of the day what motivated me to ask these questions it that I like very much defining newtypes for most of the types I use, I have completely forgotten about `type' aliasing. I'm completely happy to write Foo and unFoo all over the place to aid my type correctness, but I want a nice generic way to convert to/from newtypes but keeping it a compile-time concept. Sometimes I have unThisThat, unTheOther, unThoseWhoShantBeNamed, etc. and it I could just use fromIntegral and fromString then that would be super. Also, is 'map unFoo' optimised away at compile-time, too? I think that it would be compiled to map id. So it would still wrap a thunk around each cons. How far does it go? So, if I go around using fromIntegral/fromString (etc. for other newtype types), is it still kept compile time? After having newtypes catch dozens of type mismatches that otherwise wouldn't unified happily but were completely wrong (e.g. wrong argument order), I've found newtype to be an indispensable part of Haskell and of writing a large piece of software. Cheers

8 14

SBLP 2011 - call for papers
by Simon Thompson 05 Nov '10

05 Nov '10

SBLP 2011: Call For Papers 15th Brazilian Symposium on Programming Languages Sao Paulo, Brazil September 26-30, 2010 http://www.each.usp.br/cbsoft2011/ IMPORTANT DATES Paper abstract submission (15 lines): April 22nd, 2011 Full paper submission: April 29th, 2011 Notification of acceptance: May 30th, 2011 Final papers due: July 29th, 2011 INVITED SPEAKERS Gary T. Leavens, Univ. of Central Florida Jose Luis Fiadeiro, Univ. of Leicester INTRODUCTION The 15th Brazilian Symposium on Programming Languages, SBLP 2011, will be held in Sao Paulo, Brazil, between September 26th and 30th, 2011. SBLP provides a venue for researchers and practitioners interested in the fundamental principles and innovations in the design and implementation of programming languages and systems. The symposium will be part of the 2nd Brazilian Conference on Software: Theory and Practice, CBSoft 2011, http://www.each.usp.br/cbsoft2011/, which will host four well-established Brazilian symposia: * XXV Brazilian Symposium on Software Engineering (SBES) * XV Brazilian Symposium on Programming Languages (SBLP) * XIV Brazilian Symposium on Formal Methods (SBMF) * V Brazilian Symposium on Components, Software Architecture and Software Reuse (SBCARS) SBLP 2011 invites authors to contribute with technical papers related (but not limited) to: * Program generation and transformation, including domain-specific languages and model-driven development in the context of programming languages. * Programming paradigms and styles, including functional, object-oriented, aspect-oriented, scripting languages, real-time, service-oriented, multithreaded, parallel, and distributed programming. * Formal semantics and theoretical foundations, including denotational, operational, algebraic and categorical approaches. * Program analysis and verification, including type systems, static analysis and abstract interpretation. * Programming language design and implementation, including new programming models, programming language environments, compilation and interpretation techniques. SUBMISSIONS Submissions should be done using SBLP 2011 installation of the EasyChair conference management system at http://www.easychair.org/conferences/?conf=sblp2011. Contributions should be written in Portuguese or English. We solicit papers that should fall into one of two different categories: full papers, with at most 15 pages, or short papers, with at most 5 pages. All papers should be prepared using the Easychair template. (http://www.easychair.org/easychair.zip) In particular, we encourage the submission of short papers reporting on master dissertations or doctoral theses at early stages of their development. All accepted papers will be published in the conference proceedings. As in previous editions, a journal special issue, with selected papers from accepted contributions, is anticipated. From 2003 to 2008 there were special issues of the Journal of Universal Computer Science, published by Springer, with the post-proceedings of SBLP. The post-proceedings of SBLP 2009 and 2010 are being edited as special issues of Science of Computer Programming, published by Elsevier. GENERAL CO-CHAIRS Denise Stringhini FCI, Mackenzie Alfredo Goldman IME, USP PROGRAMME CHAIRS Christiano Braga, UFF Jose Luiz Fiadeiro, Univ. of Leicester PROGRAMME COMMITTEE * Alberto Pardo, Univ. de La Republica * Alex Garcia, IME * Alvaro Freitas Moreira, UFRGS (TBC) * Andre Santos, UFPE * Artur Boronat, Univ. of Leicester * Carlos Camarao, UFMG * Christiano Braga, UFF (co-chair) * Edward Hermann Haeusler, PUC-Rio (TBC) * Fernando Castor Filho, UFPE * Fernando Pereira, UFMG * Francisco Heron de Carvalho Junior, UFC (TBC) * Giuseppe Castagna, Paris 7 (TBC) * Jens Palsberg, UCLA * Joao Saraiva, Universidade do Minho * Johan Jeuring, Utrecht Univ. * Jonathan Aldrich, Carnegie Mellon Univ. * Jose Luiz Fiadeiro, Univ. of Leicester (co-chair) * Lucilia Figueiredo, UFOP * Luis Soares Barbosa, Univ. do Minho * Marcelo A. Maia, UFU * Marcelo d'Amorim, UFPE * Marco Tulio Valente, UFMG * Mariza A. S. Bigonha, UFMG * Martin A. Musicante, UFRN * Noemi Rodriguez, PUC-Rio * Paulo Borba, UFPE (TBC) * Peter Mosses, Swansea University * Renato Cerqueira, PUC-Rio * Ricardo Massa, UFPE * Roberto S. Bigonha, UFMG (TBC) * Roberto Ierusalimschy, PUC-Rio * Sandro Rigo, UNICAMP * Sergio Soares, UFPE (TBC) * Sergiu Dascalu, Univ. of Nevada * Simon Thompson, Univ. of Kent * Sophia Drossopoulou, Imperial College (TBC) * Varmo Vene, Univ. de Tartu (TBC) * Vladimir Di Iorio, UFV (TBC)

1 0

Is Curry alive?
by Gregory Crosswhite 05 Nov '10

05 Nov '10

Hey everyone, This is a little off-topic, but I just ran into a problem which might benefit from being attacked by a logic language, so I've been looking for a good one to try out --- and hopefully one that has a very efficient implementation since I want to iterate through billions and possibly trillions of nondeterministically generated solutions. I was thinking about using Curry, but it looks to me like the language is dead and hasn't seen much activity for a few years. Does anyone know about whether there is still much going on over there? Or, alternatively, do you have any suggestions regarding other logic language/implementations I should check out? I've also been looking at Prolog but I am having trouble seeing whether I can process N non-deterministic solutions in O(1) space (rather than first generating a O(N) size list), and I checked out Mercury but the documentation for it is a bit sparse. Thanks! Greg

