- Haskell-Cafe - Haskell.org

Re: [web-devel] Google Summer of Code: BlazeHTML RFC
by Matt Parker 02 Jun '10

02 Jun '10

> Q3: Which HTML version would you preferably use? > HTML 5. google summer of code should be about pushing the new and exciting.

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Resource module
by Arie Peterson 02 Jun '10

02 Jun '10

Dear list, For some time, I have maintained a small private module centred around the following type of "resource": > newtype Resource cap m = Resource { with :: forall a. (cap -> m a) -> m a } Interpretation: @Resource cap m@ is a resource, providing a "capability" of type @cap@, which does administration (like opening and closing handles) in the monad @m@. I would like to use a standard module instead of this howe-grown one, so I can publish things that depend on it. Does anyone know a package that provides this? Things I tried: - The type is suspiciously similar to the continuation monad transformer; in fact @Resource cap m@ is isomorphic to @forall t. ContT t m cap@. However, I can't use this latter type directly, because I would like functions on Resources like the instance > instance (Monoid cap) => Monoid (Resource cap m) , which is not possible without a newtype wrapper. - I tried using the "regions" package, since it has a very similar purpose, but this seems impossible: some resources I could not express in the form required for its class 'Resource' (methods 'open' and 'close'). If something like this is not yet around, I'll upload my version, but I'd like to reuse, if possible. Regards, Arie

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ANN: threads-0.2
by Bas van Dijk 02 Jun '10

02 Jun '10

Hello, I just released threads-0.2. The library which lets you fork threads and wait for their result. INSTALL: $ cabal update $ cabal install threads API DOCS: http://hackage.haskell.org/package/threads-0.2 DEVELOPMENT: darcs get http://code.haskell.org/~basvandijk/code/threads CHANGES: * I removed the regular 'Control.Concurrent.ThreadId' out of our own 'ThreadId α' type and renamed the type to 'Result α': data ThreadId α = ThreadId { result ∷ TMVar (Either SomeException α) , threadId ∷ Control.Concurrent.ThreadId } to: newtype Result α = Result { unResult ∷ TMVar (Either SomeException α) } * The various fork functions now return the regular 'ThreadId' paired with a 'Result α' value: forkIO ∷ IO α → IO (ThreadId α) to: forkIO ∷ IO α → IO (ThreadId, Result α) * The various wait functions now just receive a 'Result α' value: wait ∷ ThreadId α → IO (Either SomeException α) to: wait ∷ Result α → IO (Either SomeException α) * I removed: throwTo ∷ Exception e ⇒ ThreadId α → e → IO () and: killThread ∷ ThreadId α → IO () because they aren't needed anymore since we have direct access to the regular 'ThreadId'. * When you build the package with GHC the GHC specific 'forkOnIO' function is additionally build in: forkOnIO ∷ Int → IO α → IO (ThreadId, Result α) * For the rest I added documentation and made some internal changes like a simplified implementation of ThreadGroups and made small stylistic changes all over the place. FUTURE PLANS: Inspired by the 'parIO' combinator from the 'async' package, I'm thinking about adding a module which offers similar functionality: data ParGroup α new ∷ IO (ParGroup α) forkIO ∷ ParGroup α → IO α → IO ThreadId wait ∷ ParGroup α → IO (Maybe α) The idea is that when you have multiple IO computations, that each compute the same answer but differ in how fast they compute that answer, you can compute them in parallel using 'forkIO'. Finally 'wait' blocks until one of the threads has computed an answer and will return Just that answer. When all of the threads fail to compute an answer (throw an exception) wait returns Nothing. Regards, Bas

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TagSoup 0.9
by Ralph Hodgson 02 Jun '10

02 Jun '10

Hello Neil , I was using TagSoup 0.8 with great success. On upgrading to 0.9 I have this error: TQ\TagSoup\TagSoupExtensions.lhs:29:17: `Tag' is not applied to enough type arguments Expected kind `*', but `Tag' has kind `* -> *' In the type synonym declaration for `Bundle' Failed, modules loaded: TQ.Common.TextAndListHandling. where line 29 is the type declaration for 'bundle' in the following code: > module TQ.TagSoup.TagSoupExtensions where > import TQ.Common.TextAndListHandling > import Text.HTML.TagSoup > import Text.HTML.Download > import Control.Monad > import Data.List > import Data.Char > type Bundle = [Tag] [snip] > tagsOnPage :: String -> IO(String) > tagsOnPage url = do > tags <- liftM parseTags $ openURL url > let results = unlines $ map(show) $ tags > return (results) > extractTags :: Tag -> Tag -> [Tag] -> [Tag] > extractTags fromTag toTag tags = takeWhile (~/= toTag ) $ dropWhile (~/= fromTag ) tags > extractTagsBetween :: Tag -> [Tag] -> [Tag] > extractTagsBetween _ [] = [] > extractTagsBetween markerTag tags = if startTags == [] > then [] > else [head startTags] ++ (takeWhile (~/= markerTag ) $ tail startTags) > where > startTags = dropWhile (~/= markerTag ) tags I need to repair this code quickly. I am hoping you can quickly help me resolve this. Thanks. Ralph Hodgson, @ralphtq <http://twitter.com/ralphtq>

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Re: The state of Hackage: what are we doing about it?
by Bas van Dijk 01 Jun '10

01 Jun '10

On Tue, Jun 1, 2010 at 7:09 PM, Matthias Kilian <kili(a)outback.escape.de> wrote: > - "Liveness" of a library, that is: does it still get updates? Does it > build with recent versions of GHC? Note that Hackage already shows the upload date and for which versions of GHC the package does and doesn't build. > - Reverse dependencies -- how many other libraries and/or programs are > using this library (Maybe something for Hackage-2.0)? My brother Roel created a patch for this. See ticket #576: http://hackage.haskell.org/trac/hackage/ticket/576 Also see our mirror Hackage that implements that patch: http://bifunctor.homelinux.net/~roel/hackage/packages/hackage.html The server automatically synchronizes with Hackage daily. It would be great to have this in the regular Hackage however! Regards, Bas

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Re: Language Shootout reverse-complement benchmark
by Isaac Gouy 01 Jun '10

01 Jun '10

On Tue, Jun 1, 2010 at 10:25 AM, David Leimbach <leimy2k <at> gmail.com> wrote: > I'm still trying to figure out what the point of the shootout really is. >From one point of view - http://shootout.alioth.debian.org/help.php#why > If there's no dedicated folks working with a language there, trying to > make things run faster, a language will come out looking inefficient > potentially. If there isn't even one effective programmer willing to contribute tiny programs ... > There's a lot of compile flags and optimizations that can make a > difference in probably all of the languages listed on that page. The compile flags and optimizations used are shown below the program source code.

