- Haskell-Cafe - Haskell.org

A question about monad based on http://en.wikipedia.org/wiki/Monad_(category_theory)
by C K Kashyap 20 Jan '11

20 Jan '11

Hi, I was going through http://en.wikipedia.org/wiki/Monad_(category_theory) - under Formal Definition I notice that monad is a Functor T:C -> C My question is - when we think of Maybe as a functor T:C -> C .... should we think that C here refers to Hakell types? As in, (Int and Maybe Int are objects in C) and (Int -> Int and Maybe Int -> Maybe Int are arrows in C) and T is an arrow between them. Is that right? For some reason, I was imagining them as two different categories C and D. I am not able to fully understand how those diagrams translate to haskell - I can guess that T^2 -> T referes to things like concat operation but not able to relate it to bind. Regards, Kashyap

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Re: [Haskell-cafe] Building lambdabot
by Gwern Branwen 20 Jan '11

20 Jan '11

On Thu, Jan 20, 2011 at 3:24 AM, Max Bolingbroke <batterseapower(a)hotmail.com> wrote: > On 20 January 2011 01:51, Gwern Branwen <gwern0(a)gmail.com> wrote: >> It had a lot of issues which meant it wouldn't build anywhere, where >> at least the Hackage version worked at some point. I spent this >> evening working on fixing issues (rewriting the show package to use >> QuickCheck 2 rather than QuickCheck 1 was not fun!) and have uploaded >> a lambdabot and show that compile for me on GHC 6.12.1 > > The show package almost compiles on GHC 7, but: > > * You need to loosen the base upper bound to < 4.4 > * If using base >= 4, you need to depend on the syb package as well > (current version 0.3) Would this break GHC 6.12 builds? Since I do not have GHC 7 installed and, given the breakage I experienced when I installed it the other day and tried to cabal-install my usual tools, will not have it installed any time soon, I am leery of any GHC 7-related changes. data-memocombinators is only the tip of the iceberg; I believe much of lambdabot would need modifications. (Apparently Control.OldException has gone away, which alone guarantees many changes.) So there wouldn't be much point to changing show. -- gwern http://www.gwern.net

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sequence and sequence_ for iteratees
by Maciej Wos 20 Jan '11

20 Jan '11

Hello Cafe! I have written some iteratee functions that I found to be very very useful and I hope they will soon make it to the iteratee library. However, I'd like to get some feedback first, particularly about the error handling. I'm sending this here rather than to iteratee mailing list at projects.haskell.org because the latter is (and has been for a while!) down... Anyway, enumSequence and enumSequence_ are inspired by Prelude's sequence and sequence_. They are useful when one has to deal with several iteratees consuming the same data. For instance, one could use enumSequence as follows: > run $ joinIM $ enumPureNChunk [1..100] 3 $ enumSequence [I.head, I.head >>= \x -> I.head >>= \y -> return (x+y), I.last] which produces: [1,3,100] Each iteratee in the list is given the same input stream. Also, enumSequence consumes as much of the stream as the iteratee in the list that consumes the most. In the above example there is no stream left after enumSequence finishes because I.last consumes everything. As an another example, enumSequence below consumes only the first two elements of the stream and the remainder is passed to stream2list: > run $ joinIM $ enumPureNChunk [1..10] 3 $ (enumSequence [I.head, I.head >> I.head] >> stream2list) [3,4,5,6,7,8,9,10] The code for enumSequence is enclosed below (enumSequence_ is almost identical!) To make it complete though I should add some sort of error handling. I'm not quite sure however what would be the best thing to do. For instance, what should happen if the stream is finished, but one of the iteratees is not done yet? Should the whole enumSequence fail? Similarly, should the whole enumSequence fail if one of the iteratees throws an error? I guess throwing some sort of recoverable error could work. But I still need to figure out how to do that! -- Maciej ########## code ########## enumSequence :: forall m s a el . (Monad m, LL.ListLike s el, Nullable s) => [Iteratee s m a] -> Iteratee s m [a] enumSequence is = liftI step where step :: Stream s -> Iteratee s m [a] step s@(Chunk xs) | LL.null xs = liftI step | otherwise = do let is' = map (joinIM . enumPure1Chunk xs) is allDone <- lift (checkIfDone is') if allDone then uncurry idone =<< lift (collectResults is') else enumSequence (updateChunk s is') -- TODO: should return an error if not all iteratees are done step (EOF _) = uncurry idone =<< lift (collectResults . map (joinIM . enumEof) $ is) -- returns true if *all* iteratees are done; otherwise returns false checkIfDone :: [Iteratee s m a] -> m Bool checkIfDone = liftM and . mapM (\i -> runIter i onDone onCont) where onDone _ _ = return True onCont _ _ = return False -- returns a list of result values and the unconsumed part of the stream collectResults :: [Iteratee s m a] -> m ([a], Stream s) collectResults = liftM (id *** foldl1 shortest) . mapAndUnzipM (\i -> runIter i onDone onCont) where onDone a s = return (a,s) onCont _ _ = error "enumSequence: collectResults failed; all iteratees should be done" shortest :: Stream s -> Stream s -> Stream s shortest (Chunk xs) (Chunk ys) | LL.length xs > LL.length ys = Chunk ys | otherwise = Chunk xs shortest s@(EOF _) _ = s shortest _ s@(EOF _) = s -- iteratee in *done* state holds the unconsumed part of the chunk it -- was given; this chunk needs to be discarded when we move further in -- the stream updateChunk :: Stream s -> [Iteratee s m a] -> [Iteratee s m a] updateChunk s = map (\i -> joinIM $ runIter i (\a _ -> return $ idone a s) icontM)