6 12

Type inference algorithm in Prolog and Haskell
by oleg＠okmij.org 05 Nov '10

05 Nov '10

Dan Doel wrote: > Implementing type inference can be very easy in a logic language, > because most of the work in a non-logic language is implementing > unification: > http://muaddibspace.blogspot.com/2008/01/type-inference-for-simply-typed-la… Correctly implementing type inference in Prolog is much more complicated. The cited code is elegant, but it is not correct. Here's the code without Unicode embellishments: GNU Prolog 1.3.0 By Daniel Diaz Copyright (C) 1999-2007 Daniel Diaz | ?- [user]. compiling user for byte code... :- op(150, xfx, --). :- op(140, xfx, :). :- op(100, xfy, ->). :- op(100, yfx, $). Gamma -- Term : Type :- atom(Term), member(Term : Type, Gamma). Gamma -- lam(A, B) : Alpha -> Beta :- [A : Alpha|Gamma] -- B : Beta. Gamma -- A $ B : Beta :- Gamma -- A : Alpha -> Beta, Gamma -- B : Alpha. user compiled, 9 lines read - 1801 bytes written, 8746 ms % We define a boolean and an integer constants -- only to make the examples % smaller. ?- G0 = [i1:int, b1: bool], E = lam(f, lam(x, f $ (x $ i1))), G0 -- E : T. E = lam(f,lam(x,f$(x$i1))) G0 = [i1:int,b1:bool] T = (A->B)->(int->A)->B % so far, so good. ?- G0 = [i1:int, b1: bool], E = lam(f, lam(x, f $ (x $ i1) $ (x $ b1))), G0 -- E : T. no % Still good: we can't apply x both to a boolean and an integer. ?- G0 = [i1:int, b1: bool], E = lam(f, lam(x, lam(x, f $ (x $ i1) $ (x $ b1)))), G0 -- E : T. E = lam(f,lam(x,lam(x,f$(x$i1)$(x$b1)))) G0 = [i1:int,b1:bool] T = (A->B->C)->(bool->B)->(int->A)->C ? % What happened? How come we embedded an untypable term in a larger % term, and it type-checked? % Look at the type: why the first x has the type bool->B? % Because two occurrences of x in the body of the term are % different identifiers, one inner and one outer. % The type checker does not understand lexical scoping! % That is not Lambda-calculus! % It gets worse: | ?- E = lam(f, lam(x, (f $ (x $ lam(z,z))) $ (x $ (lam(u,lam(v,u)))))), [] -- E : T. % lopes forever, has to be aborted. ?- E = lam(x, x $ x), [] -- E : T. Illegal instruction: 4 Finally we've got a segmentation fault. I submit that a type-checker that accepts ill-typed terms, loops and causes segmentation faults is not correct. The member() predicate is (one of the) culprits. In a real type-checker, if we find the binding x:t in Gamma and we fail to type-check, we should not look for another binding in Gamma. We have to fail. But that is not how member works: it selects one element from the list; if it is not good, it selects another. It is also tempting to use Prolog variables for lambda-bound variables. That brings another can of worms: generally speaking, we need disequality constraints. Prolog sure makes some things easier, but some things quite more complicated. Implementing type inference for simply-typed lambda-calculus in Haskell is not that complex: http://okmij.org/ftp/Computation/FLOLAC/TInfT.hs However, we don't even have to do that. Haskell itself can do that for us: http://okmij.org/ftp/tagless-final/course/TTF.hs It boils out to three lines: class Symantics repr where lam :: (repr a -> repr b) -> repr (a->b) app :: repr (a->b) -> repr a -> repr b That is it: the type system of simply-typed lambda-calculus (which we can read as the implication fragment of minimal logic). Two rules, implication introduction and elimination, is all it takes. Now, when we try to typecheck the problematic term *TTF> lam (\x -> app x x) <interactive>:1:17: Occurs check: cannot construct the infinite type: a = a -> b Expected type: repr a Inferred type: repr (a -> b) In the second argument of `app', namely `x' In the expression: app x x we actually get a type error -- with a good error message -- rather than a segmentation fault.

1 0

Re: HSCurses - Hello World? (can't find much documentation)
by Michael Forster 05 Nov '10

05 Nov '10

On Fri, Oct 22, 2010 at 17:09:52 EDT, Ari Rahikkala wrote: > On Fri, Oct 22, 2010 at 11:18 PM, Anonymous Void <byteasphyxia at gmail.com> wrote: > > But since I've never seen how the HSCurses functions really integrate > > together, or any tutorials/examples (besides hsFishEx), > > I fear toying with it is going to be annoying until I figure it out on my > > own, especially since I'm already doing a project for someone... > > So, I was wondering if anyone would be kind enough to point me to (or write > > me) a basic example of how HSCurses works, > > maybe w/ some very simple example of interactivity (e.g. getch processing, > > screen refresh, ..) support. > > Also, maybe warn me of some of the common pitfalls and differences in > > hscurses vs ncurses in other languages. > > UI.HSCurses.Curses follows ncurses quite closely so you can get pretty > far reading the NCurses Programming HOWTO: > http://tldp.org/HOWTO/NCURSES-Programming-HOWTO/ Also, http://invisible-island.net/ncurses/ncurses-intro.html. And below is a hscurses version of the first example from that document. Cheers, Mike module Main where import Control.Exception (bracket_) import qualified UI.HSCurses.Curses as Curses import qualified UI.HSCurses.CursesHelper as CursesH start = do Curses.initScr Curses.keypad Curses.stdScr True Curses.nl False Curses.cBreak True Curses.echo True hasColors <- Curses.hasColors if hasColors then do Curses.startColor Curses.initPair (Curses.Pair 1) (CursesH.black) (CursesH.black) Curses.initPair (Curses.Pair 2) (CursesH.green) (CursesH.black) Curses.initPair (Curses.Pair 3) (CursesH.red) (CursesH.black) Curses.initPair (Curses.Pair 4) (CursesH.cyan) (CursesH.black) Curses.initPair (Curses.Pair 5) (CursesH.white) (CursesH.black) Curses.initPair (Curses.Pair 6) (CursesH.magenta) (CursesH.black) Curses.initPair (Curses.Pair 7) (CursesH.blue) (CursesH.black) Curses.initPair (Curses.Pair 8) (CursesH.yellow) (CursesH.black) return () else return () Curses.wclear Curses.stdScr return () loop num = do c <- Curses.getch Curses.attrSet Curses.attr0 (Curses.Pair (mod num 8)) Curses.refresh if Curses.decodeKey c == Curses.KeyChar 'q' then return() else loop (num + 1) end = do Curses.endWin return () main = do bracket_ start end (loop 0)

2 1

ANNOUNCE: coroutine-enumerator
by Mario Blažević 05 Nov '10

05 Nov '10

The newly released coroutine-enumerator package can be used as a bridge between the enumerator and monad-coroutine packages. It provides two-way conversion functions between an Iteratee and an Await-suspending coroutine, and also between an Enumerator and a Yield-suspending coroutine. As a little example, the following program combines the http-enumerator, monad-coroutine, and SCC packages using the coroutine-enumerator package to print out all lines from the Hackage database containing substring "enumerator": > import Control.Exception.Base (SomeException) > import Control.Monad.Trans.Class (lift) > > import Data.ByteString (ByteString) > import Data.Text.Encoding (decodeUtf8) > > import Network.HTTP.Enumerator > > import Control.Monad.Coroutine > import Control.Monad.Coroutine.SuspensionFunctors > import Control.Monad.Coroutine.Nested > import Control.Monad.Coroutine.Enumerator > > import Control.Concurrent.SCC.Sequential > > main = httpRedirect (\_ _-> coroutineIteratee consumer) =<< parseUrl address > > address = "http://hackage.haskell.org/packages/archive/pkg-list.html" > > consumer :: Coroutine (Await [ByteString]) IO (Either SomeException ((), [ByteString])) > consumer = pipe translator (consume worker) >> return (Right ((), [])) > > translator :: Functor f => Sink IO (EitherFunctor (Await [a]) f) a -> Coroutine (EitherFunctor (Await [a]) f) IO () > translator sink = do chunks <- liftParent await > if null chunks > then lift (putStrLn "END") > else putList chunks sink >> translator sink > > worker :: Consumer IO ByteString () > worker = toChars >-> foreach (line `having` substring "enumerator") > (append (fromList "\n") >-> toStdOut) > suppress > > toChars :: Monad m => Transducer m ByteString Char > toChars = oneToOneTransducer decodeUtf8 >-> coerce Alternatively, the worker coroutine can parse the XML database and print out all elements whose any attribute value contains the substring "enumerator": > worker = toChars > >-> parseXMLTokens > >-> foreach (xmlElementHavingTagWith (xmlAttributeValue `having` substring "enumerator") > `nestedIn` xmlElementContent) > (coerce >-> toStdOut) > suppress

1 0

ANNOUNCE: version 0.6 of monad-coroutine and SCC
by Mario Blažević 05 Nov '10

05 Nov '10

Packages monad-coroutine and SCC have been upgraded to version 0.6 on Hackage. The monad-coroutine package exports a generic monad transformer Coroutine: Functor s => MonadTrans (Coroutine s). A Coroutine-transformed monad can suspend at any point, returning its resumption wrapped in the functor s. There are also some functions for manipulating and running coroutines, as well as a couple of useful suspension functors such as Yield and Await. Version 0.6 of monad-coroutine makes running pairs of coroutines simpler by adding several predefined resolvers. The new version of Streaming Component Combinators (a.k.a. SCC) is mostly about performance enhancements. The coroutines now communicate through chunks of data rather than individual items, which reduces the coroutine switching overhead. The module hierarchy has been reorganized for easier use, but the exported type and function names have mostly remained the same. Both packages have been tested with GHC 6.12.3 and 7.0.