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Pugs with GHC 6.12
by Simon Thompson 01 Jun '10

01 Jun '10

Has anyone successfully compiled Pugs with GHC 6.12? The Pugs page suggests that it compiles, but it falls to link (and therefore build) when I try it. Thanks! Simon T.

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Haskell + CouchDB, anyone?
by Clint Moore 01 Jun '10

01 Jun '10

Hi there, I'm trying to use any version of couchdb with Haskell and Database.CouchDB and I'm not having a ton of luck. I've talked to others in the past that have used it without a problem so I'm wondering what it is that I'm doing wrong. I can connect, and create new databases without a problem - do any of you couchdb-using haskell people see a problem with this line? runCouchDB' $ newDoc (db "testify") $ JSObject $ toJSObject [("name",JSString $ toJSString $ "value")] I figured I would start small and just try to put something basic into a document. The rest of this email is also at a gist at http://gist.github.com/417442 Here is a list of the revisions of couchdb and the different errors I'm getting back as well as some of the code I'm using to try to test this.. Hopefully it will help. {- CouchDB 0.11.0: [error] [<0.95.0>] attempted upload of invalid JSON undefined [info] [<0.95.0>] 127.0.0.1 - - 'PUT' /honks/cmoore? 400 CouchDB Trunk as of 5/28 - 10:26 PST: [info] [<0.95.0>] 127.0.0.1 - - 'PUT' /honks? 201 [debug] [<0.114.0>] 'PUT' /honks/cmoore? {1,1} Headers: [{'Connection',"keep-alive"}, {'Content-Type',"application/json"}, {'Host',"localhost:5984"}, {'User-Agent',"hs-HTTP-4000.0.5"}] [debug] [<0.114.0>] OAuth Params: [] [error] [<0.114.0>] attempted upload of invalid JSON undefined [info] [<0.114.0>] 127.0.0.1 - - 'PUT' /honks/cmoore? 400 [debug] [<0.114.0>] httpd 400 error response: {"error":"bad_request","reason":"invalid UTF-8 JSON"} CouchDB rev 818083: [debug] [<0.107.0>] 'PUT' /honks/cmoore? {1,1} Headers: [{'Connection',"keep-alive"}, {'Content-Type',"application/json"}, {'Host',"localhost:5984"}, {'User-Agent',"hs-HTTP-4000.0.5"}] [debug] [<0.107.0>] OAuth Params: [] [debug] [<0.107.0>] Minor error in HTTP request: {invalid_json,undefined} [debug] [<0.107.0>] Stacktrace: [{mochijson2,decode,1}, {couch_httpd_db,db_doc_req,3}, {couch_httpd_db,do_db_req,2}, {couch_httpd,handle_request,5}, {mochiweb_http,headers,5}, {proc_lib,init_p_do_apply,3}] [info] [<0.107.0>] 127.0.0.1 - - 'PUT' /honks/cmoore? 500 [debug] [<0.107.0>] httpd 500 error response: {"error":"invalid_json","reason":"undefined"} CouchDB 0.10.1: [error] [<0.278.0>] attempted upload of invalid JSON undefined [info] [<0.278.0>] 127.0.0.1 - - 'POST' /testify? 400 -} module Main where import Text.JSON import Database.CouchDB import Database.CouchDB.JSON data Person = Person { name :: String, email :: String } deriving (Eq,Show) instance JSON Person where showJSON (Person nm em) = JSObject $ toJSObject [("name", showJSON nm),("email", showJSON em)] readJSON val = do obj <- jsonObject val nm <- jsonField "name" obj em <- jsonField "email" obj return (Person nm em) create = runCouchDB "localhost" 5984 $ createDB "honks" main :: IO () main = do _ <- runCouchDB "localhost" 5984 $ newNamedDoc (db "honks") (doc "cmoore") $ Person "cmoore" "cmoore(a)w.com" putStrLn "Honk." --- Later, against 0.10.1: cmoore@cathedral:~/tmp$ ghci GHCi, version 6.12.2: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. Prelude> :m +Database.CouchDB Prelude Database.CouchDB> :m +Text.JSON Prelude Database.CouchDB Text.JSON> :set prompt "> " > runCouchDB' $ createDB "testify" Loading package array-0.3.0.0 ... linking ... done. Loading package syb-0.1.0.2 ... linking ... done. Loading package base-3.0.3.2 ... linking ... done. Loading package bytestring-0.9.1.6 ... linking ... done. Loading package mtl-1.1.0.2 ... linking ... done. Loading package parsec-2.1.0.1 ... linking ... done. Loading package network-2.2.1.5 ... linking ... done. Loading package old-locale-1.0.0.2 ... linking ... done. Loading package old-time-1.0.0.4 ... linking ... done. Loading package HTTP-4000.0.8 ... linking ... done. Loading package containers-0.3.0.0 ... linking ... done. Loading package pretty-1.0.1.1 ... linking ... done. Loading package json-0.4.4 ... linking ... done. Loading package CouchDB-0.10.2 ... linking ... done. > runCouchDB' $ newDoc (db "testify") $ JSObject $ toJSObject [("name",JSString $ toJSString $ "value")] *** Exception: HTTP/1.1 400 Bad Request Server: CouchDB/0.10.1 (Erlang OTP/R13B) Date: Fri, 28 May 2010 18:46:36 GMT Content-Type: text/plain;charset=utf-8 Content-Length: 54 Cache-Control: must-revalidate

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name of this monadic combinator?
by Michael Vanier 01 Jun '10

01 Jun '10

I stumbled across this monadic combinator: mcombine :: Monad m => (a -> a -> a) -> m a -> m a -> m a mcombine f mx my = do x <- mx y <- my return (f x y) I used it to chain the outputs of two Parsec String parsers together using this operator: (<++>) :: Monad m => m String -> m String -> m String (<++>) = mcombine (++) mcombine seems like such a basic operation that it should be a library function, but I couldn't find one matching it on hoogle. Is there one? - Mike

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ANN: list-extras 0.4.0
by wren ng thornton 01 Jun '10

