- Haskell-Cafe - Haskell.org

IMAP and NNTP libraries
by Maurício 24 Jun '08

24 Jun '08

Hi, Are there mature libraries for IMAP and NNTP available to Haskell? Thanks, Maurício

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number-parameterized types and heterogeneous lists
by Harald ROTTER 24 Jun '08

24 Jun '08

Dear Haskellers, after reading Oleg Kiselyov's paper on number-parameterized types I started to play around with the class Digits that encodes decimal numbers in types. The "typed number" 10 would e.g. be defined as D1 $ D0 $ Sz I wondered if it would be possible replace the expression above by a heterogeneous list like [D1,D0] so I tried to define data Digit = forall a b.(Digits a, Digits (b a)) => Digit (a -> b a) Loading this into ghci yields: :t Digit D0 <interactive>:1:0: Ambiguous type variable `a' in the constraint: `Digits a' arising from a use of `Digit' at <interactive>:1:0-7 Probable fix: add a type signature that fixes these type variable(s) Removing the type constraints in the definition of "Digit": data Digit = forall a b.Digit (a -> b a) makes it work like this: :t Digit D0 Digit D0 :: Digit :t [Digit D0, Digit D1] [Digit D0, Digit D1] :: [Digit] "Digit", however, is far too general (it also includes e.g. \x -> [x]), but I would like it to be restricted to the Digit class. Any help is appreciated. Thanks Harald. CODE: module Test where data D0 a = D0 a data D1 a = D1 a data D2 a = D2 a data D3 a = D3 a data D4 a = D4 a data D5 a = D5 a data D6 a = D6 a data D7 a = D7 a data D8 a = D8 a data D9 a = D9 a class Digits ds where d2num :: Num a => ds -> a -> a data Sz = Sz -- zero size instance Digits Sz where d2num _ acc = acc instance Digits ds => Digits (D0 ds) where d2num dds acc = d2num (t22 dds) (10*acc) instance Digits ds => Digits (D1 ds) where d2num dds acc = d2num (t22 dds) (10*acc+1) instance Digits ds => Digits (D2 ds) where d2num dds acc = d2num (t22 dds) (10*acc+2) instance Digits ds => Digits (D3 ds) where d2num dds acc = d2num (t22 dds) (10*acc+3) instance Digits ds => Digits (D4 ds) where d2num dds acc = d2num (t22 dds) (10*acc+4) instance Digits ds => Digits (D5 ds) where d2num dds acc = d2num (t22 dds) (10*acc+5) instance Digits ds => Digits (D6 ds) where d2num dds acc = d2num (t22 dds) (10*acc+6) instance Digits ds => Digits (D7 ds) where d2num dds acc = d2num (t22 dds) (10*acc+7) instance Digits ds => Digits (D8 ds) where d2num dds acc = d2num (t22 dds) (10*acc+8) instance Digits ds => Digits (D9 ds) where d2num dds acc = d2num (t22 dds) (10*acc+9) t22 :: f x -> x t22 = undefined --data Digit = forall a b.(Digits a, Digits (b a)) => Digit (a -> b a) data Digit = forall a b.Digit (a -> b a) ------------------------------------------------------------------------------------------------- " Ce courriel et les documents qui y sont attaches peuvent contenir des informations confidentielles. Si vous n'etes pas le destinataire escompte, merci d'en informer l'expediteur immediatement et de detruire ce courriel ainsi que tous les documents attaches de votre systeme informatique. Toute divulgation, distribution ou copie du present courriel et des documents attaches sans autorisation prealable de son emetteur est interdite." " This e-mail and any attached documents may contain confidential or proprietary information. If you are not the intended recipient, please advise the sender immediately and delete this e-mail and all attached documents from your computer system. Any unauthorised disclosure, distribution or copying hereof is prohibited."

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Literal programming in Haskell with rst-literals
by Martin Blais 24 Jun '08

24 Jun '08

Hello Haskell community! I just did a marginally cool thing and I wanted to share it with you. "rst-literals" is a small program I wrote a while ago in order to write documents in reStructuredText format that would embed SQL code for data models in them, a form of literal programming for SQL if you will; I would describe my needs for the schema in prose, and reST literal-blocks were used to embed SQL code, blocks that look like this:: CLASS Employee ( firstname VARCHAR, lastname VARCHAR ) I wrote the script to be entirely generic: it parses the reST documents using the docutils code and outputs only the literal-blocks, with indentation removed; you can then run your favourite interpreter/compiler on the result (in that case, psql to initialize a database). Recently, while experimenting with Haskell, I started using both the literal (.lhs) and non-literal (.hs) styles of Haskell input, and I found the literal style a bit unpleasant to use, in particular, I don't like to have to prefix every line of code I write, despite the help that Emacs' haskell-mode provides. So I tried pulling a similar trick and embedding Haskell code in literal-blocks within reST documents, extracting that code using rst-literals, and it turns out that it works like a charm. Here is an example makefile for doing this:: .SUFFIXES: .rst .hs all: chap6 .rst.hs: rst-literals $< > $@ chap6: chap6.hs ghc --make chap6.hs An example reST document with some embedded Haskell code follows this email. Note that since rst-literals is using the docutils parser, you can make use of all of the recursive reST syntax, sections, bulleted items and much more. Only the literal-blocks are extracted, anywhere they appear. You can also easily process the reST source into HTML pages or LaTeX documents using the tools that come with docutils. You can find rst-literals here: http://furius.ca/pubcode/ Enjoy, -- Martin P.S. If there is a way to output cpp-like directives for GHC, like "#line <filename> <lineno>", it would be easy to modify rst-literals to generate those, so that compilation errors could refer to the source reST document instead of the extracted source. chap6.hs: ---------------------------------------------------------------------- =========================================== Exercises from Hutton book, Chapter 6 =========================================== .. contents:: .. 1 Introduction 2 Exercise 1 3 Exercise 2 4 Exercise 3 5 Exercise 4 6 Exercise 5 7 Exercise 6 Introduction ============ Bla bla bla blablablablablabla bla bla blabla. Bla bla bla blablablablablabla bla bla blabla. Bla bla bla blablablablablabla bla bla blabla. Bla bla bla blablablablablabla bla bla blabla. Bla bla bla blablablablablabla bla bla blabla. Bla bla bla blablablablablabla bla bla blabla. Exercise 1 ========== :: myexp :: Int -> Int -> Int myexp b 0 = 1 myexp b (n+1) = b * (myexp b n) Exercise 2 ========== (Exercise 2 consisted in derivations, so we mark the literal blocks as another type of block with "#!example", so that they don't get included in the output when only the "default" literal blocks get extracted. See rst-literals docstring for details.) Length:: #!example 1 + (length [2, 3]) 1 + 1 + (length [3]) 1 + 1 + (1) 3 Drop:: #!example drop 3 [1, 2, 3, 4, 5] [] ++ drop 3 [2, 3, 4, 5] [] ++ [] ++ drop 3 [3, 4, 5] [] ++ [] ++ [] ++ [4, 5] [4, 5] Init:: #!example init [1, 2, 3] [1] ++ init [2, 3] [1] ++ [2] ++ init [3] [1] ++ [2] ++ [] [1, 2] Exercise 3 ========== These are alternate versions of the example functions defined in the text:: and' :: [Bool] -> Bool and' [x] = x and' (x:xs) = x && and' xs concat' :: [[a]] -> [a] concat' [] = [] concat' (x:xs) = x ++ concat' xs replicate' :: Int -> a -> [a] replicate' 0 x = [] replicate' (n+1) x = (x : replicate' n x) select' :: [a] -> Int -> a select' (x:xs) 0 = x select' (x:xs) (n+1) = select' xs n elem' :: Eq a => a -> [a] -> Bool elem' _ [] = False elem' y (x:xs) | x == y = True | otherwise = elem' y xs Exercise 4 ========== The exercise asked to implement a function to merge two lists:: merge :: Ord a => [a] -> [a] -> [a] merge xs [] = xs merge [] xs = xs merge (x:xs) (y:ys) | x < y = (x : merge xs (y:ys)) | otherwise = (y : merge (x:xs) ys) Exercise 5 ========== :: msort :: Ord a => [a] -> [a] msort [] = [] msort [x] = [x] -- This is necessary to end the recursion. msort xs = merge (msort (fst hh)) (msort (snd hh)) where hh = halve xs halve :: [a] -> ([a], [a]) halve xs = (take n xs, drop n xs) where n = (length xs) `div` 2 Some main program:: main = (putStrLn . show) (halve [1..17]) Exercise 6 ========== (Too basic, I didn't bother.) ----------------------------------------------------------------------