1 0

Concurrency and ghc 6.12.3
by Ketil Malde 05 Nov '10

05 Nov '10

Hi, On experimenting with concurrency, I got somewhat surprising result. I have a program ('flower') that reads an input file and generates one or more output files. As the different output files are independent, I construct one IO action for each output requested on the command line, and just forkIO one thread for each. This seems to work fairly well on one CPU, so I decided to try on multiple CPUs, using +RTS -N. To my surprise, this made the program take several times longer (wall clock). So I tried -N2 and -N4 to see how that turned out. Results are, in minutes: format def -N2 -N4 -N i 0 0 0 0 q 2 2 2 13 f 2 5 1 14 h 8 10 2 27 s 26 17 11 - T 37 23 16 - F 47 38 42 - CPU 2543u 3027u (lost) - 36s 593s 92% 161% I had to terminate -N after five hours wall time and 158026.68s user 60682.76s system 1210% CPU. So, well, it seems this scales okay as long as there are enough threads, but scales *horribly* when you run more threads than processes. Is this a correct assessment? Would it make sense to simply cap -N to the number of forked threads? I guess I should try this with GHC7, but is there reason to believe it will perform better? -k -- If I haven't seen further, it is by standing in the footprints of giants

2 1

Is "let" special?
by Günther Schmidt 05 Nov '10

05 Nov '10

Hi all, is there something special about "let"? I don't mean only its use in haskell, but in the general context of programming languages. I've been given a few hints over time when I asked question concerning DSLs but regretfully didn't follow them up. Günther

11 12

What's happening at the next Ed Lambda
by Ollie Saunders 05 Nov '10

05 Nov '10

Hello everyone, I know the last meetup was a bit light on content so we've been working hard to make this month different. We've got, not one, but two talks for you: First up: Paul Richards will be talking about functional data structures. In particular, finger trees, which were only discovered in 2006 and offer incredible versatility and performance. Then: I'll be talking about my experiences writing Snake in Haskell using HCurses and property-based testing with QuickCheck. With a bit of luck this talk will assume very little prior knowledge of Haskell. Details: http://www.meetup.com/ed-lambda/calendar/15219997/ Looking forward to seeing you, Ollie

1 0

Re: [Haskell-cafe] Unexpected results from ExitCode
by Peter Schmitz 05 Nov '10

05 Nov '10

Dean, Thanks very much, that indeed worked. -- Peter On Wed, Nov 3, 2010 at 9:16 PM, Dean Herington <heringtonlacey(a)mindspring.com> wrote: > At 8:45 PM -0700 11/3/10, Peter Schmitz wrote: >> >> I have a program (test.hs): >> >>> module Main (main) where >>> import System.Exit >>> main :: IO ExitCode >>> main = do >>> return (ExitFailure 1) >> >> >> In another program, I invoke it via 'system': >> >>> exitCode <- system ".\\test.exe" >>> case (exitCode) of >>> ExitFailure failCnt -> do >>> putStrLn $ "-- Fail count is: " ++ show failCnt >>> exitFailure >>> ExitSuccess -> do >>> putStrLn $ "-- OK." >>> exitSuccess >> >> >> but it always gets ExitSuccess (not ExitFailure failCnt as I expected). >> (I am running under Windows.) >> >> My other use of system (such as with ghc commands) works okay, >> sometimes getting success, sometimes failure, as expected. >> >> Any suggestions much appreciated. >> Thanks, >> -- Peter > > The value returned by `main` is always discarded. You need to use > `exitWith`: > > main = exitWith (ExitFailure 1) >

1 0

Re: [Haskell-cafe] Re: HDBC/ODBC using mysql appears broken...
by John Goerzen 05 Nov '10

05 Nov '10

On 11/04/2010 04:03 PM, Neil Davies wrote: > John > > simple queries seem to work fine. > > main = do > db<- connectODBC connect'string > quickQuery db "select serial from CPE_DEVICE" []>>= print . take 3 > describeTable db "CPE_DEVICE" I'm going to guess that the MySQL ODBC driver isn't implementing the metadata stuff, which is kind of weak in ODBC in general. Can I interest you in PostgreSQL or Sqlite3? ;-) - John > > returns > > [[SqlByteString "1MD16BBV00F74"],[SqlByteString "1MD16BB700D03"],[SqlByteString "1MD16BBH02DCD"]] > *** Exception: Prelude.(!!): index too large > > if that helps.. > > Neil > > On 4 Nov 2010, at 19:41, John Goerzen wrote: > >> On 11/04/2010 01:39 PM, Neil Davies wrote: >>> Hi >>> >>> I've been trying to get the "hello world" example below to work: >>> >>> main = do >>> db<- connectODBC connect'string >>> get Tables db>>= print >> >> I wonder if you might try something other than getTables, which requires some metadata support on the backend? Some simple inserts orselects, perhaps? >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe > >

1 0

Re: HDBC/ODBC using mysql appears broken...
by John Goerzen 04 Nov '10

04 Nov '10

On 11/04/2010 01:39 PM, Neil Davies wrote: > Hi > > I've been trying to get the "hello world" example below to work: > > main = do > db<- connectODBC connect'string > get Tables db>>= print I wonder if you might try something other than getTables, which requires some metadata support on the backend? Some simple inserts orselects, perhaps?

1 0

Re: [Haskell-cafe] What is simplest extension language to implement?
by Permjacov Evgeniy 04 Nov '10

04 Nov '10

ehm. I missed something and ghc api is well documented and stable ? The problem is that haskell is overkill here and it requires esoteric extensions for some tasks (Rank2Types for reflection, for example). It is inacceptable for scripting language, faced to no-programmers. Such languages must be as plain and regular, as possible. So, statically typed languages are out of consideration: they either have poor expressive power, or have expressive, but complex type system (as haskell), or relies on hacks within language for some tasks(typecasts and duck typing in c++). Scheme may be acceptable, and possibly EcmaScript. I do not know any other acceptable standartized language. Yet, Scheme has really ugly synthax and Ecma-262 standard is 250 pages long (Of course, it is not C++ with about 1000 pages in standart, but it still hurts to read 250 pages of techdocs in foreign language). On 11/02/2010 12:21 PM, Lennart Augustsson wrote: > I don't understand. Why don't you use Haskell as the scripting language? > > On Tue, Nov 2, 2010 at 7:04 AM, Permjacov Evgeniy <permeakra(a)gmail.com> wrote: >> Let us think, that we need some scripting language for our pure haskell >> project and configure-compile-run is not a way. In such a case a >> reasonably simple, yet standartized and wide known language should be >> implemented. What such language may be? >> R(4/5/6)RS ? >> EcmaScript ? >> Some other ? >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >>