01 Jun '10

-------------------------------------------- -- list-extras 0.4.0 -------------------------------------------- A minor (but interface-changing) release for common not-so-common functions for lists. -------------------------------------------- -- Changes (since 0.3.0) -------------------------------------------- * (by Tom Lokhorst) Added Data.List.Extras.list: a function for non-recursive case matching on lists. In the spirit of `maybe` and `either` (if we don't interpret them as catamorphisms). * Changed the type of Data.List.Extras.Pair.zipBy to be correct. I have no idea where the old more restrictive type came from. -------------------------------------------- -- Links -------------------------------------------- Homepage: http://code.haskell.org/~wren/ Hackage: http://hackage.haskell.org/package/list-extras Darcs: http://code.haskell.org/~wren/list-extras/ Haddock (Darcs version): http://code.haskell.org/~wren/list-extras/dist/doc/html/list-extras/ -- Live well, ~wren

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yet another functional reactive programming tutorial :)
by Jinjing Wang 01 Jun '10

01 Jun '10

Dear list, As I'm learning frp and reading the wonderful tutorial at http://www.formicite.com/dopage.php?frp/frp.html , I'm putting up some more basic cheatsheet style tutorial for myself. http://github.com/nfjinjing/frp-guide Feel free to take advantage of it. -- jinjing

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LLVM -> Haskell
by Tom Hawkins 01 Jun '10

01 Jun '10

Is there any work being done to read LLVM object code into Haskell? I've looked through the llvm library [1], but it appears focused on code generation. -Tom [1] http://hackage.haskell.org/package/llvm

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Third Ghent Functional Programming Group Meeting: Tuesday, June 29 at 19h
by Jeroen Janssen 01 Jun '10

01 Jun '10

(apologies if you receive multiple copies) Dear all, We would like to announce that the third Ghent Functional Programming Group Meeting will take place on Tuesday, June 29, in the Technicum Building (Sint-Pietersnieuwstraat 41, 9000 Gent) of Ghent University at 19h. If you are interested in giving a talk, please do not hesitate to drop us a note! The final program will be announced shortly. For more information you can follow us at twitter (@ghentfpg) or sign up for our google group (http://groups.google.com/group/ghent-fpg) Hope to see you all then, Bart Coppens (Bart.Coppens(a)elis.ugent.be; Twitter: @bartcopp) Jasper Van der Jeugt (jaspervdj(a)gmail.com; Twitter: @jaspervdj) Jeroen Janssen (jejansse(a)gmail.com; Twitter: @jejansse) Note: if you do not want to receive any more e-mails from Ghent FPG, just send an e-mail to Jeroen Janssen (jejansse(a)gmail.com) and we will remove you from our mailinglist.

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Improving Haskell on Windows by adding it to TakeoffGW
by MH 31 May '10

31 May '10

That's a great idea and I would love to help but I have no idea where and how to start. I would love to work on making Haskell installer work on TakeofGW. Would it be possible to write down some notes describing what needs to be done? Thanks, MH

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Announce: berp, an implementation of Python 3, in Haskell
by Bernie Pope 31 May '10

31 May '10

I'm pleased to announce the first public release of berp, version 0.0.2. Berp is (the beginnings of) an implementation of Python 3, written in Haskell. It provides a compiler and an interpreter. In both cases the input Python program is translated into Haskell code. The compiler turns the Haskell code into machine code. The interpreter runs the Haskell code immediately via the GHCi interpreter. The user interface of the interpreter imitates the one provided by CPython. A cabal package is available: http://hackage.haskell.org/package/berp The source code is hosted on github: http://github.com/bjpop/berp Documentation is available on a github wiki: http://wiki.github.com/bjpop/berp/ Note: berp is known to work with GHC 6.10.4, but it may not work with 6.12.x, due to issues with GHC's memory usage when compiling the language-python package (ticket #3972 on the GHC trac). As you can see by the very low version number (0.0.2), berp is still young; there is lots of room for improvement. However, it does support a fairly wide variety of Python's features, and some extensions such as callCC and tail call optimisation. Read more about it on the github wiki mentioned above. Cheers, Bernie.

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ANNOUNCE: MonadCatchIO-foreign
by Antoine Latter 31 May '10

31 May '10

Hello Haskell, I'd like to announce a very small library in two flavors. The problem I'm trying to solve is that we have some capabilities for writing functions which are polymorphic over monad but still use IO capabilities - liftIO :: (IO a -> m a) from the packages transformers and mtl. The packages[1,2] MonadCatchIO offer similar polymorphism for the exception capabilities of the IO monad. The package MonadCatchIO-foreign offers similar polymorphism for the following functions: alloca, allocaBytes, allocaArray, allocaArray0 and withForeignPtr. It's a small thing, but I don't see why it should be written twice. Links on hackage: http://hackage.haskell.org/package/MonadCatchIO-mtl-foreign http://hackage.haskell.org/package/MonadCatchIO-transformers-foreign GitHub link: http://github.com/aslatter/MonadCatchIO-foreign Feedback, praise and adulation welcome. Antoine

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www.haskell.org web server down?
by Sean Leather 31 May '10

31 May '10

It responds to pings but not http. http://downforeveryoneorjustme.com/www.haskell.org Sean

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Why Either = Left | Right instead of something like Result = Success | Failure
by Ionut G. Stan 31 May '10

31 May '10

Hi, I was just wondering if there's any particular reason for which the two constructors of the Either data type are named Left and Right. I'm thinking that something like Success | Failure or Right | Wrong would have been a little better. I've recently seen that Scala uses a similar convention for some error notifications so I'm starting to believe there's more background behind it than just an unfortunate naming. Thanks, -- Ionuț G. Stan | http://igstan.ro

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Improving Haskell on Windows by adding it to TakeoffGW
by Hamish Mackenzie 31 May '10

31 May '10

Finally someone has created a working MinGW distribution with a package manager! http://sourceforge.net/apps/mediawiki/takeoffgw/index.php?title=Main_Page Pier11 has done a great job of making MinGW libraries, tools and apps easy to install. It may seem limited at this stage (compared to what is available on linux), but it is a darn site better than fishing around for a combination of prebuilt binaries or building everything from source. I think we should embrace this project and try to add support for installing Haskell using it (GHC, Haskell Platform, Darcs, Gtk2Hs, Leksah, ...). You can read the comments here to get a feel for just how Windows users feel about TakeoffGW... http://www.reddit.com/r/programming/comments/c9duj/takeoffgw_mingw_based_di… I tried it out myself and had Gtk2Hs and Leksah up and running in no time using TakeoffGW to install all the prerequisites. Hamish

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Re: Proposal to solve Haskell's MPTC dilemma
by Isaac Dupree 31 May '10