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expected vs inferred type confusion
by Stephen Howard 23 Jun '08

23 Jun '08

I am having issues with type signatures for the resourceHandler action below. I haven't figured out what I'm doing wrong to cause the wrong type to be returned. It looks like Haskell is inferring that I'm returning a function from this action, when from the type signature I've specified, that is not what I'm wanting. The gist of the code is based on the request_method of the passed in HttpRequest, choose an action to run and pass it the needed parameters. the error: renao.hs:6:4: Couldn't match `HttpResponse' against `IO HttpResponse' Expected type: IO HttpResponse Inferred type: IO (IO HttpResponse) In the expression: return $ (handler session request) In the result of a 'do' expression: return $ (handler session request) the code: import HttpMessage import HttpSession resourceHandler :: HttpSession -> HttpRequest -> IO HttpResponse resourceHandler session request = do return $ handler session request where handler = which_handler request which_handler r = case request_method r of Get -> get_resources Head -> last_modified Post -> update_resources Put -> create_resources Delete -> delete_resources get_resources :: HttpSession -> HttpRequest -> IO HttpResponse get_resources session request = undefined -- ... the same stubbed definition for the other handlers (last_modified, etc)

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Planet haskell
by Jamie Brandon 23 Jun '08

23 Jun '08

I was hoping to have my summer of code blog added to planet haskell but planet(a)community.haskell.org no longer seems to exist. Hopefully the owner is subscribed to this list? Jamie

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ANNOUNCE: Pipe 1.0
by Matti Niemenmaa 23 Jun '08

23 Jun '08

Greetings, I hereby announce the release of the Pipe library, a library for piping data through a pipeline of processes. A web page with (hopefully) all the necessary info and a simple example can be found at http://iki.fi/matti.niemenmaa/pipe/ The package is at Hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Pipe Feel free to comment on any part of it, either here or straight to my e-mail address. I've tested it on Windows XP and Linux (Gentoo) and GHC 6.8.2 and 6.8.3. Since the code is rather low-level it'd be interesting to hear whether it works on other systems, say the BSDs or OS X. -- Matti Niemenmaa

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Safe way to parse arguments?
by Xiao-Yong Jin 23 Jun '08

23 Jun '08

Hi, I'm wondering how usually you parse command line arguments list safely. If the given argument is wrong, the program can still print out some error information instead of giving something like Prelude.read: no parse Let's make the discussion concrete. Suppose we have the following code. -->8----------8<-- import System.Environment data Conf = Conf { number :: Int } parseArgs [] = error "need at least one argument" parseArgs (x:_) = Conf { number = read x } work (Conf { number = n }) = n * (n-1) `div` 2 main = print . work . parseArgs =<< getArgs -->8----------8<-- If the "read x" fails in line 8, the program dies with Prelude.read: no parse What is the standard way to make the error message useful? Xiao-Yong -- c/* __o/* <\ * (__ */\ <

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Lambda and closures in PHP -- could someone please comment?
by Karoly Negyesi 23 Jun '08

23 Jun '08

Hi, The PHP community is discussing the adding of closures and lambdas to the language, see the proposal at http://wiki.php.net/rfc/closures If someone with knowledge of both languages could take a quick look it would be great. Thanks a lot Karoly Negyesi Ps. I am not a member of the PHP internals team, I am just a PHP developer but I am very very interested in getting these in my favourite language.

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INVITATION
by Haskell Hall 23 Jun '08

23 Jun '08

Haskell Hall is up and running. Haskell Hall is a mailing list, a forum, where you can discuss Haskell, functional programming and anything related, freely and openly with fellow enthusiasts. We welcome people of all abilities and know-how. So, if you fancy a change from what you get on Haskell Cafe send an email to haskell-request(a)freelists.org with the word subscribe in the subject field.The first 10 members will have access to the wine cellar. :-) _________________________________________________________________ The i’m Talkathon starts 6/24/08. For now, give amongst yourselves. http://www.imtalkathon.com?source=TXT_EML_WLH_LearnMore_GiveAmongst

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another FFI question
by Galchin, Vasili 23 Jun '08

23 Jun '08

Hello, I am reading the FFI spec. The following is unclear to me. Say I have data Bonzo = A | B |C and I want to "poke" Bonzo to an Int A -> 0 B -> 1 C -> 4 I have made Bonzo an instance of Storable. How do I write the poke functions and call them? Thanks, Vasili

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ieee 754
by Sean McLaughlin 23 Jun '08

23 Jun '08

Hello, I'm considering using Haskell for a numerical application. However, I need to rely on IEEE 754 standards being implemented correctly. What is the current state of 754 in Haskell? The definition has this paragraph, which makes me suspect Haskell is not appropriate for this application: "The default floating point operations defined by the Haskell Prelude do not conform to current language independent arithmetic (LIA) standards. These standards require considerably more complexity in the numeric structure and have thus been relegated to a library. Some, but not all, aspects of the IEEE floating point standard have been accounted for in Prelude class RealFloat." Thank you, Sean

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Testing
by Haskell Hall 23 Jun '08

23 Jun '08

Testing Testing _________________________________________________________________ Need to know now? Get instant answers with Windows Live Messenger. http://www.windowslive.com/messenger/connect_your_way.html?ocid=TXT_TAGLM_W…

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Scripting OpenOffice with Haskell
by Henning Thielemann 23 Jun '08

23 Jun '08

Has someone already tried to communicate with OpenOffice via Haskell? An interface seems to exist: http://framework.openoffice.org/scripting/index.html For instance, I like to mark some cells in oocalc, preprocess them and write them to a database.

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message passing style like in Haskell?
by jinjing 22 Jun '08

22 Jun '08

Hi guys, This is my second attempt to learn Haskell :) Any way here's the code: module Dot where import Prelude hiding ( (.) ) (.) :: a -> (a -> b) -> b a . f = f a infixl 9 . So for example, 99 questions: Problem 10 (*) Run-length encoding of a list. comparing: encode xs = map (\x -> (length x,head x)) (group xs) to encode xs = xs.group.map token where token x = (x.length, x.head) I found starting with data and working my way to a solution seems to be easier to think with, or maybe it's just me ... What is your thought? Jinjing

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Pretty little definitions of left and right folds
by George Kangas 22 Jun '08

22 Jun '08

Hi, Cafe, For no practical purpose, I wrote new definitions of list folds as chains of partially applied functions, joined by composition. I took the liberty of rearranging the type signature. > foldright :: (a -> b -> b)) -> [a] -> b -> b > foldright f = chain where > chain (a:as) = (f a).(chain as) > chain _ = id > foldleft :: (a -> b -> b) -> [a] -> b -> b > foldleft f = chain where > chain (a:as) = (chain as).(f a) > chain _ = id These definitions are point free, with respect to the "initializer" argument (which is now the last argument). Also, you can see how similar they are to each other, with the difference boiling down to the order of the composition, e.g.: foldright (+) [1, 2, 3] 0 == ( (1 +).(2 +).(3 +).id ) 0 foldleft (+) [1, 2, 3] 0 == ( id.(3 +).(2 +).(1 +) ) 0 We can also see the following identities: foldright f as == foldright (.) (map f as) id foldleft f as == foldright (flip (.)) (map f as) id I like that second one, after trying to read another definition of left fold in terms of right fold (in the web book "Real World Haskell"). The type signature, which could be written (a -> (b -> b)) -> ([a] -> (b -> b)), suggests generalization to another type constructor C: (a -> (b -> b)) -> (C a -> (b -> b)). Would a "foldable" typeclass make any sense? Okay, it goes without saying that this is useless dabbling, but have I entertained anyone? Or have I just wasted your time? I eagerly await comments on this, my first posting. very truly yours, George Kangas