4 5

call for a better timing method
by Jiansen He 04 Nov '10

04 Nov '10

Hello, I want to write a timing method to test sub steps in my program. I tested my timing method as follows: ===== time.hs ===== import Data.Time time :: String -> IO a -> IO a time str a = do start <- getCurrentTime v <- a end <- getCurrentTime let diff = diffUTCTime end start putStrLn $ str ++ showNominalDiffTime (diff) return v showNominalDiffTime :: NominalDiffTime -> String showNominalDiffTime nTime = let timeString = show nTime in take (length timeString - 1) timeString ++ " sec" fun :: IO (Int, [Int]) fun = do i <- time "product: " $ return $! product [1..10000] -- testing strictness : evaluated j <- time "product list: " $ return $! map (\_ -> product [1..10000]) [1..3] -- testing strictness : not evaluated List is in hnf return (i, j) main = fun ===== In the shell ===== $ time ./time product: 0.001813 sec product list: 0.000001 sec real 0m0.006s user 0m0.000s sys 0m0.005s Notice that the ($!) operation will reduce a term to head normal form (hnf), not normal form. In the "product list" testing, the "complicated" map evaluation is reduced to a list of redex. Therefore, the result does not reflect the time needed to get the final value. How could I improve my timing method so that it will give the time to reduce a general expression to normal form? Jiansen

2 1

Re: Splittable random numbers
by Bryan O'Sullivan 04 Nov '10

04 Nov '10

On Thu, Nov 4, 2010 at 11:39 AM, Thomas DuBuisson <thomas.dubuisson(a)gmail.com> wrote: > Before we bother to do that I think it would be worth deciding what > level of performance we are trying to achieve. On my laptop (Core2 > 2.5Ghz) I generate 4MB of random values in less than 900ms (HashDRBG). > What is StdGen getting, which I know people consider slow? I measured StdGen last year as generating around a million values per second. My mwc-random library is about 100 times faster when running in the ST monad.

1 0

Re: [Haskell-cafe] DSL libraries
by Dupont Corentin 04 Nov '10

04 Nov '10

Hello, With your version it compiles. Thanks. Corentin On Thu, Nov 4, 2010 at 7:03 PM, Sebastiaan Visser <s(a)fvisser.nl> wrote: > Dupont, > > The `AwesomePrelude' (we apologize for this name) is a very experimental > project that is not a workable library or executable at all. You can better > consider it to be a `bunch of code' with some interesting ideas and some > interesting potential. > > At least my branch[1] runs on my machine and I'm still able to produce > javascript code. > > But don't expect it to solve any of your problems. ;-) We (at least I) > still have some ideas for this project and hope we can someday revive it. > > Cheers, > Sebastiaan > > [1] https://github.com/sebastiaanvisser/AwesomePrelude > > On Nov 4, 2010, at 6:51 PM, Dupont Corentin wrote: > > Nobody had the compilation messages I had? > > > > ---------- Forwarded message ---------- > > From: Dupont Corentin <corentin.dupont(a)gmail.com> > > Date: Tue, Nov 2, 2010 at 2:30 PM > > Subject: [Haskell-cafe] DSL libraries (Was: Map constructor in a DSL) > > To: steffen <steffen.siering(a)googlemail.com>, haskell-cafe(a)haskell.org > > > > > > Hello Steffen, > > can you compile AwesomePrelude? > > I've got error messages (see below). > > > > By the way, I've looked at some DSLs made in Haskell, if I don't mistake > there are lots of similarities between them. > > There similarities could be put in a library to help the implementors of > a DSL, more or less like AwesomePrelude. > > > > Is there already packages on Hackage dealing with that? > > > > Cheers, > > Corentin > > > > AwesomePrelude compilation error: > > I'm using GHC 6.12.1. > > > > > cd tomlokhorst-AwesomePrelude-9819315 > > > cabal install > > (...) > > Warning: Lang.Haskell: Instances of type and data families are not yet > supported. Instances of the following families will be filtered out: H > > > > > > Then, when trying with a very simple GATD in GHCI, I've got: > > > > <interactive>: HSAwesomePrelude-0.1.0.o: unknown symbol > `AwesomePreludezm0zi1zi0_ > > CompilerziLiftDefinitions_inline_closure' > > ghc: unable to load package `AwesomePrelude-0.1.0' > > > > > > > > On Thu, Oct 28, 2010 at 2:02 PM, steffen <steffen.siering(a)googlemail.com> > wrote: > > I think you would love to have a look at AwesomePrelude[1] or a fork > > of AwesomePrelude using associated types[2] > > Some more background information by Tom Lokhorst [3][4]. > > > > [1] http://github.com/tomlokhorst/AwesomePrelude > > [2] http://github.com/urso/AwesomePrelude > > [3] http://tom.lokhorst.eu/2009/09/deeply-embedded-dsls > > [4] http://tom.lokhorst.eu/2010/02/awesomeprelude-presentation-video > > > > On 28 Okt., 12:09, Dupont Corentin <corentin.dup...(a)gmail.com> wrote: > > > Thank you for your rich responses. > > > > > > Indeed I think I miss some thinks in my DSL, that would make things > easier > > > to deal with lists and first class functions. > > > I don't really know what for now. > > > Perhaps a List Constructor? Or a constructor on functions like yours > Ryan? > > > EAp :: Exp ref (a -> b) -> Exp ref a -> Exp ref b > > > It's from which DSL? It is accessible on the net? > > > > > > Chris suggested me that I can only define the Foldr constructor and > deduce > > > Map from it. > > > But maybe I have to add a List constructor for that. > > > > > > But in the suggestions from Ryan and Brandon I don't understand why I > should > > > add an extra type parameter and what it is! > > > > > > Steffen: Wow nice. I'll integrate that ;) > > > > > > I'm also looking at the Atom's DSL to get inspiration. > > > Something I don't understand in it is that it has two languages, on > typed: > > > > > > data E a where > > > VRef :: V a -> E a > > > Const :: a -> E a > > > Cast :: (NumE a, NumE b) => E a -> E b > > > Add :: NumE a => E a -> E a -> E a > > > etc. > > > > > > And, along with it, an untyped counterpart: > > > > > > -- | An untyped term. > > > data UE > > > = UVRef UV > > > | UConst Const > > > | UCast Type UE > > > | UAdd UE UE > > > etc. > > > > > > What that for? What's the use of having beautiful GADT if you have to > > > maintain an untyped ADT aside?? > > > > > > Corentin > > > > > > _______________________________________________ > > > Haskell-Cafe mailing list > > > Haskell-C...@haskell.orghttp:// > www.haskell.org/mailman/listinfo/haskell-cafe > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > >

1 0

Non-hackage cabal source
by Tony Morris 04 Nov '10

04 Nov '10

I am trying to set up an apache server as an additional source to hackage for haskell packages. I have added to my ~/.cabal/config file: remote-repo: myhackage:http://myhackage/packages I am able to successfully make the HTTP requests: http://myhackage/packages/00-index.tar.gz http://myhackage/packages/MyPackage/0.0.1/MyPackage-0.0.1-tar.gz I can execute: $ tar -tvf 00-index.tar.gz -rw-r--r-- root/root 1491 2010-11-03 15:16 ./MyPackage/0.0.1/MyPackage.cabal However, when I try to unpack my package with cabal: $ cabal unpack MyPackage Downloading MyPackage-0.0.1... cabal: Failed to download http://myhackage/packages/package/MyPackage-0.0.1.tar.gz : ErrorMisc "Unsucessful HTTP code: 404" Why is cabal even making this request? Why is it not making the request to http://myhackage/packages/MyPackage/0.0.1/MyPackage-0.0.1.tar.gz Thanks for any tips. -- Tony Morris http://tmorris.net/