31 May '10

On 05/26/10 15:42, Carlos Camarao wrote: > What do you think? I think you are proposing using the current set of instances in scope in order to remove ambiguity. Am I right? ..I read the haskell-cafe thread so far, and it looks like I'm right. This is what I'll add to what's been said so far: Your proposal appears to allow /incoherent/ instance selection. This means that an expression can be well-typed in one module, and well-typed in another module, but have different semantics in the two modules. For example (drawing from above discussion) : module C where class F a b where f :: a -> b class O a where o :: a module P where import C instance F Bool Bool where f = not instance O Bool where o = True k :: Bool k = f o module Q where import C instance F Int Bool where f = even instance O Int where o = 0 k :: Bool k = f o module Main where import P import Q -- (here, all four instances are in scope) main = do { print P.k ; print Q.k } -- should result, according to your proposal, in -- False -- True -- , am I correct? Also, in your paper, example 2 includes > m = (m1 * m2) * m3 and you state > In Example 2, there is no means of specializing type variable c0 occurring in the > type of m to Matrix. I suggest that there is an appropriate such means, namely, to write m = (m1 * m2 :: Matrix) * m3 . (Could the paper address how that solution falls short? Are there other cases in which there is more than just a little syntactical convenience at stake?, or is even that much added code too much for some use-case?) -Isaac

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How to fix this problem in FreeBSD ?
by zaxis 31 May '10

31 May '10

/usr/ports/devel/hs-ghc-paths>sudo make install clean ===> hs-ghc-paths-0.1.0.5_1 depends on executable: ghc - found ===> Configuring for hs-ghc-paths-0.1.0.5_1 [1 of 1] Compiling Main ( Setup.hs, Setup.o ) Setup.hs:18:22: Not in scope: `preMakefile' *** Error code 1 Stop in /usr/ports/devel/hs-ghc-paths. Sincerely! ----- e^(π⋅i) + 1 = 0 -- View this message in context: http://old.nabble.com/How-to-fix-this-problem-in-FreeBSD---tp28714271p28714… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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gemcutter src?
by Jens Petersen 31 May '10

31 May '10

On 16 May 2010 05:13, Henning Thielemann <schlepptop(a)henning-thielemann.de> wrote: > http://resources.businessobjects.com/labs/cal/gemcutter-techpaper.pdf Does anyone have the url to the source code? (I guess businessobjects was acquired by SAP.) Jens

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Reproducible Bug in Emacs' haskell-mode
by aditya siram 30 May '10

30 May '10

Hi all, I have a reproducible bug in Emacs' Haskell mode where Emacs gets stuck in an endless loop. I am using GNU Emacs 23.0.93.1. To reproduce: 1. Use a bad Language pragma eg. {-# LANGUAGE FlexibleTypes #-} 2. Load a GHCI instance with C-c C-l. GHCI outputs the following error: unsupported extension: FlexibleTypes 3. Fix the pragma: {-# FlexibleInstances -#} 4. Try to load the file again with C-c C-l and Emacs gets stuck. -deech

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Work on Video Games in Haskell
by Ryan Trinkle 29 May '10

29 May '10

iPwn Studios is seeking Haskell developers for its debut title, BloodKnight. * No prior game development experience is required, but you must be very comfortable working in Haskell. * Compensation is negotiable; profit-sharing may be available in some cases. * To apply, or for more information, contact me at ryan(a)ipwnstudios.com. BloodKnight is an action-roleplaying game inspired by games like Diablo and Fallout. It is currently in the final stages of development, and will be released later this year on a variety of smartphone platforms, including iPhone and Android. iPwn Studios is a start-up company located in Boston, MA. We believe in giving back to the Haskell community, so we've open-sourced our ghc-iphone project, which allows GHC to produce binaries for the iPhone. Check it out at http://projects.haskell.org/ghc-iphone/. Ryan Trinkle iPwn Studios

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Math questions
by Mujtaba Boori 29 May '10

29 May '10

Hello I am try to solve this equation Define a higher order function that tests whether two functions , both defined on integers , coincide for all integers between 1 and 100 how can I solve this ? is there any thing in Haskell library to solve this kind ? -- Mujtaba Ali Alboori

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Existentials and SYB [Was: GADTs and Scrap your Boilerplate]
by oleg＠okmij.org 29 May '10

29 May '10

Oscar Finnsson wrote: > I got the GADT > > data DataBox where > DataBox :: (Show d, Eq d, Data d) => d -> DataBox > > and I'm trying to get this to compile > > instance Data DataBox where > gfoldl k z (DataBox d) = z DataBox `k` d > gunfold k z c = k (z DataBox) -- not OK As has been pointed out, DataBox is an ordinary existential data type, only written using the new-style notation. For clarity, let us define our DataBox with the conventional notation: > data DataBox = forall d. (Show d, Eq d, Data d) => DataBox d So, the question becomes of using existentials with Scrap your Boilerplate. On one hand, it is unnervingly trivial to incorporate existentials into the SYB framework. Let us consider the signature of gunfold, the problematic member of the (Data a) type class. -- | Unfolding constructor applications gunfold :: (forall b r. Data b => c (b -> r) -> c r) -> (forall r. r -> c r) -> Constr -> c a We have to eventually produce a value of the type (c DataBox). We observe that our work is done if we manage to produce one value of the type DataBox. We then apply the second argument to gunfold (which `lifts' from any type r to the type c r), and we are done. To produce an existential value, we need one witness of the constraints Show, Eq and Data. For example, the type Int is such witness. Thus, we are done indeed. Here is the complete code: > {-# LANGUAGE ExistentialQuantification, Rank2Types #-} > > module F where > > import Data.Generics > > data DataBox = forall d. (Show d, Eq d, Data d) => DataBox d > > instance Typeable DataBox where > typeOf _ = mkTyConApp (mkTyCon "DataBox") [] > > instance Data DataBox where > gfoldl k z (DataBox d) = z DataBox `k` d > gunfold k z c = z (DataBox (42::Int)) One may complain that we have fixed not only the type of the argument to DataBox but also the value. We should let gunfold to `produce' the value. Here is a sketch: > enDataI :: (Int -> DataBox) > enDataI = DataBox > > enDataB :: (Bool -> DataBox) > enDataB = DataBox > > instance Data DataBox where > gfoldl k z (DataBox d) = z DataBox `k` d > gunfold k z c = (if True then k (z enDataI) else k (z enDataB)) Now, the particular Int value to supply to DataBox will be produced by the function k. The above code used the constant True to fix the choice of the type to Int. Generally, one could use the third argument of gunfold to help make the choice. That is, we could incorporate the choice of the type in the value of Constr.