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an example from YAHT doesn't run
by trifod＠gmail.com 21 Jun '08

21 Jun '08

Hello, On pdf page 43 of YAHT it provides a code example. I typed this into a file named "Guess.hs" and tried unsuccessfully to load it into ghci. I am running ghci version 6.8.2 This is the output it gave me: " [1 of 1] Compiling Main ( Guess.hs, interpreted ) Guess.hs:19:12: parse error on input `doGuessing' Failed, modules loaded: none. " I've attached the code module. As far as I can tell it is identical to the example in YAHT. Did I type it wrong, is the tutorial wrong, or something else? thanks again, Jared Langson

2 1

HDBC converting a date sql value to UTCTime
by George Moschovitis 21 Jun '08

21 Jun '08

Hello, I am HDBC with the Postgres driver. I am trying to read a simple date column from the DB: stmt <- DB.prepare conn "SELECT date FROM my_table ORDER BY date DESC LIMIT 1" DB.execute stmt [] DB.commit conn rows <- fetchAllRows' stmt let ((d:_):_) = rows the d variable is of type: SqlEpochTime 1213736400 I am wondering how to convert this value to an UTCTime value. Any help appreciated (I am a newbie). -g. -- http://gmosx.me.gr http://joy.gr http://cull.gr http://nitroproject.org http://phidz.com http://joyerz.com

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What's wrong with the classes/insances?
by Pieter Laeremans 21 Jun '08

21 Jun '08

HI, What 's wrong with this: type Id = String class Catalog a where listItems :: a -> IO [String] getItem :: a -> Id -> IO (Maybe String) class Item a where getCatalog :: Catalog catalog => a -> catalog data Catalog c => Content c = Content {auteur :: String, inhoud:: String, catalog::c} instance Catalog c => Item (Content c) where getCatalog (Content _ _ c) = c I get this as error from ghci: Couldn't match expected type `catalog' against inferred type `c' `catalog' is a rigid type variable bound by the type signature for `getCatalog' at ../Sites/liberaleswebsite/www.liberales.be/cgi-bin/Test.hs:16:26 `c' is a rigid type variable bound by the instance declaration at ../Sites/liberaleswebsite/www.liberales.be/cgi-bin/Test.hs:20:17 In the expression: c In the definition of `getCatalog': getCatalog (Content _ _ c) = c In the definition for method `getCatalog' Failed, modules loaded: none. thanks in advance, P -- Pieter Laeremans <pieter(a)laeremans.org> "The future is here. It's just not evenly distributed yet." W. Gibson

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Wrapping FTGL in FFI calls
by Jefferson Heard 21 Jun '08

21 Jun '08

I've been looking for awhile now for a simple way to get truetype fonts into my visualizations so I can abandon the hideous GLUT fonts and make things that look like they were developed in the 1990s instead of back in the days of TRON. I found FTGL, but I'm mostly a Haskell developer these days, and resent having to go back to C just to write a simple application. So I was wondering if anyone had ever wrapped the FTGL library in Haskel FFI or whether those out there who are experts on the FFI think at first glance it should be readily wrappable by a rank amateur at FFI such as myself. http://ftgl.sourceforge.net/docs/html/ -- Jeff

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Haddock compilation problem
by Ronald Guida 21 Jun '08

21 Jun '08

I just upgraded to ghc-6.8.3, using a linux binary, and I am having a problem compiling Haddock. Haddock 2.1.0 and Haddock 2.0.0.0 both fail to build under ghc-6.8.3, but they both build successfully with ghc-6.8.2. I don't know if this is a Haddock problem, or a GHC problem, or perhaps something else entirely? Here is the error I'm getting. It is the same error for either version of Haddock. [15 of 24] Compiling Haddock.GHC.Typecheck ( src/Haddock/GHC/Typecheck.hs, dist/build/haddock/haddock-tmp/Haddock/GHC/Typecheck.o ) src/Haddock/GHC/Typecheck.hs:82:4: Constructor `HsModule' should have 7 arguments, but has been given 8 In the pattern: HsModule _ _ _ _ _ mbOpts _ _ In a pattern binding: HsModule _ _ _ _ _ mbOpts _ _ = unLoc parsed In the definition of `mkGhcModule': mkGhcModule (mod, file, checkedMod) dynflags = GhcModule {ghcModule = mod, ghcFilename = file, ghcMbDocOpts = mbOpts, ghcHaddockModInfo = info, ghcMbDoc = mbDoc, ghcGroup = group, ghcMbExports = mbExports, ghcExportedNames = modInfoExports modInfo, ghcNamesInScope = fromJust $ modInfoTopLevelScope modInfo, ghcInstances = modInfoInstances modInfo} where HsModule _ _ _ _ _ mbOpts _ _ = unLoc parsed (group, _, mbExports, mbDoc, info) = renamed (parsed, renamed, _, modInfo) = checkedMod

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Checking packages before upload to Hackage
by Henning Thielemann 21 Jun '08

21 Jun '08

Is there some command which tests a tar.gz created by 'Setup.lhs sdist' by unpacking the archive to say /tmp and compiling and documenting the sources? Hackage should recommend this tool before package upload.

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Quantitative Trading Developer Position at Hutchin Hill Capital
by Neil Mehra 20 Jun '08

20 Jun '08

Job Description: Quantitative Trading Developer Description: * Newly formed multi-strategy hedge fund, Hutchin Hill Capital, located in midtown Manhattan, seeks a Quantitative Trading Developer; * Focus on building infrastructure, applications, and technical support for quantitative equity trading business; * Candidate should have demonstrated interest or passion for functional programming-with experience using Haskell, F#, Kdb+/q, OCaml, ML, and related technologies; * Position reports directly to portfolio manager, and will work closely with other technology and research professionals. Responsibilities Include: * Developing data management, quant research, and analytical tools for model-driven equity trading strategies; * Building production environment for real-time content capture, trade generation, and risk management; * Deploying equity trading technology, methods, and protocols, including electronic execution algorithms and market impact analysis; * Conducting quantitative portfolio research using cutting-edge tools and techniques. Required Experience/Education: * 2 - 5 years in IT development or quantitative analytics; * Advanced degree or equivalent experience in a technical field such as computer science, mathematics, physics, or engineering; Other Requirements: * Exceptional technical and analytical problem-solving; * Desire to master the latest technology, hardware, and quantitative methods; * Willingness to work in a fast-paced, high pressure environment; * Team-oriented and highly resourceful; * Entrepreneurial drive. Please contact quant2(a)hutchinhill.com<mailto:quant2@hutchinhill.com> with resume if interested. __________ This communication, including any attachment(s), may contain confidential and/or privileged information. If you are not the intended recipient or have received this communication in error, please destroy/delete this email and any attachments. We do not waive confidentiality or privilege by mistransmission. This communication does not constitute an offer of employment, or an offer, or the solicitation of an offer, to buy or sell any security or investment advisory services, and is not an official confirmation of any transaction.