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Wondering about networking programming in Haskell.
by Magicloud Magiclouds 20 Jan '11

20 Jan '11

Hi, Originally, my idea for networking programming is like: read some bytes, judge if this is enough for a packet (defined in certain protocol), if not, read more until the packet is complete, then parse it into a message. Now we have lazy in Haskell. Can I do it as: buffer <- hGetContents handle (message1, leftData1) <- parse1 buffer (message2, leftData2) <- parse2 leftData2 ... -- 竹密岂妨流水过山高哪阻野云飞

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Formal Program verification in Meta-Haskell
by Alexander McPhail 20 Jan '11

20 Jan '11

Hi, I have no understanding of template haskell and thus this message is uninformed speculation. Would it not be possible to write a function verifier in TH that, unlike QuickCheck which provides bounds on a function being probably approximately correct, is given a list over properties which are proven over a quasi-quoted syntax tree? For example: > multiply :: Num a => a -> a -> a > multiply_group :: Proof Bool > multiply_group = group multiply 1 > group :: (a -> a -> a) -> a -> Proof Bool > group f ident = [proof| f, [associative, identity 1]] where `associative` and `ident` are proof-constructing functions that act over the `f` syntax tree. (Maybe some invokation of an Isabelle-like theorem prover or an SMT-like solver as part of the 'proof' module). Then we might even be able to list properties with class declarations. The properties get verified at compile time. > class Monoid a where > mzero :: a > msum :: a -> a -> a > property a [associative msum, identity mzero msum] Any thoughts? Vivian

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Call for Submissions: Special Poetry and Fiction Edition of the Monad.Reader
by Brent Yorgey 20 Jan '11

20 Jan '11

Submissions are now being accepted for a special POETRY AND FICTION edition of the Monad.Reader. Poems about Haskell, short stories about Haskell, poems about poems about Haskell, elaborate monad tutorial allegories, Haskell/Dinosaur Comics crossovers: all fair game! If you want a bit of inspiration, check out the links at the end of this message [1,2,3]. But you are a creative bunch, I'm sure you can come up with all sorts of fun ideas and formats. The submission deadline will be **Friday, February 18** but feel free to send submissions at any time between now and then. Questions That May Be Asked Frequently (QTMBAF) ----------------------------------------------- Q. Are you serious? A. Absolutely. Q. The deadline is only four weeks away! It will take me at least five weeks to write my incredible masterpiece! A. Are you serious? Q. Will you take every submission or will there be some sort of selection process? A. I hope to publish every submission, although not without going through a process of editing and suggested revisions. Ultimately, it depends on the number of submissions. Q. Can I submit the same poem multiple times to increase my chances? A. I have written a sophisticated, 5000-line Haskell program to detect this and alter your submission in a subtle yet highly embarrassing (for you) way. Q. Seriously, this is a dumb idea. A. Please try to phrase your question in the form of a question. Q. Seriously, is this a dumb idea? A. No, I'm pretty sure it is awesome. Submission Details ------------------ Submissions should be sent to (byorgey at cis.upenn.edu) Simple text submissions are fine for short submissions such as poetry. For longer submissions, LaTeX is appreciated. See the guidelines here to get started: http://themonadreader.wordpress.com/contributing/ If submitting in LaTeX is a hardship, text submissions will be accepted. Submissions in Microsoft Word format will be deleted and then overwritten fifty times with random bits. [1] http://haskell.org/haskellwiki/Humor/Poetry [2] http://haskell.org/haskellwiki/Haiku [3] http://www.cis.upenn.edu/~byorgey/haskell-alphabet/index.html