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currying combinators
by Günther Schmidt 29 May '10

29 May '10

Hi all, I've posted some code on hpaste http://www.hpaste.org/fastcgi/hpaste.fcgi/view?id=25694 in which I attempt to develop a "currying combinator" library. I'm stuck at some point and would appreciate any help. Günther

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Google Summer of Code: BlazeHTML RFC
by Jasper Van der Jeugt 28 May '10

28 May '10

Dear all, BlazeHtml started out on ZuriHac 2010. Now, Jasper Van der Jeugt is working on it as a student to Google Summer of Code for haskell.org. His mentors are Simon Meier and Johan Tibell. The goal is to create a high-performance HTML generation library. In the past few weeks, we have been exploring the performance and design of different drafts of this library. Now, the time has come to ask some questions to the Haskell community — more specifically the future users of BlazeHtml as well as current users of other HTML generation libraries. We have written an RFC to gather feedback from the community: HTML version: http://jaspervdj.be/posts/2010-05-27-blazehtml-rfc.html Plain version: http://github.com/jaspervdj/BlazeHtml/raw/develop/doc/RFC.lhs The easiest way of sending us feedback, comments or criticism is replying to the haskell-cafe thread here. Alternatively, drop a comment at the bottom of the HTML version or at reddit. Looking forward to your feedback, Kind regards, Simon Meier Jasper Van der Jeugt

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Steven S. Skiena of "The Algorithm Design Manual", Second Edition, 2008 has sent me a message that if there is a Haskell page of algorithms he will link to it.
by Casey Hawthorne 28 May '10

28 May '10

Hi: Steven S. Skiena of "The Algorithm Design Manual", Second Edition, 2008 has sent me a message that if there is a Haskell page of algorithms he will link to it. So, is there such a page and/or is there some collection(s) of algorithms and data structures some where that I can massage into the shape of his book? :) -- Regards, Casey

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Re: [Haskell] Re: [Haskell-cafe] Work on Video Games in Haskell
by Ketil Malde 28 May '10

28 May '10

<bf3(a)telenet.be> writes: > Or maybe this would be a nice research topic: how to generate C code > that looks like it’s human written… Nah, that's too easy: just add a sprinkling of buffer overflows, undefined behavior, and off-by one index errors. -k -- If I haven't seen further, it is by standing in the footprints of giants

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ANNOUNCE: Haskell Communities and Activities Report (18th ed., May 2010)
by Janis Voigtländer 28 May '10

28 May '10

On behalf of the many, many contributors, I am pleased to announce that the Haskell Communities and Activities Report (18th edition, May 2010) is now available in PDF and HTML formats: http://haskell.org/communities/05-2010/report.pdf http://haskell.org/communities/05-2010/html/report.html Many thanks go to all the people that contributed to this report, both directly, by sending in descriptions, and indirectly, by doing all the interesting things that are reported. I hope you will find it as interesting a read as I did. If you have not encountered the Haskell Communities and Activities Reports before, you may like to know that the first of these reports was published in November 2001. Their goal is to improve the communication between the increasingly diverse groups, projects, and individuals working on, with, or inspired by Haskell. The idea behind these reports is simple: Every six months, a call goes out to all of you enjoying Haskell to contribute brief summaries of your own area of work. Many of you respond (eagerly, unprompted, and sometimes in time for the actual deadline ;-) to the call. The editor collects all the contributions into a single report and feeds that back to the community. When I try for the next update, six months from now, you might want to report on your own work, project, research area or group as well. So, please put the following into your diaries now: ---------------------------------------- End of October 2010: target deadline for contributions to the November 2010 edition of the HC&A Report ---------------------------------------- Unfortunately, many Haskellers working on interesting projects are so busy with their work that they seem to have lost the time to follow the Haskell related mailing lists and newsgroups, and have trouble even finding time to report on their work. If you are a member, user or friend of a project so burdened, please find someone willing to make time to report and ask them to "register" with the editor for a simple e-mail reminder in October (you could point me to them as well, and I can then politely ask if they want to contribute, but it might work better if you do the initial asking). Of course, they will still have to find the ten to fifteen minutes to draw up their report, but maybe we can increase our coverage of all that is going on in the community. Feel free to circulate this announcement further in order to reach people who might otherwise not see it. Enjoy! Janis Voigtlaender <hcar at haskell.org>

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Optimizations and constant unboxed values
by Pierre-Etienne Meunier 28 May '10

28 May '10

Hi Cafe, Profiling a function that I thought ultra simple revealed that it consumed more than half the execution time of my code. After noticing that GHC did not unbox all I thought it did, I rewrote it with primitive types, and it did a little better, but not much. Then, examining the core (with of course -O3 on) revealed things like : (GHC.Prim.*## (GHC.Prim.-## 1.0 (GHC.Prim.**## 2.0 -53.0)) (GHC.Prim.**## 2.0 1024.0)) or case GHC.Prim.<## x_aB9 (GHC.Prim.**## 2.0 -1021.0) of _ {... Then I wondered if this was really the last stage of GHC's optimizations, as constants are not yet propagated. Or maybe GHC does not propagate constants, in which case I'd really like to write my 1-2^53 and 2^-1021 once and for all in my program. But since unsafeCoerce# does not work between doubles and words ( there is a trac ticket about it : http://hackage.haskell.org/trac/ghc/ticket/4092 ), I do not even think this is possible (and of course, specifying a double in decimal notation with ## is not precise enough for this program). Any clue about how to do it ? Thanks, PE

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Turn off verbose template haskell output
by Michael Snoyman 28 May '10

28 May '10

Hi all, Is there a GHC flag to turn off all of the "Loading package..." messages from Template Haskell? Thanks, Michael

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Chuch encoding of data structures in Haskell
by Günther Schmidt 28 May '10

28 May '10

Hi all, I'm exploring the use of church encodings of algebraic data types in Haskell. Since it's hard to imagine being the first to do so I wonder if folks here could point me to some references on the subject. I'm looking for examples of church encodings in Haskell a little bit beyond Church Booleans and Church Numerals. Günther

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Re[2]: [Haskell-cafe] Re: Unicode vs. System.Directory
by Bulat Ziganshin 28 May '10

28 May '10

Hello Andy, Friday, May 28, 2010, 1:05:59 AM, you wrote: > Looks my file-manager: > http://farm5.static.flickr.com/4027/4584389024_782b1e09ee_o.png can you please share windows and linux executables and source code? > I have finish all necessary GIO APIs at > http://patch-tag.com/r/AndyStewart/gio-branch/home > I will try to merge those APIs to next release version of gtk2hs when i > have spare time. it seems that i can just merge these sources into my program for a while. but what is the license? believe it or not, but i have commercial project on the march -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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Announce : New haskell-llvm mailing list.
by Erik de Castro Lopo 28 May '10