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This is a bug?
by Samuel Silva 20 Jun '08

20 Jun '08

Hello I'm using HaXml library to handle XML. How I solve this issue when call "readXml": >in <score-partwise>, In a sequence: > in <part>, In a sequence: > in <measure>, Too many elements inside <measure> at > file ../../../parsers/elite2.xml at line 75 col 15 > Found excess: > > >Done. I don't understood this error. I'm using HaXml-1.19.4 with GHC-6.8.2 -- Don't hug that, Hugs ME! Samuel Silva

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Re: (Lazy) SmallCheck and peano numbers
by Levi Stephen 20 Jun '08

20 Jun '08

On Fri, Jun 20, 2008 at 3:30 AM, Benedikt Huber <benjovi(a)gmx.net> wrote: > Levi Stephen schrieb: >> >> Hi, >> >> I have the following definitions >> >> type Zero >> type Succ a >> >> so that I can muck around with a Vector type that includes its length >> encoded in its type. >> >> I was wondering whether it was possible to use SmallCheck (or >> QuickCheck) to generate random Peano numbers? Is there an issue here >> in that what I actually want to generate is a type rather than a >> value? >> >> I do have >> >> reifyInt :: Int -> (forall a. ReflectNum a => a -> b) -> b >> >> but, I'm not sure if this can help me when I need to generate other >> values based upon that type (e.g., two vectors with the same size >> type) > > Hi Levi, > > For QuickCheck, I know it is possible as long as you do not need to use type > level functions in your tests. For example, using Alfonso's type-level and > parametrized-data packages, one can write: > >> instance (Nat n, Arbitrary a) => Arbitrary (FSVec n a) where >> arbitrary = >> liftM (unsafeVector (undefined :: n)) $ >> mapM (const arbitrary) [1..toInt (undefined :: n)] > >> propLength :: forall n a. (Nat n) => FSVec n Integer -> Bool >> propLength (FSVec xs) = P.length xs == toInt (undefined :: n) > >> propLengthEqual :: forall n a. (Nat n) => >> FSVec n Integer -> FSVec n Integer -> Bool >> propLengthEqual v1 v2 = length v1 == length v2 > >> tests1 = forM_ [0..100] $ \n -> reifyIntegral n $ \(t :: ty) -> >> quickCheck (propLength :: FSVec ty Integer -> Bool) >> tests2 = forM_ [0..100] $ \n -> reifyIntegral n $ \(t :: ty) -> >> quickCheck (uncurry propLengthEqual :: >> (FSVec ty Integer,FSVec ty Integer) -> Bool) Thanks for the example code. Ideally it would be great to have n generated also. Any thoughts on whether something like propLengthEqual :: forall n a. (Nat n) => n -> FSVec n Integer -> FSVec n Integer -> Bool propLengthEqual _ v1 v 2 = length v1 == length v2 with an arbitrary instance for generate all Nat n's is possible? Is something like instance (forall n. Nat n) => Arbitrary n possible/legal haskell? and would it for the above test? > > best regards, > > benedikt > Thanks, Levi

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Meaning of "ribbonsPerLine" at Text.PrettyPrint.HughesPJ ?
by Alfonso Acosta 20 Jun '08

20 Jun '08

Hi, Can anyone give a good explanation of what ribbonsPerLine means? Maybe it would be better to simply ask for the meaning of ribbon in this context. The documentation is totally meaningless to me: "reibbonsPerLine: Ratio of ribbon length to line length". I asked at #haskell and frankly, I was surprised by the answer: fons: can anyone tell me what the heck does ribbonsPerLine mean in Text.PrettyPrint.HughesPJ? fons: > renderStyle style{lineLength=10, ribbonsPerLine=1} $ sep [text "foo" ,text "bar"] lambdabot: "foo bar" fons: > renderStyle style{lineLength=10, ribbonsPerLine=2} $ sep [text "foo" ,text "bar"] lambdabot: "foo\nbar" fons: ok, somehow it affects the line width ... byorgey: fons: I can't explain it, all I know is that you must set it to 1 or else it does bizarre things fons: hahah, ok fons: byorgey: that's funny considering its default value is 1.5 byorgey: if you set it to 1 then lineLength means what you think it should byorgey: fons: EXACTLY Cheers, Fons

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Re: (Lazy) SmallCheck and peano numbers
by Levi Stephen 20 Jun '08

20 Jun '08

> If so, you could write a SmallCheck Series instance as follows. > > instance Serial (Vec Zero a) where > series = cons0 nil > > instance (Serial a, Serial (Vec n a)) => Serial (Vec (Succ n) a) where > series = cons2 (|>) > If we have the property > > prop_vector :: Vec (Succ (Succ Zero)) Bool -> Bool > prop_vector (V xs) = xs == xs > > we can check it, and only 2 element vectors will be tested I have some code up now at http://hpaste.org/8420 It looks pretty much like what you've written above and is what I am doing for the moment, but for various size vectors from zero to four. > Now, it seems what you really want to do is define polymorphic > properties like > > prop_poly :: Vec n Bool -> Vec n Bool -> Bool > > and have SmallCheck check all equal-sized vectors. If so, good > question! :-) Yep, this would be exactly what I'm after. Thanks for a better write up and background on what I'm after than I gave :) I'm thinking the property might be written as prop_poly:: size -> Vec size Bool -> Vec size Bool -> Bool, so that the size is generated randomly and the types take care of ensuring the vectors generated are of the same size. Levi

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Re: [Haskell-cafe] Lambda and closures in PHP -- could someone please comment?
by Alexander Wagner 20 Jun '08

20 Jun '08

On Wednesday 18 June 2008, Luke Palmer wrote: > I don't know that much PHP, but I find the "lexical" keyword to be a > nuisance. What are the semantics if the lexical keyword is omitted? In PHP Variables are local by default, the keyword global defines them as references to the global namespace. There are some exceptions, like the availability of $this in methods or a couple of predefined superglobal variables that are implicitly global in all functions. <?php $globalVar = 1; $param = "notshared"; $localVar = "notshared"; function foo ($param) { global $globalVar; $localVar = (int) $param; return $localVar + $localVar; } ?> Given how much it works like "global", the keyword "lexical" seems quite consistent with the style of PHP. Gesundheit Wag -- There’s a phrase we live by in America: “In God We Trust”. It’s right there where Jesus would want it: on our money. - Steven Colbert

1 0

cabal-install failure
by Darrin Thompson 20 Jun '08

20 Jun '08

Cabal-install is looking good. It now, for the record, has only two deps outside of Cabal-1.4. I installed cabal-install-0.5 on ubuntu with the haskell.org linux binary for ghc 6.8.2. I then tried cabal-install yi Got this output: ... many successful installs ... Registering vty-3.0.1... Reading package info from "dist/installed-pkg-config" ... done. Saving old package config file... done. Writing new package config file... done. Downloading yi-0.3... [1 of 1] Compiling Main ( Setup.hs, dist/setup/Main.o ) Setup.hs:9:0: Warning: Deprecated use of `showPackageId' (imported from Distribution.Simple, but defined in Distribution.Package): use the Text class instead Linking dist/setup/setup ... Warning: defaultUserHooks in Setup script is deprecated. Configuring yi-0.3... Warning: Instead of 'ghc-options: -DDYNAMIC -DFRONTEND_VTY' use 'cpp-options: -DDYNAMIC -DFRONTEND_VTY' setup: alex version >=2.0.1 && <3 is required but it could not be found. cabal: Error: some packages failed to install: yi-0.3 failed during the configure step. The exception was: exit: ExitFailure 1 Is this a cabal problem of package problem? I would have expected it to fail immediately instead of discover this problem so far into process. Thanks. -- Darrin

4 9

type constructor confusion
by Stephen Howard 20 Jun '08

20 Jun '08

hello list, loading the code below into ghci gives me this error: HttpMessage.hs:36:20: Not in scope: type constructor or class `HttpRequest' The troublesome line is the definition of the cookie function at the end of the code. I've made HttpRequest and HttpResponse constructors of HttpMessage as I've sketched out an action that might return either one. Given that I thought what I was doing below was defining these as type constructors (maybe I have my vocabulary mixed up) I don't know why ghci is returning this error. module HttpMessage ( HttpMessage(..), HttpRequestMethod(..), cookie ) where import Data.Map as M data HttpHeaders = HttpHeaders !(M.Map String String ) data HttpMessage = HttpRequest { headers :: HttpHeaders, body :: String, request_method :: HttpRequestMethod, uri :: URI, http_version :: Float, cookies :: !(M.Map String String) } | HttpResponse { headers :: HttpHeaders, body :: String, status :: HttpStatus } data HttpRequestMethod = Get | Head | Post | Put | Delete | Unsupported String data HttpStatus = OK | NotFound | NotAuthorized | ServerError data URI = URI { protocol :: String, domain :: String, port :: String, path :: String, query :: !(M.Map String String), anchor :: String } cookie :: String -> HttpRequest -> Maybe String cookie n request = M.lookup n (cookies request)