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Haskell Weekly News: Issue 165 - January 19, 2011
by Daniel Santa Cruz 20 Jan '11

20 Jan '11

Welcome to issue 165 of the HWN, a newsletter covering developments in the [1]Haskell community. This release covers the week of January 9 to 15, 2011. Announcements Brent Yorgey [2]announced the release of issue 17 of The Monad.Reader, containing the following three articles: List Leads Off with the Letter Lambda by Douglas M. Auclair, The InterleaveT Abstraction: Alternative with Flexible Ordering by Neil Brown, and The Reader Monad and Abstraction Elimination by Petr Pudlak. Andy Stewart [3]sent an invite to anyone interested in joining the Manatee team. Manatee is a Haskell integrated environment written in Haskell. Follow the link to see some videos posted for a better idea of what the tool is. Niklas Broberg [4]announced the release of version 1.10.1 of haskell-src-extensions. Huibiao Zhu [5]sent a call for papers to the 13th International Conference on Formal Engineering Methods to be held in Durham, UK on October 25-28 2011. Read on to see what was hot in the Haskell community around the net... Quotes of the Week * ion: ddarius: Yeah, abstract concepts absolutely hate being anthropomorphized. * gwern: there are no beginnings or ends to the circular list; but a cons cell thunked in Amador... * tac-tics: If you wish to create the universe from scratch, you must first invent the continuation. * sproingie: hm. Â i can't remember how to use @remember * NOTE: Just say @remember :) I'm grepping the logs to find these things * Kaidelong: <Kaidelong> take 6 "bananas" <lambdabot> "banana" <Kaidelong> clearly lambdabot is keeping the bananas to itself and lying about it * hpc: functor and applicative are easy, since you can remember them as "those things what look like function application, sort of" Top Reddit Stories * RFC: migrating GHC development to git Domain: haskell.org, Score: 51, Comments: 66 On Reddit: http://www.reddit.com/r/haskell/comments/ezgvs/rfc_migrating_ghc_developmen… Original: http://www.haskell.org/pipermail/glasgow-haskell-users/2011-January/019752.… * [CCC] On how Haskell’s Arrows are NOT just function compositions with a fancier name Domain: ro-che.info, Score: 38, Comments: 4 On Reddit: http://www.reddit.com/r/haskell/comments/f3e06/ccc_on_how_haskells_arrows_a… Original: http://ro-che.info/ccc/12.html * foldl vs foldl', and why you should always use the latter Domain: reddit.com, Score: 35, Comments: 8 On Reddit: http://www.reddit.com/r/haskell/comments/f25iv/foldl_vs_foldl_and_why_you_s… Original: http://www.reddit.com/r/haskell/comments/f150x/whats_wrong_with_my_haskell_… * ~Haskell 2011 Domain: haskell.org, Score: 32, Comments: 5 On Reddit: http://www.reddit.com/r/haskell/comments/ezmzh/haskell_2011/ Original: http://www.haskell.org/pipermail/haskell/2011-January/022497.html * The Monad.Reader: Issue 17 Domain: themonadreader.wordpress.com, Score: 30, Comments: 10 On Reddit: http://www.reddit.com/r/haskell/comments/ezft9/the_monadreader_issue_17/ Original: http://themonadreader.wordpress.com/2011/01/09/issue-17/ * Painless NP-complete problems: an embedded DSL for SMT solving Domain: donsbot.wordpress.com, Score: 27, Comments: 2 On Reddit: http://www.reddit.com/r/haskell/comments/f3fp6/painless_npcomplete_problems… Original: http://donsbot.wordpress.com/2011/01/16/painless-np-complete-problems-an-em… * Oregon Programming Languages Summer School: Types, Semantics, and Verification Domain: lists.seas.upenn.edu, Score: 22, Comments: 20 On Reddit: http://www.reddit.com/r/haskell/comments/f15rm/oregon_programming_languages… Original: http://lists.seas.upenn.edu/pipermail/types-announce/2011/002241.html * Announcing Web Application Interface 0.3.0 (now with more speed) Domain: docs.yesodweb.com, Score: 