28 May '10

Hi all, I have set up a mailing list for discussion of haskell and LLVM related topics. All discussion on using Haskell with LLVM is relevant. That includes the haskell-llvm bindings on Hackage, custom LLVM bindings and directly generating LLVM IR code from haskell and passing that through the LLVM tools. In particular, this list is a place to share code, information, techniques and user experiences. The subscription page is here: http://projects.haskell.org/cgi-bin/mailman/listinfo/haskell-llvm Cheers, Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

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Multidimensional generalization of the vector package
by Gregory Crosswhite 28 May '10

28 May '10

Hey everyone, I have been thinking about how to generalize the vector package to multiple dimensions, and I'd like to share my ideas with you all in the hope of having people smarter than me share their thoughts on how it could be improved --- *especially* regarding how to make it efficient! The standard way to represent a memory layout is to use a stride vector --- i.e., a vector which lets you get the (flat) index in memory of a given element by dotting the stride vector with the multidimensional index vector. This has the advantage that one can efficiently manipulate it to represent views with arbitrary slices, subranges, subranges plus striping (i.e., take every k-th element), and transpositions. Basically the only thing it can't do is represent a weird view where we, say, take every index along a particular dimension that is a prime number. Having said this, it might be useful to abstract the memory layout in order to preserve generality. Thus, I was thinking that the layout of the multidimensional vector could be represented as a type class with an associated type family: class MemoryLayout layout where type LayoutIndex layout :: * indexOf :: layout -> (LayoutIndex layout) -> Int firstIndex :: layout -> (LayoutIndex layout) -> Int lastIndex :: layout -> (LayoutIndex layout) -> Int withinBounds :: layout -> (LayoutIndex layout) -> Bool ... etc. It is important that we be able to iterate through the array efficiently. One way that this could be done is through the use of cursors. class MemoryLayout layout where ... type LayoutCursor layout :: * ... getCursorAt :: layout -> (LayoutIndex layout) -> (LayoutCursor cursor) getFirstCursor :: layout -> (LayoutCursor cursor) getLastCursor :: layout -> (LayoutCursor cursor) indexOfCursor :: layout -> (LayoutCursor cursor) -> Int advanceCursor :: layout -> (LayoutCursor cursor) -> (LayoutCursor cursor) In general these unfortunately cannot be integers because integers don't contain enough information to tell us about where we need to go next inside a non-contiguous array. The alternative is to use a data structure, but then we may potentially have lots of potentially wasteful allocations, deallocations, and copying going on unless we can somehow cause the cursor to be updated destructively. Fortunately, much of the time our array will be contiguous, so we can have a special case fast-path for this case: class MemoryLayout layout where ... layoutIsContiguous :: layout -> Bool layoutIsContiguousWhenTraversedInRowMajorOrder :: layout -> Bool layoutIsContiguousWhenTraversedInColumnMajorOrder :: layout -> Bool Then we know that we can just iterate through the array in the order of the indices. It turns out that it is not hard to compute whether a layout is contiguous efficiently. For example, in the code that implements cuts and slicing we can actually check to see if the new view is still contiguous and simply cache this in the layout. For restricted cases of layouts (when there aren't transpositions) it isn't too hard to just scan through the layout and see if it is contiguous. Another way to iterate through the array is to provide this capability by constructing functions that traverse through the array for us and call some user-supplied function on each element. This actually can be a lot more efficient (since by using recursive algorithms it can store the information that it needs on the stack rather than using heap allocations) but it is much more restrictive, and it requires special cases for traversing multiple arrays in parallel. Probably the most general way to sum/fold/etc. over particular dimensions (i.e., given a 4D vector, sum over the first and third dimensions leaving the second and fourth dimensions) is simply to generalize the notion of cursors, so that rather than iterating over elements we are iterating over views of the array. class (MemoryLayout layout1, MemoryLayout layout2) => MemoryLayoutSubviewable layout1 layout2 where type MemoryLayoutViewSetSlicer layout1 layout2 :: * type MemoryLayoutViewSet layout1 layout2 :: * getViewSet :: (MemoryLayoutViewSetSlicer layout1 layout2) -> layout1 -> (MemoryLayoutViewSet layout1 layout2) advanceViewSet :: (MemoryLayoutViewSet layout1 layout2) -> (MemoryLayoutViewSet layout1 layout2) currentView :: MemoryLayoutViewSet layout1 layout2 -> layout2 This way we can hand getViewSet some "slicer" that selects some dimensions to be reduced but not others and get back a set of views that we can iterate over, providing a generalization of the sum and fold functionality provided by repa. Although these framework allows us to stay general, in practice working with shapes that are fixed-length vectors (I personally would prefer the Vec package over the hand-rolled one in repa) tends to be easiest, because it allows one to write recursive algorithms employing type families that can define views for arrays with arbitrary dimensions. Also, in practice one can construct type aliases any helper functions such as i1 a = a :. () i2 a b = a :. b :. () i3 a b c = a:. b:. c:. () . . . type I0 = () type I1 = Int : I0 type I2 = Int :. I1 . . . etc. so that we construct values via (i2 x y) rather than (x :. y :. ()). Any thoughts? Cheers, Greg

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Unicode vs. System.Directory
by Arie Peterson 27 May '10

27 May '10

After upgrading to haskell-platform-2010.1.0.0, with the improved unicode support for IO in ghc-6.12, I hoped to be able to deal with filenames containing non-ascii characters. This still seems problematic, though: $ ls m×n♯α $ ghci GHCi, version 6.12.1: http://www.haskell.org/ghc/ :? for help Prelude> :m +System.Directory Prelude System.Directory> getDirectoryContents "." >>= mapM_ putStrLn .. mÃnâ¯Î± . I hope this passes through the various email systems unharmed; on my terminal, the output of 'ls' contains shiny unicode characters, while 'ghci' garbles up the filename. (My locale is en_GB.utf8.) Similar problems arise with functions such as 'copyFile', which refuses to handle filenames with non-ascii characters (unless wrapping it with encoding functions). Is this a known problem? I searched ghc's trac, but there are no relevant bugs for the component 'libraries/directory'. I have parts of a unicode-aware layer on top of System.Directory laying around somewhere. I was rather hoping to ditch it, but I can polish it and put it on hackage, if people are interested. Kind regards, Arie

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double2Float is faster than (fromRational . toRational)
by Daniel van den Eijkel 27 May '10