4 6

Gtk2Hs and GCs
by Tim Newsham 20 Jun '08

20 Jun '08

I don't see a way to fetch an existing standard GC from a widget. In other Gtk bindings I usually do this by fetching the widget's style and then grabbing one of the gc's (such as fg_gc[STATE_NORMAL]). In Gtk2Hs docs: http://www.haskell.org/gtk2hs/docs/gtk2hs-docs-0.9.12/Graphics-UI-Gtk-Gener… it looks like you can only get Colors, not the whole GC. I can't seem to find any other way short of making a whole new GC (as is done in Gtk2Hs/demos/graphic/Drawing.hs). Am I missing something? Is there a reason the GC's arent retrievable? Tim Newsham http://www.thenewsh.com/~newsham/

2 1

do and ifthenelse indentation in GHC
by Stefan Monnier 20 Jun '08

20 Jun '08

http://hackage.haskell.org/trac/haskell-prime/wiki/DoAndIfThenElse says (and my memory seems to agree) that GHC has been changed to accept things like readnums f n = do eof <- hIsEOF f if eof then return n else do l <- hGetLine f readnums f (n + read l) where the `then' and `else' are aligned with the `if' rather than being slightly more indented as the Haskell 98 standard requires. Yet, when I try it with GHCi 6.8.2 I get an error: Prelude> :load "/home/monnier/tmp/foo.hs" [1 of 1] Compiling Main ( /home/monnier/tmp/foo.hs, interpreted ) /home/monnier/tmp/foo.hs:6:18: parse error (possibly incorrect indentation) Failed, modules loaded: none. Prelude> Does anybody know what's up with that? Stefan

1 0

Burn ACM
by Achim Schneider 20 Jun '08

20 Jun '08

for monetary reasons. I just wanted my voice to be heard, you may pass on now. -- (c) this sig last receiving data processing entity. Inspect headers for past copyright information. All rights reserved. Unauthorised copying, hiring, renting, public performance and/or broadcasting of this signature prohibited.

1 0

reactive and <<loop>>
by stefan kersten 20 Jun '08

20 Jun '08

hi, i'm having problems with a very simple example using conal elliott's 'reactive' library: module Main where import Control.Applicative import Control.Concurrent import Control.Monad import Data.Reactive import System.Random main :: IO () main = do (e, snk) <- mkEvent forkIO $ forever ((getStdRandom random :: IO Double) >>= snk >> threadDelay 10000) runE (print `fmap` withPrevE e) return () which starts to output this after a while: reactive_loop.hs: <<loop>> this is with ghc 6.8.1 on osx 10.4. any ideas what might be going wrong? many thanks, <sk>

1 0

(Lazy) SmallCheck and peano numbers
by Levi Stephen 20 Jun '08

20 Jun '08

Hi, I have the following definitions type Zero type Succ a so that I can muck around with a Vector type that includes its length encoded in its type. I was wondering whether it was possible to use SmallCheck (or QuickCheck) to generate random Peano numbers? Is there an issue here in that what I actually want to generate is a type rather than a value? I do have reifyInt :: Int -> (forall a. ReflectNum a => a -> b) -> b but, I'm not sure if this can help me when I need to generate other values based upon that type (e.g., two vectors with the same size type) Thanks, Levi

3 2

network-related segfaults
by Brent Yorgey 20 Jun '08

20 Jun '08

Hi all, Has anyone else experienced segfaults related to network stuff? Observe: [11:50:56 brent@xenophon:~]$ ghci -ignore-dot-ghci GHCi, version 6.8.2: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. Prelude> :m +Text.HTML.Download Prelude Text.HTML.Download> txt <- openURL "http://www.google.com" Loading package bytestring-0.9.1.0 ... linking ... done. Loading package mtl-1.1.0.0 ... linking ... done. Loading package parsec-3.0.0 ... linking ... done. Loading package network-2.1.0.0 ... linking ... done. Loading package array-0.1.0.0 ... linking ... done. Loading package containers-0.1.0.1 ... linking ... done. Loading package tagsoup-0.6 ... linking ... done. Segmentation fault (core dumped) This just started happening yesterday. I think I may have just upgraded some packages via cabal-install but I'm not sure which ones. Now is a time when I wish all my terminal sessions were automatically logged. (Hmm, perhaps I should look into that...) -Brent

1 1

Re: Bit Streams
by Dominic Steinitz 20 Jun '08

20 Jun '08

OK, so today I tried to write my first program using the Binary library. And I've hit a snag: It appears the library will only handle data that is byte-aligned. So if I try to write three Bool values, it uses three bytes, not three bits. Before I sit down and spend 3 months designing my own library from scratch, does anybody know of an existing library that allows you to do what Binary does, but with single-bit precision? [I presume Binary is byte-aligned for efficiency...] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/binary-strict

6 10

News of the day: C fails to stay self-hosting
by Achim Schneider 19 Jun '08

19 Jun '08

http://article.gmane.org/gmane.comp.gcc.patches/165063 -- (c) this sig last receiving data processing entity. Inspect headers for past copyright information. All rights reserved. Unauthorised copying, hiring, renting, public performance and/or broadcasting of this signature prohibited.

1 0

Design suggestion for Data.Binary.Defer
by Neil Mitchell 19 Jun '08

19 Jun '08

Hi, I'm in the process of updating the Deferred Binary library, http://www-users.cs.york.ac.uk/~ndm/binarydefer/. The idea is that its does serialisation, but certain elements can be marked as deferred - instead of being written in the current file stream, they are merely pointed at and if needed, that pointer will be followed. Example:("hello","world"), where the first field is marked as deferred would write out: [6]"world""hello" i.e. [skip 6 characters if you want hello], the "world" data, the "hello" data we previously promised to put here. When reading, the "hello" would only be seeked to and read if necessary. So, its like binary, but some fields are lazy. The question is how to indicate which fields are lazy. There are three schemes I can think of: == Simple Instances == put (a,b) = putDefer a >> put b get = do a <- getDefer; b <- get; return (a,b) If the put/get and putDefer/getDefer items do not line up perfectly it will go very wrong at runtime - probably resulting in random values being created. You also can't derive the instances automatically, with something like Derive or DrIFT. == Complex Instances == This is the current scheme, based on lazy pattern matching and exceptions - very confusing, probably low performance. deferBoth = [\~(a,b) -> unit (,) <<~ a << b] Where <<~ a means write out the a field lazily, and << b means write out the b field strictly. The advantage over the simple instances is that a field being deferred is declared once. == Lazy Data Type == Instead of customizing the instance, you can write a data Defer a = Defer a type, and then instead of the original tuple write: (Defer "hello","world") But now the code must unwrap the Defer before accessing "hello", but the instance becomes much simpler, and can be derived. == The Question == Is there a simple way of tagging fields in a constructor as deferred, just once for reading and writing, and ideally outside the instance definition and not requiring additional code to unwrap? I can't think of any, but there may be something I have missed. Thanks Neil

