June 2011 - Haskell-Cafe

Parsing Haskell in Parsec
by Alex Rozenshteyn 16 Jun '11

16 Jun '11

I'm trying to write a parser for a small functional language in Parsec, as part of a larger project of porting this<http://www.cs.jhu.edu/%7Escott/pl/book/dist/>from ocaml to Haskell. I was wondering if there was a parser for Haskell written in Parsec that I could use as a reference. -- Alex R

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Build failure on Hackage, missing syb
by José Pedro Magalhães 16 Jun '11

16 Jun '11

Hi all, What causes the following build failure on Hackage? *** setup configure > > Configuring instant-generics-0.3.2... > cabal-setup: At least the following dependencies are missing: > syb <0.4 > This is in package instant-generics-0.3.2<http://hackage.haskell.org/packages/archive/instant-generics/0.3.2/logs/fai…>. The syb package built fine on Hackage. Previously, packages depending on syb built fine too (e.g. http://hackage.haskell.org/package/syz) So why is instant-generics-0.3.2 not building? Thanks, Pedro

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Beginners Question: Problem with Data Type Declaration
by kaffeepause73 16 Jun '11

16 Jun '11

I try to create an own data type containing "Vector Double" from the H-Matrix package. The code: ## data PowerSig = PowerSig Int Double Vector Double main = do let p5 = PowerSig 1 0.1 (fromList [1,2,3]) ## When compiling with ghci, I get however the following message: `Vector' is not applied to enough type arguments Expected kind `?', but `Vector' has kind `* -> *' In the type `Vector' In the definition of data constructor `PowerSig' In the data type declaration for `PowerSig' I don't understand the compilers message and couldn't find any wiki-page/other material to help me. The Data Declaration somehow seems not to understand that "Vector Double" belongs together, but brackets don't help either. Cheers Phil -- View this message in context: http://haskell.1045720.n5.nabble.com/Beginners-Question-Problem-with-Data-T… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Haskell Weekly News: Issue 186
by Daniel Santa Cruz 16 Jun '11

16 Jun '11

Glad you are reading this issue! This is issue 186, not 180... Seems that the last couple of issues got stuck on 180. Hopefully you will find pointers to interesting things happening in the Haskell community. This issue covers the week of June 05 to 11, 2011. Announcements Hakim Cassimally shared with us slides and screencast from Geekup's first ever "Functional Programming Night." http://goo.gl/1oCO8 Johannes Waldmann sent an invite to a Local Haskell Meeting (Stammtisch) in Leipzig, Germany on June 27. http://goo.gl/UM9ci We are very proud of the two Simons, Simon Peyton Jones and Simon Marlow. Isaac Potoczny-Jones relayed the announcement from ACM SIGPLAN of the SIGPLAN Programming Language Software Award. Many congrats! http://goo.gl/sAuVv Quotes of the Week * augustss: So JHC has bad code. GHC had a lot of that a long time ago, and they have slowly been converting to the pure way. It's the only sensible way. * <geheimdienst> monochrom: i thought it maybe would end executing the current do block ... <monochrom> use "when". <mm_freak_> use "goto" <hpc> use Java * cmccann: programming is a lot easier if you avoid working with dates/times, text, or numbers. I say stick to category theory, it's simpler * xplat: the underlying graph of a category is transitive. transitive graphs have no bridges. this is why trolls always have problems with category theory. Top Reddit Stories * SIGPLAN Programming Languages Software Award, to Simon Peyton Jones and Simon Marlow for GHC Domain: permalink.gmane.org, Score: 115, Comments: 12 On Reddit: http://goo.gl/d1yww Original: http://goo.gl/ym5q9 * Haskell gets a new super-fast hash table library Domain: gregorycollins.net, Score: 80, Comments: 20 On Reddit: http://goo.gl/gNcRa Original: http://goo.gl/O6oc6 * Pinpointing space leaks in big programs : Inside T5 Domain: blog.ezyang.com, Score: 47, Comments: 4 On Reddit: http://goo.gl/T3ZVs Original: http://goo.gl/4rL1y * iPad app that is a lisp interpreter written in haskell by David Pollak, creator of the Scala web framework Lift. Domain: github.com, Score: 37, Comments: 16 On Reddit: http://goo.gl/eXf03 Original: http://goo.gl/c859K * Memory footprints of some common data types Domain: blog.johantibell.com, Score: 36, Comments: 23 On Reddit: http://goo.gl/bjvFC Original: http://goo.gl/QVrIN * Gilad Bracha: Types are Anti-Modular Domain: gbracha.blogspot.com, Score: 22, Comments: 22 On Reddit: http://goo.gl/rnbeF Original: http://goo.gl/44kCd * Biggest / most popular / most trafficked web app written in Haskell? Janrain, TypLAB, or ... ? Domain: reddit.com, Score: 19, Comments: 15 On Reddit: http://goo.gl/eMn8Y Original: http://goo.gl/hUzFc * Three Haskell web frameworks Domain: blogs.linux.ie, Score: 17, Comments: 3 On Reddit: http://goo.gl/5HGd6 Original: http://goo.gl/oR6Zu * Parallel GHC project: new opportunity for an organisation to participate Domain: well-typed.com, Score: 17, Comments: 0 On Reddit: http://goo.gl/rppqC Original: http://goo.gl/CB4ZG * Building Ajax Sites With Snap - Imagining Heist-Async Domain: blog.dbpatterson.com, Score: 17, Comments: 0 On Reddit: http://goo.gl/o8jTF Original: http://goo.gl/j1y2o Top StackOverflow Questions * why write type declarations in Haskell? votes: 11, answers: 4 Read on SO: http://goo.gl/SqM4y * Creating functions over Enumerations votes: 11, answers: 5 Read on SO: http://goo.gl/0lguE * Understanding a recursively defined list (fibs in terms of zipWith) votes: 10, answers: 3 Read on SO: http://goo.gl/kbVtC * Speeding up SHA256 in Haskell votes: 10, answers: 1 Read on SO: http://goo.gl/whGhV * Poor performance with transpose and cumulative sum in Repa votes: 10, answers: 1 Read on SO: http://goo.gl/abxuD * Would you mind to explain the code in the forum ? votes: 9, answers: 1 Read on SO: http://goo.gl/ftoI3 About the Haskell Weekly News To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. Until next time, Daniel Santa Cruz