27 May '10

Dear Haskellers, I just want to share an observation. I had to convert a Double to a Float value in an inner loop of an application, and I used somethin like this: xf = (fromRational $ toRational xd) :: Float The program works on windows but it did not on OSX - it was too slow. Now, after big headaches and much frustration, I replaced the code above with this line (why didn't I come up with this earlier?): xf = double2Float xd and now everything works just fine. I am not really surprised by the speed-up (and no-one should be), but I am still surprised how often such kinds of unobvious problems occur while programming in Haskell. So I write this email just to remind me and you to look out for such pitfalls. Best, Daniel

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Re: [Gtk2hs-users] Problem when installing gtk2hs.
by Magicloud Magiclouds 27 May '10

27 May '10

I only have one alex installed under ~/.cabal/bin. It seems working with shell environment, but not cabal. On Thu, May 27, 2010 at 1:40 PM, Axel Simon <Axel.Simon(a)in.tum.de> wrote: > Hi Magicloud, > > On May 27, 2010, at 4:11, Magicloud Magiclouds wrote: > >> Hi, I have met similar problem before, and I do not know what to do. >> >> # cabal install --reinstall gtk2hs-buildtools >> Resolving dependencies... >> Configuring gtk2hs-buildtools-0.9... >> cabal: alex is required but it could not be found. >> cabal: Error: some packages failed to install: >> gtk2hs-buildtools-0.9 failed during the configure step. The exception was: >> ExitFailure 1 >> # alex -v >> Alex version 2.3.3, (c) 2003 Chris Dornan and Simon Marlow > > I don't think this is due to us. Are there perhaps other alex installations > on your path? Or in ~/.cabal/bin? > > Axel > > >> -- >> 竹密岂妨流水过 >> 山高哪阻野云飞 >> >> >> ------------------------------------------------------------------------------ >> >> _______________________________________________ >> Gtk2hs-users mailing list >> Gtk2hs-users(a)lists.sourceforge.net >> https://lists.sourceforge.net/lists/listinfo/gtk2hs-users > > -- 竹密岂妨流水过山高哪阻野云飞

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Problem when installing gtk2hs.
by Magicloud Magiclouds 27 May '10

27 May '10

Hi, I have met similar problem before, and I do not know what to do. # cabal install --reinstall gtk2hs-buildtools Resolving dependencies... Configuring gtk2hs-buildtools-0.9... cabal: alex is required but it could not be found. cabal: Error: some packages failed to install: gtk2hs-buildtools-0.9 failed during the configure step. The exception was: ExitFailure 1 # alex -v Alex version 2.3.3, (c) 2003 Chris Dornan and Simon Marlow -- 竹密岂妨流水过山高哪阻野云飞

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Re: [Haskell-cafe] [reactive] A pong and integrate
by Patai Gergely 26 May '10

26 May '10

> I did not look thoroughly at elerea, but at least, when I tried its sample > "dungeons of wor" it worked properly ;) Elerea has its own 'beauty' though. I suggest unpacking the source of dow and executing it in ghci, the problem will be obvious as you play at length. Unfortunately, Elerea doesn't really work well as an EDSL, however much I'd like it to. Its main issue is that unreferenced entities keep using CPU time until they are garbage collected -- this comes from the necessity to keep them referentially transparent, i.e. behaving the same way regardless of how they are observed --, and currently I don't see any way to solve that without implementing a dedicated runtime for dynamic stream processing. So there, no FRP library without a catch for you. ;) Gergely -- http://www.fastmail.fm - A fast, anti-spam email service.

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MultiParamClasses question
by Eugeny N Dzhurinsky 26 May '10

26 May '10

Hello, all! I'm trying to create set of polymorphic functions for working with custom containers. I decided to try out typeclass and define generic function, which uses the methods from the typeclass. The quick and naive solution is listed below: ================================================================================ {-# OPTIONS_GHC -XMultiParamTypeClasses -XTypeSynonymInstances #-} import Data.List as L class Storage k t a b where stExists :: k -> t -> b -> Bool stAdjust :: k -> t -> ( a -> a ) -> b -> b stInsert :: k -> t -> a -> b -> b stList :: b -> [a] type IxPair k t = ( k, t, String, String) data Pair = Pair { name, value :: String } type IxPairParser k t = Pair -> Maybe (IxPair k t) type RecordUpdateF r = String -> String -> r -> r convertPairsToRecords :: (Storage k t a b) => b -> RecordUpdateF a -> IxPairParser k t -> a -> [Pair] -> [a] convertPairsToRecords storg updateRecF parsePairF initRec = stList . processWithPairs where processWithPairs = foldl' ( (. parsePairF) . updateStorage ) storg updateStorage st Nothing = st updateStorage st ( Just (idx, sType, name, value) ) | stExists idx sType st = stAdjust idx sType (updateRecF name value) st | otherwise = stInsert idx sType (updateRecF name value initRec) st ================================================================================ so I want to provide methods for checking if a record with given key exists, update a record, insert a record and get list of records. Sounds similar as for Map, but I want also to be able to operate on map of maps, or lists, or whatever. I don't really see any problem with the code above, however GHC 6.12.1 does think that I am doing something weird. And it gives me this error log: ================================================================================ test.hs:19:60: Could not deduce (Storage k t a1 b) from the context (Storage k1 t1 a1 b) arising from a use of `stList' at test.hs:19:60-65 Possible fix: add (Storage k t a1 b) to the context of the type signature for `convertPairsToRecords' In the first argument of `(.)', namely `stList' In the expression: stList . processWithPairs In the definition of `convertPairsToRecords': convertPairsToRecords storg updateRecF parsePairF initRec = stList . processWithPairs where processWithPairs = foldl' ((. parsePairF) . updateStorage) storg updateStorage st Nothing = st updateStorage st (Just (idx, sType, name, value)) | stExists idx sType st = stAdjust idx sType (updateRecF name value) st | otherwise = stInsert idx sType (updateRecF name value initRec) st test.hs:21:53: Could not deduce (Storage k1 t1 a b) from the context (Storage k1 t1 a1 b) arising from a use of `updateStorage' at test.hs:21:53-65 Possible fix: add (Storage k1 t1 a b) to the context of the type signature for `convertPairsToRecords' In the second argument of `(.)', namely `updateStorage' In the first argument of `foldl'', namely `((. parsePairF) . updateStorage)' In the expression: foldl' ((. parsePairF) . updateStorage) storg ================================================================================ Can somebody please advice, what am I doing in wrong way? Thank you all in advance! -- Eugene Dzhurinsky

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Declaring a tuple of instances of Enums as an instance of the Enum class
by R J 26 May '10