5 6

ANN: dataenc 0.11 and omnicodec 0.2
by Magnus Therning 19 Jun '08

19 Jun '08

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 A few days ago I uploaded dataenc 0.11[1] and omnicode 0.2[2]. The short description of the (most visible) changes would be the added support for Base85 encoding and decoding in dataenc. The dataenc package (version 0.11) Data encoding library currently providing Uuencode, Base64, Base64Url, Base32, Base32Hex, Base16, and Base85. Other versions0.9, 0.10.1, 0.10.2 Dependenciesbase LicenseLGPL CopyrightMagnus Therning, 2007 AuthorMagnus Therning Maintainermagnus(a)therning.org CategoryCodec Exposed modulesCodec.Binary.DataEncoding, Codec.Binary.Base16, Codec.Binary.Base32Hex, Codec.Binary.Base32, Codec.Binary.Uu, Codec.Binary.Base64Url, Codec.Binary.Base64, Codec.Binary.Base85 Upload dateThu Jun 12 15:18:19 PDT 2008 Uploaded byMagnusTherning Built onghc-6.8 The omnicodec package (version 0.2) Two simply command line tools built on dataenc. Other versions0.1 Dependenciesbase, dataenc (>=0.11), directory, filepath, haskell98 LicenseGPL CopyrightMagnus Therning, 2007 AuthorMagnus Therning Maintainermagnus(a)therning.org CategoryCodec Executablesodec, oenc Upload dateThu Jun 12 15:19:07 PDT 2008 Uploaded byMagnusTherning Built onghc-6.8 /M [1]: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/dataenc [2]: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/omnicodec -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.9 (GNU/Linux) iEYEARECAAYFAkhaKtsACgkQiMWTaatN+6Ru6wCdFBrMfmFWbqTbkWhml8idgbb7 e8sAoN58Xil4ZnbFmSH7xPUtrPUCyZq9 =vlsA -----END PGP SIGNATURE-----

1 0

Haskell's type system
by Ron Alford 19 Jun '08

19 Jun '08

I'm trying to wrap my head around the theoretical aspects of haskell's type system. Is there a discussion of the topic separate from the language itself? Since I come from a rather logic-y background, I have this (far-fetched) hope that there is a translation from haskell's type syntax to first order logic (or an extension there-of). Is this done? Doable? -Ron

6 5

Using cabal-install
by Daniel Fischer 19 Jun '08

19 Jun '08

Is there a tutorial/user's guide for cabal-install somewhere? My limited google skills haven't found one and I really need it, because there MUST be a better way to get a package and its dependencies built and the haddock documentation generated, installed and hyperlinked than manually unpacking each .tar.gz and running cabal haddock there. Thanks, Daniel

3 3

Bit streams
by Andrew Coppin 19 Jun '08

19 Jun '08

OK, so today I tried to write my first program using the Binary library. And I've hit a snag: It appears the library will only handle data that is byte-aligned. So if I try to write three Bool values, it uses three bytes, not three bits. Before I sit down and spend 3 months designing my own library from scratch, does anybody know of an existing library that allows you to do what Binary does, but with single-bit precision? [I presume Binary is byte-aligned for efficiency...]

6 6

Problem building GHC 6.8.3
by Daniel Fischer 19 Jun '08

19 Jun '08

Sorry, probably stupid questions for knowledgeable folks, but after having built for a couple of hours, make died with if ifBuildable/ifBuildable base; then \ cd base && setup/Setup haddock --html-location='../$pkg' \ --hyperlink-source; \ fi Preprocessing library base-3.0.2.0... Running hscolour for base-3.0.2.0... Preprocessing library base-3.0.2.0... Running Haddock for base-3.0.2.0... Warning: The documentation for package rts-1.0 is not installed. No links to it will be generated. Setup: Haddock's internal GHC version must match the configured GHC version make[1]: *** [doc.library.base] Fehler 1 make[1]: Leaving directory `/home/dafis/HasCom/ghc-6.8.3/libraries' make: *** [stage1] Fehler 2 So, what is Haddock's internal GHC version? Is it the version, haddock was built with? And what is the configured GHC version, is it the one I'm trying to build or the one I'm building with? If the former, I seem to have landed in a circulus vitiosus, if the latter, that is the GHC, haddock was built with, too. For the record: dafis@linux:~/HasCom/ghc-6.8.3> haddock --version Haddock version 2.1.0, (c) Simon Marlow 2006 Ported to use the GHC API by David Waern 2006-2008 How should I proceed now? Thanks, Daniel

3 2

First Call for Papers: DSL WC
by Emir Pasalic 19 Jun '08

19 Jun '08

IFIP Working Conference on Domain Specific Languages (DSL WC) July 15-17, 2009, Oxford CALL FOR PAPERS Domain-specific languages are emerging as a fundamental component of software engineering practice. DSLs are often introduced when new domains such as web-scripting or markup come into existence, but it is also common to see DSLs being introduced and adopted for traditional domains such as parsing and data description. Developing software using DSLs has many benefits. DSLs are often designed based on existing notations that are already in use by experts in a given domain. As such, successful DSLs often reduce or eliminate the effort needed to transform the concept or innovation produced by the domain expert into an executable artifact or even a deliverable software product. DSL implementations can capture and mechanize a significant portion of the repetitive and mechanical tasks that a domain expert traditionally needed to perform in order to produce an executable. DSLs can in many cases capture and make widely available special expertise that only top specialists in a given domain might have. By capturing expert knowledge and reducing repetitive tasks, DSLs often also lead to software that is significantly more portable, more reliable and more understandable than it would otherwise be. DSLs can be viewed as having a dual role to general-purpose languages: whereas general purpose languages try to do everything as well as possible, DSLs are designed to find a domain where they can solve some class of problems -- no matter how small -- in the best possible way. Widely known examples of DSLs include Matlab, Verilog, SQL, LINQ, JavaScript, PERL, HTML, Open GL, Tcl/Tk, Macromedia Director, Mathematica/Maple, AutoLisp/AutoCAD, XSLT, RPM, Make, lex/yacc, LaTeX, PostScript, Excel, among many others. But while these tools have been widely successful, they still fall short of realizing the full idea behind them. The goal of this conference is to explore the extent to which incorporating modern principles of language design and software engineering can benefit existing and future domain-specific languages. The ultimate goal of using DSLs is to improve programmer productivity and software quality. Often, this is achieved by reducing the cost of initial software development as well as maintenance costs. These improvements - programs being easier to write and maintain - materialize as a result of domain-specific guarantees, analyses, testing techniques, verification techniques, and optimizations. * Paper Criteria Papers are sought addressing the research problems, fundamental principles, and practical techniques of DSLs, including but not limited to: - Foundations, including semantics, formal methods, type theory, and complexity theory - Language design, ranging from concrete syntax to semantic and typing issues - Software engineering, including domain analysis, software design, and round-trip engineering - Software processes, including metrics for software and language evaluation - Implementation techniques, including parsing, compiling, and program generation - Program analysis and automated transformation - Reverse engineering, re-engineering, design discovery, automated refactoring - Hardware/software codesign - Programming environments, including visual languages, debuggers, and testing infrastructure - Teaching DSLs and the use of DSLs in teaching - Case studies, including engineering, bioinformatics, hardware specification languages, parallel computing languages, real-time and embedded systems, and networked and distributed domains Papers will be judged on the depth of their insight and the extent to which they translate specific experience into general lessons for domain-specific language designers and implementers, and software engineers. Papers can range from the practical to the theoretical; where appropriate, they should refer to actual languages, tools, and techniques, provide pointers to full definitions and implementations, and include empirical data on results. * Important Dates - December 14th, 2008: Abstract submission due. Firm, will not have any extensions - December 21st, 2008: Paper submission deadline. Firm, will not have any extensions - February 23rd, 2009: Author notification of decisions - March 22nd, 2009: Camera ready manuscripts due * Program Committee - Jon Bentley, Avayalabs - Martin Erwig, Oregon State University - Jeff Gray, University of Alabama at Birmingham - Robert Grimm, New York University - Jim Grundy, Intel Strategic CAD Labs - Tom Henzinger, EPFL - Sam Kamin, UIUC - Dick Kieburtz, Portland State University - Ralf Lämmel, University of Koblenz - Julia Lawall, University of Copenhagen - Benjamin Pierce, University of Pennsylvania - Vivek Sarkar, Rice University - Jeremy Siek, University of Colorado at Boulder - José Nuno Oliveira, University of Minho - Doaitse Swierstra, Utrecht University - Walid Taha (Chair), Rice University - Eelco Visser, Delft University - William Waite, University of Colorado at Boulder - Stephanie Weirich, University of Pennsylvania * Organizers - General Chair: Jeremy Gibbons, Oxford University - Publicity Chair: Emir Pasalic, LogicBlox