4 3

Import proposal from Haskell'?
by John Smith 04 Nov '10

04 Nov '10

Type-directed name resolution was proposed by SPJ for Haskell', but not for GHC. Is there any particular process for importing tickets from Haskell' to GHC? http://hackage.haskell.org/trac/haskell-prime/ticket/129

1 0

Unexpected results from ExitCode
by Peter Schmitz 04 Nov '10

04 Nov '10

I have a program (test.hs): > module Main (main) where > import System.Exit > main :: IO ExitCode > main = do > return (ExitFailure 1) In another program, I invoke it via 'system': > exitCode <- system ".\\test.exe" > case (exitCode) of > ExitFailure failCnt -> do > putStrLn $ "-- Fail count is: " ++ show failCnt > exitFailure > ExitSuccess -> do > putStrLn $ "-- OK." > exitSuccess but it always gets ExitSuccess (not ExitFailure failCnt as I expected). (I am running under Windows.) My other use of system (such as with ghc commands) works okay, sometimes getting success, sometimes failure, as expected. Any suggestions much appreciated. Thanks, -- Peter

2 1

Haskell Weekly News: Issue 157 - November 03, 2010
by Daniel Santa Cruz 04 Nov '10

04 Nov '10

Welcome to issue 157 of the HWN, a newsletter covering developments in the [1]Haskell community in the week of October 24 - 30. Seems that things quieted down a bit this week. We saw 85 new or updated packages in Hackage, 29 (-5) stories posted to Reddit, 25 (-22) questions posted to StackOverflow, and 302 (-20) messages posted on the -cafe mailing list. Maybe some of the effects of ICFP have died down a bit? Don Stewart posted some interesting numbers comparing different programming communities in Reddit. As we can see, we seem to be very well represented: /r/python 16.3k users /r/javascript 8k users /r/ruby 6k users /r/php 5.7k users /r/haskell 5.5k users /r/cpp 5.3k users /r/lisp 3.9k users /r/perl 3.5k users /r/java/ 3k users /r/erlang 1.8k users /r/c_programming 1.7k users /r/scheme 1.2k users /r/clojure 1.2k users /r/asm 1.1k users /r/scala 1k users We hope to see pictures, videos, and blog posts from the people that are attending BelHac this weekend! So, what was hot last week? Announcements Anton Kholomiov [2]announced his new csound-expression combinator libary. It features liberation from id-style csound code, haskore-like composition structures, type-safe composable opcodes and simple instrument interface. Maciej Piechotka [3]announced a new version of iteratee-compress, which provides compressing and decompressing enumerators including flushing. Currently only gzip is available, but bzip is planned. Tim Docker [4]announced an update to his Chart (v0.14) library. This is a library for drawing 2D data plots. Eugene Kirpichov [5]announced an update to his timeplot library. This library is a tool for visualizing time series from log files. He also provided us with some [6]pretty pictures. Niklas Broberg couldn't find a library to manipulate Java source code, so he [7]announced the creation of language-java. One of the [8]bigger updates last week was brought to us by the darcs team. It contains many improvements over the 2.4 series. Most notable are the performance improvements in record and pull, and the --bisect option for trackdown. Way to go guys! John Lato [9]announced the initial release of mutable-iter, an extension to the iteratee package which uses mutable buffers. Top Reddit Stories * Neat Haskell commenting trick Domain: self.haskell Score: 77, Comments: 6 On Reddit: http://www.reddit.com/r/haskell/comments/dyvsi/neat_haskell_commenting_tric… Original: /r/haskell/comments/dyvsi/neat_haskell_commenting_trick/ * Haskell Halloween! Domain: i.imgur.com Score: 53, Comments: 5 On Reddit: http://www.reddit.com/r/haskell/comments/dyznc/haskell_halloween/ Original: http://i.imgur.com/t7qWf.jpg * OCaml for Haskellers (Inside 245s) Domain: blog.ezyang.com Score: 41, Comments: 27 On Reddit: http://www.reddit.com/r/haskell/comments/dw2cy/ocaml_for_haskellers_inside_… Original: http://blog.ezyang.com/2010/10/ocaml-for-haskellers/ * MTL 2.0 has been uploaded good bye old mtl, hello monads-fd! Domain: hackage.haskell.org Score: 40, Comments: 11 On Reddit: http://www.reddit.com/r/haskell/comments/dx8ml/mtl_20_has_been_uploaded_goo… Original: http://hackage.haskell.org/package/mtl-2.0.0.0 * Linear vs. compositional type checking Domain: gergo.erdi.hu Score: 34, Comments: 10 On Reddit: http://www.reddit.com/r/haskell/comments/dwip1/linear_vs_compositional_type… Original: http://gergo.erdi.hu/blog/2010-10-23-the_case_for_compositional_type_checki… * Tour of a real toy Haskell program Domain: mainisusuallyafunction.blogspot.com Score: 29, Comments: 1 On Reddit: http://www.reddit.com/r/haskell/comments/dwjvo/tour_of_a_real_toy_haskell_p… Original: http://mainisusuallyafunction.blogspot.com/2010/10/tour-of-real-toy-haskell… * Ride the snake: Calling Python libraries from Haskell Domain: john-millikin.com Score: 26, Comments: 1 On Reddit: http://www.reddit.com/r/haskell/comments/dxj0e/ride_the_snake_calling_pytho… Original: http://john-millikin.com/articles/ride-the-snake/ * Announcing: Snap Framework v0.2.15 Domain: snapframework.com Score: 25, Comments: 3 On Reddit: http://www.reddit.com/r/haskell/comments/dy94z/announcing_snap_framework_v0… Original: http://snapframework.com/blog/2010/10/27/snap-0.2.15-released * Yesod 0.6.0 Released Domain: docs.yesodweb.com Score: 23, Comments: 8 On Reddit: http://www.reddit.com/r/haskell/comments/dwn5v/yesod_060_released/ Original: http://docs.yesodweb.com/blog/yesod-0-6/ * darcs 2.5 released Domain: wiki.darcs.net Score: 21, Comments: 3 On Reddit: http://www.reddit.com/r/haskell/comments/dyo7d/darcs_25_released/ Original: http://wiki.darcs.net/Releases/2.5 Top StackOverflow Questions * type constructor or class error Question (votes: 0, answers: 4): Read on SO: http://stackoverflow.com/questions/4060486/type-constructor-or-class-error * parse error in else Question (votes: 0, answers: 1): Read on SO: http://stackoverflow.com/questions/4053638/parse-error-in-else * xmonad could not find module `XMonad': broken package Question (votes: 0, answers: 1): Read on SO: http://stackoverflow.com/questions/4037101/xmonad-could-not-find-module-xmo… * Haskell: I/O and Returning From a Function Question (votes: 1, answers: 3): Read on SO: http://stackoverflow.com/questions/4056867/haskell-i-o-and-returning-from-a… * Collapse arrow to list Question (votes: 1, answers: 1): Read on SO: http://stackoverflow.com/questions/4030439/collapse-arrow-to-list * how to force the evaluation of this expression? Question (votes: 1, answers: 1): Read on SO: http://stackoverflow.com/questions/4027097/how-to-force-the-evaluation-of-t… * How to get type parameters from SYBs dataTypeOf Question (votes: 1, answers: 1): Read on SO: http://stackoverflow.com/questions/4007693/how-to-get-type-parameters-from-… * How to sort the list by its accountID using quick sort in Haskell. Question (votes: 1, answers: 2): Read on SO: http://stackoverflow.com/questions/4006811/how-to-sort-the-list-by-its-acco… * Parsec parse many question Question (votes: 2, answers: 2): Read on SO: http://stackoverflow.com/questions/4058765/parsec-parse-many-question * Type error using Criterion Question (votes: 2, answers: 2): Read on SO: http://stackoverflow.com/questions/4033103/type-error-using-criterion HackageDB Updates Who contributed, and what got uploaded? * AdamVogt: xmonad-extras * AlexSuraci: atomo * AndreasVoellmy: nettle-openflow * AndrewCoppin: AC-EasyRaster-GTK, AC-Terminal * BenLippmeier: buildbox, buildbox-tools * BryanOSullivan: text-icu * ChrisDone: throttle * ChrisKuklewicz: hprotoc, protocol-buffers-descriptor * ChristiaanBaaij: clash * ClaudeHeilandAllen: gmndl * DanDoel: bytestring-show * DanielDiaz: DTC * DougBeardsley: heist * ErikHesselink: flock * EugeneKirpichov: timeplot * GaneshSittampalam: HTTP * GregoryCollins: snap-core, snap-server * GregoryWeber: sifflet, sifflet-lib * GrzegorzChrupala: morfette * HeinrichApfelmus: operational * IlyaPortnov: todos * JacobStanley: network-info * JanRochel: SoccerFun * JasperVanDerJeugt: hakyll * JeremyShaw: web-routes-happstack * JinjingWang: practice-room * JohanTibell: network * JohnGoerzen: twidge * JohnLato: ListLike, hCsound, iteratee, iteratee-mtl, mutable-iter, sndfile-enumerators * JohnMacFarlane: texmath * JohnMillikin: cpython, expat-enumerator, libxml-enumerator, libxml-sax, ncurses, yajl-enumerator * JudeNagurney: augeas * KetilMalde: bio, dephd, flower * KiYungAhn: yices * MaciejPiechotka: iteratee-compress * MarcosViera: AspectAG * MattBrown: MissingPy * MaxBolingbroke: ansi-terminal * MichaelIlseman: yaml-light * MichaelSnoyman: authenticate, hamlet, neither, persistent, persistent-postgresql, persistent-sqlite, wai, yesod, yesod-auth, yesod-examples * MichelBoucey: ip6addr * NiklasBroberg: language-java * PetrRockai: shellish * ReinierLamers: darcs * RichardSmith: linux-ptrace, posix-waitpid * RossPaterson: mtl * SebastianFischer: weighted-regexp * SinisaBidin: websockets * ThomasDuBuisson: crypto-api, hsXenCtrl, pureMD5 * ThomasSchilling: murmur-hash * TimDocker: Chart * TomHawkins: improve * TonyMorris: JSON-Combinator * VincentHanquez: cryptohash * YoshikuniJujo: mtlparse, regexpr About the Haskell Weekly News New editions are posted to [10]the Haskell mailing list as well as to [11]the Haskell Sequence and [12]Planet Haskell. [13]RSS is also available, and headlines appear on [14]haskell.org. To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. The code used to produce this version of the newsletter is not yet publicly available, as it is a complete hack designed to get things started again. Until next time, Daniel Santa Cruz References 1. http://haskell.org/ 2. http://hackage.haskell.org/package/csound-expression 3. http://groups.google.com/group/haskell-cafe/browse_thread/thread/faf1da04ba… 4. http://groups.google.com/group/haskell-cafe/browse_thread/thread/0759530428… 5. http://groups.google.com/group/haskell-cafe/browse_thread/thread/4215122a77… 6. http://www.haskell.org/haskellwiki/Timeplot 7. http://groups.google.com/group/haskell-cafe/browse_thread/thread/4c9b3e1499… 8. http://groups.google.com/group/haskell-cafe/browse_thread/thread/229c751ebc… 9. http://article.gmane.org/gmane.comp.lang.haskell.general/18274 10. http://www.haskell.org/mailman/listinfo/haskell 11. http://sequence.complete.org/ 12. http://planet.haskell.org/ 13. http://sequence.complete.org/node/feed 14. http://haskell.org/