26 May '10

Say I've got a type "Month" declared as an instance of the "Enum" class, and a type "MonthPair" declared as a pair of months: data Month = January | February | March | April | May | June | July | August | September | October | November | December deriving (Eq, Enum, Ord, Show) type MonthPair = (Month, Month) deriving (Enum) The "deriving" on "MonthPair" gives me the error "parse error on input 'deriving'". Why is this error generated? Is there a syntax error, or is there a conceptual problem with enumerating a Cartesian product, such as Month x Month? The cardinality of the Cartesian product is finite (including the bottom values, cardinality = 1 + (12 + 1)*(12 + 1) = 170), and so the product is amenable at least to some arbitrary enumeration (such as Cantor's diagonal method). Thanks. _________________________________________________________________ The New Busy think 9 to 5 is a cute idea. Combine multiple calendars with Hotmail. http://www.windowslive.com/campaign/thenewbusy?tile=multicalendar&ocid=PID2…

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Re: [Haskell-cafe] [ANNOUNCE] SyntaxMacros-0.1
by Oscar Finnsson 26 May '10

26 May '10

> We are pleased to announce the first release of SyntaxMacros [1]. Very interesting. This is something I've been interested in since I first heard of Ziggurat. In which order do you recommend I try to learn the packages: HList -> AspectAG -> SyntaxMacros? I tried to read the SyntaxMacros-paper but had a hard time understanding which parts belonged to the package and which parts belonged to your new mini-language. Do you got the mini-language as a separate download? You mention in the paper that the source of the paper is in literate Haskell and that the code is available but I can't find it at the wiki. I found some code (Expr.hs, Main.hs) in the examples-folder in the packages source code (SyntaxMacrox-0.1.tar.gz) but it doesn't seem to match 1:1 to the code in the paper (couldn't for example find extSpecChars in the paper or updateFinalEnv in the source code). -- Oscar

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Re: Cabal sqlite3 installation problems
by Ben 26 May '10

26 May '10

i have this exact problem, on linux x86_64, as well. b Hok Shun Poon fushunpoon at googlemail.com Sun May 23 19:51:20 EDT 2010 Previous message: memory useage of data types in the time package Next message: Text.Regex library with bytestrings? Messages sorted by: [ date ] [ thread ] [ subject ] [ author ] Hello, This is a question about how cabal-install finds 'installed foreign libraries' when trying to satisfy dependencies for packages like hdbc-sqlite3 on Windows. Sadly, the current setup means that I must use Windows 7 to develop code. I've already followed steps on http://wiki.github.com/jgoerzen/hdbc/frequentlyaskedquestions to place the .dll and the .h files in the correct places; it's just that cabal never seems to find it, despite being given --extra-lib-dirs= and --extra-include-dirs= directives. The message is as follows: > cabal install hdbc-sqlite3 --extra-lib-dirs=D:\dev\libs\database\sqlite Resolving dependencies... Configuring HDBC-sqlite3-2.3.0.0... cabal: Missing dependency on a foreign library: * Missing C library: sqlite3 This problem can usually be solved by installing the system package that provides this library (you may need the "-dev" version). If the library is already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where it is. cabal: Error: some packages failed to install: HDBC-sqlite3-2.3.0.0 failed during the configure step. The exception was: ExitFailure 1 The strangest thing is, I've successfully made cabal build this package before (with a different version of Haskell Platform)... what's going on? I'm using cabal-install version 0.8.2 and Cabal library version 1.8.0.2. Hok

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Haskell, Queries and Monad Comprehension
by Günther Schmidt 26 May '10

26 May '10

Hi all, is there anybody currently using Haskell to construct or implement a query language? I have read a couple of papers on "Monad Comprehension Calculus" and similar but none using Haskell nor any other existing programming language to build an actual implementation. Most papers give some sort of "Pseudo code", but I couldn't find any meat. Günther

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Re: [Haskell-cafe] Haskell, Queries and Monad Comprehension
by Günther Schmidt 26 May '10

26 May '10

Hello Torsten, well thank you for taking the time and answering that. It seems that the one thing I am good for, as far as contributions to this list go, is coaxing answers out of our Functional Pros. :) That is good news then, I was getting frustrated reading fantastic papers which eventually were nothing more than teasers. So, will Ferry then be usable from within Haskell? Best regards Günther Am 25.05.10 21:28, schrieb Torsten Grust: > Günther, > > we are currently underway building the second version (as in: > done right this time) of Ferry, a query compiler that > > (1) accepts queries over ordered, nested lists, > (2) compiles these queries into an intermediate algebraic form, then > (3) emits (small bundles of) SQL queries that evaluate the input > query over your off-the-shelf RDBMS. > > We've used Ferry's first version to build new LINQ to SQL providers > for Microsoft's LINQ as well as a new code generator for Phil Wadler's > Links. > > The Ferry compiler itself is built in Haskell. Surf to www.ferry-lang.org > for more information, screencasts, papers, talks, and contact us for > more details. We will be happy to share Ferry's Haskell code once in > digestable shape (soon). > > Cheers, > --Torsten > > On May 24, 2010, at 03:20 , Günther Schmidt wrote: >> Hi all, >> >> is there anybody currently using Haskell to construct or implement a query language? >> >> I have read a couple of papers on "Monad Comprehension Calculus" and similar but none using Haskell nor any other existing programming language to build an actual implementation. >> >> Most papers give some sort of "Pseudo code", but I couldn't find any meat. >> >> >> >> Günther >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> > >

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Exception: : changeWorkingDirectory: does not exist (No such file or directory)
by Anatoly Yakovenko 25 May '10

25 May '10

anyone else seeing this behavior? anatolyy@anatolyy-linux ~ $ ghci GHCi, version 6.12.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. > 1/2 0.5 *** Exception: : changeWorkingDirectory: does not exist (No such file or directory) > 1/2 *** Exception: : changeWorkingDirectory: does not exist (No such file or directory) >

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TDD in Haskell
by Ionut G. Stan 25 May '10

25 May '10

Hi, I'm doing TDD in pretty much all of the languages that I know, and I want to introduce it early in my Haskell learning process. I wonder though, if there's some established process regarding TDD, not unit testing. I've heard of QuickCheck and HUnit, but I've also read that QuickCheck is used mostly for testing pure code, while HUnit for impure code? What framework lends itself better for writing tests before the actual production code? Can you point out to some resources regarding this? Oh, and a small off-topic question? Is it considered a good practice to use implicit imports in Haskell? I'm trying to learn from existing packages, but all those "import all" statements drive me crazy. Thanks, -- Ionuț G. Stan | http://igstan.ro

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