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Fwd: [iteratee] how to do nothing .. properly
by Sergey Mironov 15 Jun '11

15 Jun '11

Forgot reply to all ---------- Forwarded message ---------- From: Sergey Mironov <ierton(a)gmail.com> Date: 2011/6/16 Subject: Re: [Haskell-cafe] [iteratee] how to do nothing .. properly To: John Lato <jwlato(a)gmail.com> Thanks for explanations! /ErrorT String (Iteratee s m) a/ definitely does the work. But wrapping Enumeratee is harder since /ErrorT String (Enumeratee s s' m) a/ doesn't compiles. Maybe most users actually don't need to wrap Enumeratee, will check it. Heh, throwRecoverableErr name tells me "use me, I am all you need":) Well, one just need to grok the interface.. Anyway, iteratees are much deeper and powerful than I expected when switched to it from Data.Binary. I'll better open new threads for asking more questions. Again thanks, Sergey PS sorry for delayed answer, had vacation without email access 2011/6/5 John Lato <jwlato(a)gmail.com>: > Yes, this is expected. 'throwErr' is only meant to be used when the error > should be non-recoverable, and the stream would often be invalid then, so > throwErr doesn't take any steps to preserve it. You could retain the rest > of the stream with getChunk and use throwRecoverableErr though. > > Wrapping an iteratee with ErrorT is fine, and I use this approach often. I > typically would use an explicit Either rather than ErrorT, but the two > approaches are exactly the same. > > Note that wrapping the other way, Iteratee s (ErrorT e m), can sometimes > cause problems, and is best avoided unless you really know what you're > doing. This may change in a future release. > > John > > On Thu, Jun 2, 2011 at 4:27 PM, Sergey Mironov <ierton(a)gmail.com> wrote: >> >> I am glad to help! Looks like upgrading to 0.8.5.0 also fixes initial >> problem that involved me into testing! >> >> I'll take the opportunity and ask another thing about iteratee: Is it >> expected behavior that throwErr consumes all data in current chunk? I >> wish it to stop in place and let after-checkErr code to continue the >> parsing. Well, I already found solution (or workaround?) - I wrap >> Iteratee with ErrorT monad and use ErrorT's raiseError instead of >> throwErr. Is it correct? >> >> Here is example code >> >> instance Exception Int >> >> iter4 = do >> I.dropWhile (/= 3) >> h<-I.head >> throwErr $ toException $ (-4::Int) -- doesn't meter what exactly to >> throw >> return h >> >> -- catch the error with checkErr >> iter5 = do >> (_,b)<-countBytes $ I.checkErr $ iter4 >> s <- I.stream2list >> return (b,s) >> >> print5 = enumPure1Chunk [1..10] (iter5) >>= run >>= print >> >> >> Thanks a lot! >> Sergey >> >> 2011/6/2 John Lato <jwlato(a)gmail.com>: >> > Hi Sergey, >> > >> > I've got an explanation; quite surprisingly it's a bug in >> > enumPure1Chunk. >> > Even though it is an odd case, I'm surprised that it hasn't come up >> > before >> > now since enumPure1Chunk appears frequently. >> > >> > I've just uploaded 0.8.5.0 which has the fix. There's now an additional >> > Monoid constraint on enumPure1Chunk, unfortunately. >> > >> > Thanks very much for reporting this. >> > >> > John L >> > >> > On Thu, Jun 2, 2011 at 10:02 AM, Sergey Mironov <ierton(a)gmail.com> >> > wrote: >> >> >> >> Ok. I've checked iteratee-0.8.3.0 and 0.8.4.0. Results are same. >> >> >> >> Sergey >> >> >> >> 2011/6/2 John Lato <jwlato(a)gmail.com>: >> >> > Hi Sergey, >> >> > I can't explain this; maybe it's a bug in enumWith? I'll look into >> >> > it. >> >> > Thanks, >> >> > John >> >> > >> >> >> >> >> >> Message: 20 >> >> >> >> >> >> Date: Thu, 2 Jun 2011 02:46:32 +0400 >> >> >> From: Sergey Mironov <ierton(a)gmail.com> >> >> >> Subject: [Haskell-cafe] [iteratee] how to do nothing .. properly >> >> >> To: haskell-cafe(a)haskell.org >> >> >> Message-ID: <BANLkTimMFRWgH9Nopt-eua+L7jQcGq+u=g(a)mail.gmail.com> >> >> >> Content-Type: text/plain; charset=ISO-8859-1 >> >> >> >> >> >> Hi. Would anybody explain a situation with iter6 and iter7 below? >> >> >> Strange thing - first one consumes no intput, while second consumes >> >> >> it >> >> >> all, while all the difference is peek which should do no processing >> >> >> (just copy next item in stream and return to user). >> >> >> What I am trying to do - is to write an iteratee consuing no input, >> >> >> but returning a constant I give to it. I thought (return a) should >> >> >> do >> >> >> it, but it seems I was wrong as return actually consumes all >> >> >> unparsed >> >> >> stream. iter6 experience tells me that (peek>>return a) is what I >> >> >> need, but it's completely confusing and not what I expected. >> >> >> >> >> >> Thanks, >> >> >> Sergey >> >> >> >> >> >> import Data.Iteratee as I >> >> >> import Data.Iteratee.IO >> >> >> import Control.Monad >> >> >> import Control.Exception >> >> >> import Data.ByteString >> >> >> import Data.Char >> >> >> import Data.String >> >> >> >> >> >> -- countBytes :: (..., Num b) => Iteratee s m a -> Iteratee s m (a, >> >> >> b) >> >> >> countBytes i = enumWith i I.length >> >> >> >> >> >> iter6 = do >> >> >> h <- countBytes $ (peek >> return 0) >> >> >> s <- I.stream2list >> >> >> return (h,s) >> >> >> >> >> >> iter7 = do >> >> >> h <- countBytes $ (return 0) >> >> >> s <- I.stream2list >> >> >> return (h,s) >> >> >> >> >> >> print6 = enumPure1Chunk [1..10] (iter6) >>= run >>= print >> >> >> print7 = enumPure1Chunk [1..10] (iter7) >>= run >>= print >> >> >> >> >> >> >> >> >> Here is example ghci session >> >> >> >> >> >> *Main> print6 >> >> >> ((0,0),[1,2,3,4,5,6,7,8,9,10]) >> >> >> -- read 0 items, returns 0 >> >> >> *Main> print7 >> >> >> ((0,10),[]) >> >> >> -- read 10 items (???) returns 0 >> >> >> *Main> >> >> >> >> >> >> >> >> >> >> >> >> ------------------------------ >> >> >> >> >> >> _______________________________________________ >> >> >> Haskell-Cafe mailing list >> >> >> Haskell-Cafe(a)haskell.org >> >> >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >> >> >> >> >> >> >> >> End of Haskell-Cafe Digest, Vol 94, Issue 3 >> >> >> ******************************************* >> >> > >> >> > >> > >> > > >

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Simple string tokenizer?
by Dmitri O.Kondratiev 15 Jun '11

15 Jun '11

Sorry if this question was asked billions of times already, but I can not find a simple string tokenizer. All I need is to split a line in chunks at specified delimiter such as (,), nothing more. Don't want to write it myself for two reasons: 1) have to write lots of other code in very short time 2) don't want undermine idea of code reuse ) Where to get latest Data.String.Utils library that I saw somewhere on the web and that has a nice and simple 'split' function? Thanks!