1 0

Generalized monadic exception handling with monad-peel
by Anders Kaseorg 04 Nov '10

04 Nov '10

I just released the monad-peel library to Hackage. http://hackage.haskell.org/package/monad-peel MonadPeelIO is a simple class that allows lifting monadic control operations, such as those in Control.Exception and Foreign.Marshal.Alloc, through layers of monad transformers. It comes with instances for every transformer in the transformers package except ContT, and uses no Haskell extensions. class MonadTrans t => MonadTransPeel t where peel :: (Monad m, Monad n, Monad o) => t n (t m a -> m (t o a)) class MonadIO m => MonadPeelIO m where peelIO :: m (m a -> IO (m a)) I’ve included a wrapped version of Control.Exception with types generalized to all monads in MonadPeelIO, for example: catch :: (MonadPeelIO m, Exception e) => m a -> (e -> m a) -> m a catch a handler = do k <- peelIO join $ liftIO $ Control.Exception.catch (k a) (\e -> k $ handler e) I’m hoping this package can become a suitable replacement for MonadCatchIO, whose implementation of ‘finally’ was recently shown to be broken with certain monads (see “MonadCatchIO, finally and the error monad” on this list). The home page has links to a Git repository and has references to older mailing list threads providing some background: http://andersk.mit.edu/haskell/monad-peel/ Thanks to everyone who contributed to those threads and helped shape the ideas that led to this package (and if it turns out more ideas are needed, thanks for those too!). Anders

2 2

Bug in HTTP (bad internal error handling)
by Bit Connor 03 Nov '10

03 Nov '10

Hello, I have reported this problem to the maintainer of the HTTP package about 2 weeks ago, but have not yet received a response, so I am reporting it here. I am using a recent git check out of HTTP 4000.0.10 The bufferReadLine function from Network.TCP has a bug in how it handles an IOException after it catches one. It checks for isEOFError e, and that branch of code seems to be ok. The problem is how it handles other types of IO errors. One type of such an error, for example, is: <socket: 117>: Data.ByteString.hGetLine: resource vanished (Connection reset by peer) After it catches this error, the function returns (line 376): return (fail (show e)) The "fail" is running in the Either monad (The Result type = Either). This calls the default Monad implementation of fail, which is just a call to plain old error. This basically causes the entire program to crash. A simple fix, is to instead just reraise the original error: ioError e This will propagate the error through as a normal IOException, which can then be caught by the caller (such as the caller of simpleHTTP). By looking at the library code, the same bug seems to be in other places (bufferGetBlock, bufferPutBlock), but I have only observed the bug actually occurring in bufferReadLine. Actually, it appears that simpleHTTP isn't actually supposed to throw an IOException, and it is instead supposed to return a ConnError result. So the real fix is to fix the code to make this happen. But I've found simpleHTTP to throw an IOException in a lot of circumstances, so for now I think the fix above is a good immediate solution. Thanks, Bit Connor

7 11

Benchmark
by André Batista Martins 03 Nov '10

03 Nov '10

Hi, exist some benchmark "active" to use in my libraries, i want test memory usage and performance? Cheers, André

3 3

Re: [Haskell-cafe] ffmpeg-tutorials fails to build
by John Lato 03 Nov '10

03 Nov '10

> > > Message: 10 > Date: Wed, 3 Nov 2010 00:38:53 -0200 > From: Victor Oliveira <rhapsodyv(a)gmail.com> > > Hi, > > Info: > ghc 6.12.3 (installed from haskell plataform) > Mac os x snow leopard 10.6.4 > ffmpeg instaled from ports - ffmpeg @0.6.1 > hs-ffmpeg 0.3.4 - installed from cabal-install > > When I try compile this package, I get this erros: > >

1 0

Re: [Haskell-cafe] [ANNAUNCE] ghcjs-0.1.0 Haskell to Javascript compiler
by Martijn Schrage 03 Nov '10