1 0

Haskell Weekly News: Issue 73 - June 18, 2008
by Brent Yorgey 19 Jun '08

19 Jun '08

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20080618 Issue 73 - June 18, 2008 --------------------------------------------------------------------------- Welcome to issue 73 of HWN, a newsletter covering developments in the [1]Haskell community. The Google Summer of Code is in full swing, preparations are underway for ICFP and the eleventh ICFP Programming Contest, and cabal-install is oh-so-sexy. It's an exciting time to be a part of the Haskell community! Community News Andy Gill has completed his move from Portland, OR to Kansas. Luke Palmer (luqui) has [2]begun work for [3]Anygma, Peter Verswyvelen's startup using Haskell (among other languages) to ``generate easy-to-use tools for creating audio-visual 2D/3D content.'' Congrats to Andy and Luke on their new beginnings! Announcements Final CFP: 2008 Haskell Symposium. Andy Gill [4]announced the final call for papers for the [5]2008 Haskell Symposium. The deadline is the 23rd of this month; please submit a paper! cabal-install. Duncan Coutts [6]announced the release of [7]cabal-install-0.5, along with the release of Cabal-1.4 to support it. It features an improved command line interface, smarter upgrading, and is made of win. If you are still stuck in the dark ages of runhaskell Setup configure blah blah, then the imperative monkeys have already won. ICFP programming contest. Tim Chevalier [8]announced the eleventh annual [9]ICFP programming contest, to be held from Friday, July 11, 2008 to Monday, July 14, 2008. Are you ready? c.h.o trac. Ian Lynagh [10]announced that it is now possible for projects on [11]community.haskell.org to [12]create themselves a trac, providing a bug tracking system and wiki. random-access-list. Stephan Friedrichs [13]announced an [14]implementation of Chris Okasaki's random-access lists, providing typical list operations (cons, head, tail) in O(1) and indexed random-access in O(log n). GHC version 6.8.3. Ian Lynagh [15]announced a [16]new patchlevel release of [17]GHC, containing a number of bugfixes relative to 6.8.2. Printf-TH. Marc Weber [18]announced that he has taken over maintenance of the [19]Printf-TH library, which implements a printf function via [20]Template Haskell, in order to guarantee that wrong argument types or the wrong number of arguments will result in compile time errors. Mueval. Gwern Branwen [21]announced the release of the [22]mueval package, providing a standalone executable for evaluating Haskell expressions based on the GHC API. Topkata. Christoph Bauer [23]announced the release of [24]Topkata, a simple OpenGL game written in Haskell. The goal is to guide a ball through a labyrinth to the opposite corner. Haddock Trac. David Waern [25]announced a new [26]bug-tracker and wiki for the [27]Haddock project. Fortress talk. Jeff Polakow [28]announced that a [29]talk on [30]Fortress, a new OO/functional language from Sun, will take place on Wednesday, June 25 at 6:30pm in Manhattan, New York, USA. ieee-0.2. Patrick Perry [31]announced the release of [32]ieee, a library that provides approximate comparison of floating point numbers based, NaN-aware minimum and maximum, and a type class for approximate comparisons. Google Summer of Code Hoogle 4. Neil Mitchell (ndm) is [33]working on Hoogle 4, recently adding support for generating Hoogle databases to [34]Haddock, using the GHC API. This week he plans to work on database creation and text searches. DPH physics engine. Roman Cheplyaka (Feuerbach) is [35]working on a physics engine using [36]Data Parallel Haskell, recently adding rotations, represented by quaternions. Next he plans to handle collisions properly with respect to rotation, and to add documentation. Generic tries. Jamie Brandon is writing a library for efficient maps using generalized tries. He has come up with a preliminary API and is [37]asking for feedback. Cabal dependency framework. Andrea Vezzosi (Saizan) is working on a make-like dependency analysis framework for Cabal, recently refining the core model, that has built its first sources in the testing environment. The next step will be dealing with preprocessor chaining. Language.C. Benedikt Huber (visq) is working on Language.C, a standalone parser/pretty printer library for C99. The test suite is finished, the parser and pretty printer support most GNU extensions, and all failing tests of gcc.dg are documented. GHC API. Thomas Schilling (nominolo) is working on improvements to the GHC API. GHC plugins. Maximilian Conroy Bolingbroke is working on dynamically loaded plugins for GHC. Discussion Low-level array performance. Dan Doel began a [38]discussion about the [39]fannkuch benchmark and the current state of Haskell support for fast low-level array operations. 1/0. Evan Laforge began a lively [40]discussion about Infinity, NaN, and Haskell's support for the IEEE floating-point standard. Documenting the impossible. Andrew Coppin began a [41]discussion on the relative merits of {-# IMPOSSIBLE #-} pragmas, calls to 'error' and 'assert', the use of tools like [42]Catch, and other methods of annotating impossible cases. Blog noise [43]Haskell news from the [44]blogosphere. * [45]PE Problem #1 in Haskell * [46]osfameron: Countdown words game solver in Haskell * [47]Algebraic Data Types in JavaScript * [48]Finance and Haskell * [49]Well-Typed.Com: New Cabal and cabal-install releases * [50]Neil Mitchell: GSoC Hoogle: Week 3 * [51]Max Bolingbroke: Compiler Plugins For GHC: The First Week * [52]Dan Piponi (sigfpe): Categories of polynomials and comonadic plumbing * [53]Roman Cheplyaka: Status report: week 3 * [54]Thomas DuBuisson (TomMD): Static Buffers Considered Harmful Quotes of the Week * ddarius: Here's the short guide to Haskell for OO programmers: Haskell isn't at all an OO language. * swalters: I'm starting to believe that learning haskell is mostly about carefully crafting small and clever functions and then finding out that they are already part of the standard library. About the Haskell Weekly News New editions are posted to [55]the Haskell mailing list as well as to [56]the Haskell Sequence and [57]Planet Haskell. [58]RSS is also available, and headlines appear on [59]haskell.org. Headlines are available as [60]PDF. To help create new editions of this newsletter, please see the information on [61]how to contribute. Send stories to byorgey at seas dot upenn dot edu. The darcs repository is available at darcs get [62]http://code.haskell.org/~byorgey/code/hwn/ . References 1. http://haskell.org/ 2. http://luqui.org/blog/archives/2008/06/18/new-job/ 3. http://anygma.com/ 4. http://article.gmane.org/gmane.comp.lang.haskell.general/16249 5. http://haskell.org/haskell-symposium/2008 6. http://article.gmane.org/gmane.comp.lang.haskell.cabal.devel/3267 7. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/cabal-install 8. http://article.gmane.org/gmane.comp.lang.haskell.general/16238 9. http://icfpcontest.org/ 10. http://article.gmane.org/gmane.comp.lang.haskell.general/16241 11. http://community.haskell.org/ 12. http://community.haskell.org/admin/using_project.html#trac 13. http://article.gmane.org/gmane.comp.lang.haskell.general/16237 14. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/random-access-li… 15. http://article.gmane.org/gmane.comp.lang.haskell.glasgow.user/14645 16. http://haskell.org/ghc/docs/6.8.3/html/users_guide/release-6-8-3.html 17. http://www.haskell.org/ghc/ 18. http://article.gmane.org/gmane.comp.lang.haskell.cafe/41293 19. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Printf-TH 20. http://www.haskell.org/th/ 21. http://article.gmane.org/gmane.comp.lang.haskell.cafe/41265 22. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/mueval 23. http://article.gmane.org/gmane.comp.lang.haskell.cafe/41263 24. http://home.arcor.de/chr_bauer/topkata.html 25. http://article.gmane.org/gmane.comp.lang.haskell.cafe/41249 26. http://trac.haskell.org/haddock 27. http://www.haskell.org/haddock/ 28. http://article.gmane.org/gmane.comp.lang.haskell.cafe/41235 29. http://lisp.meetup.com/59 30. http://research.sun.com/projects/plrg/ 31. http://article.gmane.org/gmane.comp.lang.haskell.cafe/41201 32. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ieee 33. http://neilmitchell.blogspot.com/2008/06/gsoc-hoogle-week-3.html 34. http://www.haskell.org/haddock/ 35. http://physics-dph.blogspot.com/2008/06/status-report-week-3.html 36. http://www.haskell.org/haskellwiki/GHC/Data_Parallel_Haskell 37. http://article.gmane.org/gmane.comp.lang.haskell.libraries/9259 38. http://article.gmane.org/gmane.comp.lang.haskell.glasgow.user/14637 39. http://shootout.alioth.debian.org/gp4/benchmark.php?test=fannkuch 40. http://article.gmane.org/gmane.comp.lang.haskell.cafe/41368 41. http://article.gmane.org/gmane.comp.lang.haskell.cafe/41295 42. http://www-users.cs.york.ac.uk/~ndm/catch/ 43. http://planet.haskell.org/ 44. http://haskell.org/haskellwiki/Blog_articles 45. http://jdfrens.blogspot.com/2008/06/pe-problem-1-in-haskell.html 46. http://osfameron.vox.com/library/post/countdown-words-game-solver-in-haskel… 47. http://w3future.com/weblog/stories/2008/06/16/adtinjs.xml 48. http://blog.gmosx.com/2008/06/finance-and-haskell.html 49. http://blog.well-typed.com/2008/06/new-cabal-and-cabal-install-releases/ 50. http://neilmitchell.blogspot.com/2008/06/gsoc-hoogle-week-3.html 51. http://blog.omega-prime.co.uk/2008/06/15/compiler-plugins-for-ghc-the-first… 52. http://sigfpe.blogspot.com/2008/06/categories-of-polynomials-and-comonadic.… 53. http://physics-dph.blogspot.com/2008/06/status-report-week-3.html 54. http://tommd.wordpress.com/2008/05/25/static-buffers-considered-harmful/ 55. http://www.haskell.org/mailman/listinfo/haskell 56. http://sequence.complete.org/ 57. http://planet.haskell.org/ 58. http://sequence.complete.org/node/feed 59. http://haskell.org/ 60. http://code.haskell.org/~byorgey/code/hwn/archives/20080618.pdf 61. http://haskell.org/haskellwiki/HWN 62. http://code.haskell.org/~byorgey/code/hwn/