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wxHaskell for Haskell Platform 2011.2.0.1
by Eric Kow 15 Jun '11

15 Jun '11

Hi all, With green light from maintainer Jeremy O'Donoghue, I've uploaded a trivial update to wxHaskell, relaxing the dependencies so that it builds with the latest Haskell Platform and does not try to install other versions of packages. What is wxHaskell? ---------------------------------------------------------------------- wxHaskell is a Haskell library for writing GUIs that work on multiple platforms. It provides a reasonable compromise between getting a native user interface and saving effort. It uses the wxWidgets library, which in turn wraps around the native GUI toolkit for the target platform and tries to follow platform-specific customs and conventions so that you don't have to worry about them. Here are the sort of results the minor details wxWidgets seems to get right on Mac OS X: - system preferences are under the Preferences menu for the application - file open/save and print dialogues are the actual ones used by Mac applications instead of some half-baked Aqua coloured monstrosity It may not be perfect, but it's not bad considering you don't even have to think about it. wxHaskell is available under a relaxed LGPL license, which basically means you can release both open sourced and closed source with it. About this release ---------------------------------------------------------------------- This is just a minor release bumping some dependencies. Installation comes in two basic steps: 1. Install wxWidgets 2.8 - On MacOS X, Homebrew works well for me (if you update to get patches to build wxWidgets with Unicode enabled). - On Ubuntu/Debian, I use apt-get wxgtk. - I have not personally tried it on Windows, but I've hear reports that it's easy to install there as well. 2. cabal install wx See http://www.haskell.org/haskellwiki/WxHaskell for more information What's next? ---------------------------------------------------------------------- Jeremy has been doing quite a lot of work on wxHaskell. Among other things [1], he's been: - updating it for what will eventually be wxWidgets 3.0 - trying to resolve issues restarting the UI from GHCi - improved Event handling (for Mac users, the upshot is that wxHaskell would work with the wxWidgets that ships with OS X, meaning "cabal install wx" from the HP would "just work") There are also mutterings about pushing for inclusion in the Haskell Platform, but there is still a lot of work of to do before we get there [2]. Help wanted :-) Enjoy! Eric [1] http://thread.gmane.org/gmane.comp.lang.haskell.wxhaskell.devel/612 [2] http://thread.gmane.org/gmane.comp.lang.haskell.wxhaskell.devel/616 -- Eric Kow <http://erickow.com>

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Re: [Haskell-cafe] Simple string tokenizer?
by Thedward Blevins 15 Jun '11

15 Jun '11

On Jun 15, 2011 11:22 AM, "Dmitri O.Kondratiev" <dokondr(a)gmail.com> wrote: > > Sorry if this question was asked billions of times already, but I can not find a simple string tokenizer. > All I need is to split a line in chunks at specified delimiter such as (,), nothing more. Don't want to write it myself for two reasons: 1) have to write lots of other code in very short time 2) don't want undermine idea of code reuse ) > Sounds like you want `Data.List.Split.splitOn` from the 'split' package ( http://hackage.haskell.org/package/split ).

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Please help : Data.Time.Format parseTime
by Dmitri O.Kondratiev 15 Jun '11

15 Jun '11

I am trying to convert data string to time: import Data.Time import Data.Time.Format import Locale ds = "10/11/2009 7:04:28 PM" t = parseTime defaultTimeLocale "%D %H:%M:%S %p" ds :: Maybe UTCTime and get "Nothing". What is wrong? Thanks ! Dmitri.

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GHC 7.0.3 / Win32: Data.Time library?
by Dmitri O.Kondratiev 15 Jun '11

15 Jun '11

It looks like GHC, 7.0.3 "Haskell Hierarchical Libraries" documentation generated for Win32, that goes with Haskell Platform installation package, does not have a section on Data.Time module. How can that be? I need to convert a string of the form ",10/11/2009 7:04:28 PM" to Haskell type that can be used for comparing and abstracting dates. What is the simplest way to day that? Thanks!

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