03 Nov '10

On 02-11-10 18:57, Aaron Gray wrote: > On 2 November 2010 14:00, Martijn Schrage <martijn(a)oblomov.com > <mailto:martijn@oblomov.com>> wrote: > > On 01-11-10 22:35, Aaron Gray wrote: >> >> Right, FF seems okay now on Vista too. >> >> Chrome :- >> ... > It works on Firefox, Chrome and Safari now, both on Windows and > Mac. The IE problems seem a bit more subtle, as the script goes > into an infinite loop. > > > Great, thanks, I will check it out later when I get some time. > > Aaron Ok, after a bit of nasty IE Javascript debugging, I got it to work on IE8 as well. This only holds for the example at http://tryout.oblomov.com/ghcjs/ghcjs.html though. The ghcjs sources still need to be updated. Until then, you might run into some other problems using IE when building more fancy examples. -- Martijn

1 0

darcs hacking sprint 5 report
by Eric Kow 03 Nov '10

03 Nov '10

Hi everybody, Here's a little report from the most recent Darcs hacking sprint (held 15-17 October in Orleans) http://blog.darcs.net/2010/10/darcs-hacking-sprint-5-report.html -- Eric Kow <http://www.nltg.brighton.ac.uk/home/Eric.Kow> For a faster response, try +44 (0)1273 64 2905 or xmpp:kowey@jabber.fr (Jabber or Google Talk only)

1 0

-fwhole-program (for gcc) in cabal files?
by Johannes Waldmann 03 Nov '10

03 Nov '10

Hi. In a project with Haskell + FFI, I recently split my C program into modules (files). I don't want to lose optimization though, so I want to use something like gcc -c -o complete.o -fwhole-program -combine *.c (doing whole program optimization/inlining etc., I hope). How do I write this in the cabal file? It seems ghc compiles each C source individually. Or should I use "-flto" on individual compilations? (That's what Gimple was invented for, actually?) But does this play nicely with ghc's linker? (Which is gcc, so it should be fine?) J.W.

1 0

Scrap your rolls/unrolls
by Max Bolingbroke 03 Nov '10

03 Nov '10

Hi Cafe, In generic programming, e.g. as in "Data Types a la Carte" and Compos, you often wish to work with data types in their fix-point of a functor form. For example: """ data ListF a rec = Nil | Cons a rec newtype List a = Roll { unroll :: ListF a (List a) } """ In some presentations, this is done by factoring the fix point like so: """ newtype FixF f = Roll { unroll :: f (FixF f) } type List a = FixF (ListF a) """ This is all well and good, but it means when working with data types defined in this manner you have to write Roll and unroll everywhere. This is tedious :-( Something I have long wished for is equirecursion, which would let you write this instead: """ type List a = ListF a (List a) """ Note that we now have a type instead of a newtype, so we won't need to write any injection/extraction operators. However, currently GHC rightly rejects this with a complaint about a cycle in type synonym declarations. However, you can use type families to hide the cycle from GHC: """ type List a = ListF a (Force (ListThunk a)) data ListThunk a type family Force a type instance Force (ListThunk a) = List a """ Unfortunately, this requires UndecidableInstances (the type instance RHS is not smaller than the instance head). And indeed when we turn that extension on, GHC goes into a loop, so this may not seem very useful. However, we can make this slight modification to the data type to make things work again: """ data ListF a rec = Nil | Cons a (Force rec) type List a = ListF a (ListThunk a) """ Note that the application of Force has moved into the *use site* of rec rather than the *application site*. This now requires no extensions other than TypeFamilies, and the client code of the library is beautiful (i.e. has no rolls/unrolls): """ example, ones :: List Int example = Cons 1 (Cons 2 Nil) ones = Cons 1 ones """ We can factor this trick into a fix point combinator that does not require roll/unroll: """ type Fix f = f (FixThunk f) type List a = Fix (ListF a) data FixThunk f type family Force a type instance Force (FixThunk f) = Fix f """ The annoying part of this exercise is the the presence of a "Force" in the functor definition (e.g ListF) means that you can't make them into actual Functor instances! The fmap definition gives you a function of type (a -> b) and you need one of type (Force a -> Force b). However, you can make them into a category-extras:Control.Functor.QFunctor instance (http://hackage.haskell.org/packages/archive/category-extras/0.53.5/doc/html…) by choosing the "s" Category to be: """ newtype ForceCat a b = ForceCat { unForceCat :: Force a -> Force b } instance Category ForceCat where id = ForceCat id ForceCat f . ForceCat g = ForceCat (f . g) """ Rather than the usual "Hask" Category which is used in the standard Functor. This is somewhat unsatisfying, but I'm not sure there is a better way. I haven't seen this trick to "emulate equirecursion" before. Has it showed up anywhere else? Cheers, Max

5 8

ffmpeg-tutorials fails to build
by Victor Oliveira 03 Nov '10

03 Nov '10

Hi, Info: ghc 6.12.3 (installed from haskell plataform) Mac os x snow leopard 10.6.4 ffmpeg instaled from ports - ffmpeg @0.6.1 hs-ffmpeg 0.3.4 - installed from cabal-install When I try compile this package, I get this erros: I'm build it to get a starting point to use hs-ffmpeg. Anyone have the same problem? Any ideias? Thanks, Victor

1 0

profiling and strictness (was: Re: [Haskell-cafe] An interesting paper from Google)
by Evan Laforge 03 Nov '10

03 Nov '10

> Any time you see something "inexplicable" like lots of time being attributed > to something simple like "get", it means that something isn't strict enough > and "get" is having to force a bunch of lazy evaluations to do its job. > Since you're using State.Strict but lift-ing to get there, I'd first look at > the strictness of the monad you're lift-ing from. (I'm assuming > State.Strict does what the label says, but it's possible that it's not > strict in the way you need; strictness is kinda tricky.) Ahh, thanks, this makes sense to me. The monads involved are an ErrorT and one other strict StateT. When the monads say they are strict, they seem to mean that >>= will force the evaluation of the monadic mechanics of the first argument when the second is demanded. By "monadic mechanics" I mean the work done by >>= itself, so for StateT it binds the result of the first argument with either 'let' or '~(a, s') <-' instead of 'case' or '(a, s') <-'. So in my understanding, a lazy State will build thunks on every >>= and they won't ever be forced until you pull the final value and pass it to a strict function in IO like print or an FFI call, and then they are forced recursively, at which point there is a stack overflow unless it's a short sequence. Talking about strictness seems complicated to me because it's not really about what is actually forced, it's about who is forced by who (i.e., when 'a' is forced then 'b' will be) and whether it's forced in sequence or nested. So a strict State is also not forced until the strict IO function, but it turns into a sequential series of thunks rather than a nested one. If this is accurate, why would anyone want to use the lazy State? Anyway, State doesn't provide the really critical bit of strictness, which is in the actual state being modified. So 'State.modify' will inevitably lead to stack overflow if you don't intersperse 'get's in there. What is needed is both $! on State.put (or define your own strict modify that puts $! on put), and strictness annotation of the fields of the record that is being modified (provided it's a record and not a scalar type). In any case, ErrorT has a strict >>= by necessity, both States have a strict >>=, and all modifys and puts (I think) are strict, as are all the record fields. In any case, a simple 'get' should only have to force the state to whnf, so it shouldn't matter if the fields are huge thunks or not, so really all that should matter is if the state is already in whnf or is a huge nest of thunks. Which a strict >>= should prevent, right? This is right next to a 'lookup' function which is called twice as many times and should be doing more work since it actually looks in a Map, but is credited with less cpu and alloc. A lift . get should turn into -- ErrorT a <- do a <- StateT (\s -> return (s, s)) -- 'get' of the outer StateT return (a, s) -- >>= belongs to inner StateT return (Right a) Anyway, it's sort of a mishmash of semi inlined functions since this is especially nested and hard to understand with transformers, but it looks like a lot of applications of (\a -> (a, s)) type functions, so (,) constructors and a 'Right' constructor, inside of 'case's to make sure this are evaluations and not allocations (and I remember from the STG paper that a constructor inside a case need not even be constructed). So I don't totally understand where the forcing is happening to make credit the 'get' with so much work. BTW, I have a theory about what the '0' might mean... perhaps if a function is inlined away, it still appears in the profile list, but with 0 entries. > Moral of the story: time is accounted to the function that forces > evaluation of lazy thunks, not to the thunks themselves or the function that > created the lazy thunks. (I think the latter is impossible without passing > around a lot of expensive baggage, and in any case doesn't tell you anything > useful; unexpected functions taking a lot of time, on the other hand, tells > you right away that there's excessive laziness in the invocation somewhere > and gives you a starting point to track it down.) Indeed, this is a good point that I hadn't realized in quite that way before, but tracking it down is still the tricky part.