1 0

Fast sorting with Bytestring
by gsauthof＠TechFak.Uni-Bielefeld.DE 19 Jun '08

19 Jun '08

Hi, I played a bit around with the nice bytestring package. At some point I implemented a simple sorting program, because I needed line-sorting a file with a custom line-compare function. I was a bit surprised, that the resulting code is very fast. A lot of faster than sorting via GNU sort (with the standard line-compare function). Then I got suspicious and tested standard GNU sort against a trivial standard sort implementation in Haskell using bytestring. The Haskell implementation looks like: > module Main > where > import qualified Data.ByteString.Lazy.Char8 as B > import Data.List > main = do > c <- B.getContents > let l = B.lines c > let r = sort l > B.putStr $ B.unlines r Sorting a file of '[]'-strings I get: time ./sort < brackets > /dev/null # ~ 0.01 s time sort < brackets > /dev/null # ~ 14 s Ok, strane ... Well, let's test with some 'normal' text: time ./sort < bible > /dev/null # ~ 0.4 s time sort < bible > /dev/null # ~ 0.56 s Ok, not that different. But with Haskell you often expect to get very slow code compared to an implementation in C. And I am surprised, that the Haskell is fast _and_ nice to read - because for example the ultra fast 'wc -l' Haskell implementation from the Haskell-Wiki uses some insider-knowledge about bytestring, and looks to a beginner not that intuitive, I guess. ./sort is the shown Haskell implementation. I used ghc 6.8.2, installed bytestring 0.9.1.0 as a user-local package (don't now I this superseeds the global bytestring package), compiled via ghc -O2. The tests are run at a Pentium M 1.3 GHz computer. As GNU sort I tested the version from coreutils 6.9. You can get the test files from: http://www.techfak.uni-bielefeld.de/~gsauthof/stuff/brackets.gz http://www.techfak.uni-bielefeld.de/~gsauthof/stuff/bible.gz (obviously, you have to gunzip them after downloading ...) Of course, the naive Haskell implementation doesn't care about locales (i.e. collating locale sequences), but this shouldn't explain the observed differences sorting the brackets file. Best regards Georg Sauthoff -- Fortune : 'Real programmers don't comment their code. It was hard to write, it should be hard to understand.' ;)

3 4

working with Random.randoms
by Stephen Howard 19 Jun '08

19 Jun '08

Hi List, I am a newcomer doing my obligatory struggling with Haskell's type system, and I've got a nut I've not been able to crack. Given: import Random random_test :: Int -> String random_test n = do g <- getStdGen take n (randoms g)::String I'm expecting that I ought to be able to pass this action an integer and get back a random string that long (given, not all characters may be printable). But GHCI tells me: RandomTest.hs:7:4: Couldn't match `[]' against `IO' Expected type: [] Inferred type: IO In a 'do' expression: g <- getStdGen In the definition of `random_test': random_test n = do g <- getStdGen take n (randoms g) :: String And yet, when I run these lines in GHCI by hand, things seem to work (though the string is the same set of random characters each time, another bit that I need to solve): Prelude> :module Random Prelude Random> g <- getStdGen Prelude Random> take 5 (randoms g)::String "\1025049\315531\882767\1032009\334825" I'm guessing that randoms is returning an IO type but I'm not sure how to go about extracting the String to return to the calling action. Changing the type signature to Int -> IO String only gives me a different error. Where am I going wrong? thanks, Stephen

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Re: [Haskell-cafe] Lambda and closures in PHP -- could someone please comment?
by PR Stanley 19 Jun '08

19 Jun '08

Blimey! Talk about rearranging the deckchairs :-) With respect, do you not think it'd be wiser for the community to deal with a total lack of proper type checking, the lack of any notion of the general concept of a function, or a whole range of other fundamental issues in PHP first before worrying about closures and lambda functions? I'm no language purist but PHP really does take the biscuit. My personal views and not in any way representing the list . Paul At 17:45 17/06/2008, you wrote: >Hi, > >The PHP community is discussing the adding of closures and lambdas to >the language, see the proposal at http://wiki.php.net/rfc/closures > >If someone with knowledge of both languages could take a quick look it >would be great. > >Thanks a lot > >Karoly Negyesi > >Ps. I am not a member of the PHP internals team, I am just a PHP >developer but I am very very interested in getting these in my >favourite language. >_______________________________________________ >Haskell-Cafe mailing list >Haskell-Cafe(a)haskell.org >http://www.haskell.org/mailman/listinfo/haskell-cafe

15 27

Something like scan1
by Achim Schneider 18 Jun '08

18 Jun '08

Is there a generalisation of scan1, such that eg. foo (+) [0,1] (1,2) = [0,1,1,2,3,5,8,13,...] ? I came up with it while thinking about the equivalence of laziness and strictness resp. pull and push. To be more specific, I was thinking about ArrowLoop and how that beast generalises over time-varying values. It's some kind of helixoid fixed point (which might not help at all if your visualisation is incompatible with mine). -- (c) this sig last receiving data processing entity. Inspect headers for past copyright information. All rights reserved. Unauthorised copying, hiring, renting, public performance and/or broadcasting of this signature prohibited.

2 3

Haskell data types and Storable class
by Galchin, Vasili 18 Jun '08

18 Jun '08

Hello, I am reading the FFI spec. Something is unclear to me. Let's assume we have data A = {b:B, .......} and data B ={ .............} both of which belong to class Storable. In the A instance of Storable I want to do a poke and peek of "b" into and out-of, respectively, B. How do I write the poke and peek function of "b" in the "instance Storable A"? thank you, Vasili

2 1