1 1

problems installing latest parsec
by Michael Litchard 03 Nov '10

03 Nov '10

Here's what cabal says I have installed * parsec Synopsis: Monadic parser combinators Latest version available: 3.1.0 Latest version installed: 2.1.0.1 Homepage: http://www.cs.uu.nl/~daan/parsec.html License: BSD3 but here is what happens when I try to upgrade cabal upgrade parsec Resolving dependencies... No packages to be installed. All the requested packages are already installed. If you want to reinstall anyway then use the --reinstall flag. mlitchard@kether:~/projects/perf/autoperf/session_creator$ Any clues as to what I can try next to fix things would be appreciated.

2 2

trying to use Tag Soup - fromAttrib
by Michael Litchard 03 Nov '10

03 Nov '10

I have the following TagOpen [TagOpen "a" [("href","/launchWebForward.do?resourceId=4&policy=0&returnTo=%2FshowWebForwards.do")]] I would like to get the attribute resourceId=4 from that. My understanding is that fromAttrib is the right thing to use. But I'm having difficulty understanding the type signature fromAttrib :: (Show str, Eq str, StringLike str) => str -> Tag str -> str I'm sure if I did, the way to use it would be clear. Could someone help me make sense of this?

5 6

Linker errors when using the Hipmunk library
by Ulrik Rasmussen 03 Nov '10

03 Nov '10

Hello, I have installed the Hipmunk physics library (5.2.0.1) using cabal-install, but whenever I try to compile a program that uses it (e.g. the HipmunkPlayground package), I get the following linker error: > $ cabal install hipmunkplayground > Resolving dependencies... > Configuring HipmunkPlayground-5.2.0... > Preprocessing executables for HipmunkPlayground-5.2.0... > Building HipmunkPlayground-5.2.0... > [1 of 1] Compiling Main ( Playground.hs, dist/build/HipmunkPlayground/HipmunkPlayground-tmp/Main.o ) > Linking dist/build/HipmunkPlayground/HipmunkPlayground ... > /home/ulrik/.cabal/lib/Hipmunk-5.2.0.1/ghc-6.12.3/libHSHipmunk-5.2.0.1.a(Callbacks.o): In function `sr0l_info': > (.text+0x1062): undefined reference to `cpArbiterIsFirstContact' > /home/ulrik/.cabal/lib/Hipmunk-5.2.0.1/ghc-6.12.3/libHSHipmunk-5.2.0.1.a(Callbacks.o): In function `srBn_info': > (.text+0x37f8): undefined reference to `cpArbiterGetShapes' > collect2: ld returned 1 exit status > cabal: Error: some packages failed to install: > HipmunkPlayground-5.2.0 failed during the building phase. The exception was: > ExitFailure 1 The problem does not seem to be with HipmunkPlayground, since loading Hipmunk in ghci and evaluating `initChipmunk' also generates an error: > Prelude> import Physics.Hipmunk > Prelude Physics.Hipmunk> initChipmunk > Loading package StateVar-1.0.0.0 ... linking ... done. > Loading package array-0.3.0.1 ... linking ... done. > Loading package containers-0.3.0.0 ... linking ... done. > Loading package transformers-0.2.2.0 ... linking ... done. > Loading package Hipmunk-5.2.0.1 ... linking ... <interactive>: /home/ulrik/.cabal/lib/Hipmunk-5.2.0.1/ghc-6.12.3/HSHipmunk-5.2.0.1.o: unknown symbol `cpArbiterIsFirstContact' > ghc: unable to load package `Hipmunk-5.2.0.1' Does anyone have a clue as to what might be wrong? I am using Ubuntu 10.10, gcc 4.4.5 and ghc 6.12.3. Thanks :) /Ulrik Rasmussen

2 1

Am I using type families well?
by Yves Parès 03 Nov '10

03 Nov '10

Hello, I'm trying to make a simple monad (built on operational's ProgramT) for resource loading. I have classes featuring type families : {-# LANGUAGE TypeFamilies, FlexibleContexts, GADTs #-} -- | A ResourceId is something that identifies a resource. -- It should be unique for one resource, and should be used to find the location (the path) of the resource, -- possibly by using a configuration datatype class (Ord id) => ResourceId id where type LocOf id type CfgOf id retrieveLoc :: CfgOf id -> id -> LocOf id -- | Class describing a resource of type @rsc@ class (ResourceId (IdOf rsc)) => Resource rsc where type IdOf rsc load :: LocOf (IdOf rsc) -> IO (Maybe rsc) -- ^ Called when a resource needs to be loaded unload :: rsc -> IO () -- ^ Idem for unloading -- | Then, the operations that the loader can perform data EDSL id a where Load :: id -> EDSL id () IsLoaded :: id -> EDSL id Bool Unload :: id -> EDSL id () -- | The loader monad itself type RscLoader rsc m a = ProgramT (EDSL (IdOf rsc)) m a -- | And finally, how to run a loader runLoader :: (Monad m, Resource rsc) => CfgOf (IdOf rsc) -> RscLoader rsc m a -> m a runLoader cfg loader = viewT loader >>= eval M.empty where eval :: (Monad m, Resource rsc) => M.Map (IdOf rsc) rsc -> ProgramViewT (EDSL rsc) m a -> m a eval _ (Return x) = return x eval rscs (instr :>>= k) = case instr of Load id -> do let loc = retrieveLoc cfg id -- open and load from loc will go here viewT (k ()) >>= eval rscs -- -- -- Other cases yet to come... Well, there is no way I can get it type-check. I think I must be misusing the type families (I tried with multi-param typeclasses and functional dependencies, but it ends up to be the same kind of nightmare...).

4 7

Can a fundep force parametricity
by Brandon Moore 02 Nov '10

02 Nov '10

I would like to make a two-parameter type class where some choices of the first type uniquely determine the second type, and other choices of the first type promise to work with any choice of the second type. This code seems a bit suspicious, but is accepted thanks to UndecidableInstances. >{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances, > UndecidableInstances #-} > >class C a b | a -> b where > update :: b -> a -> a >instance C (Maybe a) a where > update x m = fmap (const x) m > >instance C Int b where > update _ n = n The application is a program transformation problem, where I'm using distinct types for Statements, Expressions, etc, but want a single environment that contains the bindings for variables of each different type for implementing rewriting rules. I'm trying to make a type class for binding things in the environment. Some of the terms, such as Statement, have a type parameter for semantic analysis information, so the Environment type has the type parameter as well. Then the instance a statement variable needs to require that the info type of the statement matches that of the environment, but the operation for binding an expression should be polymorphic in the info type of the environment. I usually have enough type information around that type checking succeeds without the fundep, so it's not currently a big problem. Brandon

2 1