20, Comments: 2 On Reddit: http://www.reddit.com/r/haskell/comments/f0e6f/announcing_web_application_i… Original: http://docs.yesodweb.com/blog/announcing-wai-0-3/ * A bran new functional programming (Haskell inter alia) jobs site (corrected link) Domain: functionaljobs.com, Score: 19, Comments: 11 On Reddit: http://www.reddit.com/r/haskell/comments/ezqst/a_bran_new_functional_progra… Original: http://functionaljobs.com/ * [PDF] Orthogonal Serialization For Haskell Domain: mathematik.uni-marburg.de, Score: 17, Comments: 3 On Reddit: http://www.reddit.com/r/haskell/comments/f1o8t/pdf_orthogonal_serialization… Original: http://www.mathematik.uni-marburg.de/~berthold/papers/mainIFL10-withCopyrig… Top StackOverflow Questions * Monads as adjunctions votes: 15, answers: 3 Read on SO: http://stackoverflow.com/questions/4697320/monads-as-adjunctions * What is a VM and why do dynamic languages need one? votes: 11, answers: 8 Read on SO: http://stackoverflow.com/questions/4640809/what-is-a-vm-and-why-do-dynamic-… * Is Haskell a Lisp? votes: 9, answers: 5 Read on SO: http://stackoverflow.com/questions/4696618/is-haskell-a-lisp * Are there any connections between Haskell and LINQ? votes: 8, answers: 4 Read on SO: http://stackoverflow.com/questions/4683506/are-there-any-connections-betwee… * Haskell newbie on types votes: 7, answers: 4 Read on SO: http://stackoverflow.com/questions/4664429/haskell-newbie-on-types * I can't seem to figure out type variables mixed with classes. votes: 5, answers: 2 Read on SO: http://stackoverflow.com/questions/4653102/i-cant-seem-to-figure-out-type-v… * Emacs align-regexp on = but not == votes: 5, answers: 1 Read on SO: http://stackoverflow.com/questions/4653575/emacs-align-regexp-on-but-not * Current status of automatic parallelism in Haskell [closed] votes: 5, answers: 3 Read on SO: http://stackoverflow.com/questions/4701207/current-status-of-automatic-para… About the Haskell Weekly News You can find an html version of this newsletter at: http://contemplatecode.blogspot.com/2011/01/haskell-weekly-news-issue-165.h… To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. I'm in dire need of finding good "quotes of the week". If you happen to come across any, please don't hesitate to send it along. Until next time, Daniel Santa Cruz References 1. http://haskell.org/ 2. http://permalink.gmane.org/gmane.comp.lang.haskell.general/18419 3. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/85224 4. http://permalink.gmane.org/gmane.comp.lang.haskell.general/18427 5. http://permalink.gmane.org/gmane.comp.lang.haskell.general/18428

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Building lambdabot
by Joe Bruce 20 Jan '11

20 Jan '11

I've had a bit of an adventure trying to build and run lambdabot<http://hackage.haskell.org/package/lambdabot-4.2.2.1> on my box. 'cabal install lambdabot' does not work. It states it's not GHC 6.12 (and certainly not 7.0) compatible, but I tried 6.12 anyway and got nowhere. Next, I tried 6.10 but failed on the unix dependency (in the same manner as this thread<http://web.archiveorange.com/archive/v/nDNOvQMXUGwfkyy0ITet>). The thread is unresolved, so I've not tried modifying the cabal build-depends. I'm hoping to find someone who has a working lambdabot setup who can point me in the right direction. What further information about my struggles should I provide? Thanks, Joe

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Community.haskell.org is down
by Neil Mitchell 19 Jan '11

19 Jan '11

http://www.downforeveryoneorjustme.com/http://community.haskell.org Currently community is down for me. I remember some infrastructure/website strike team was set up, but I couldn't find where to get in contact with them? Thanks, Neil

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planet@community.haskell.org and Mailman Images
by Jon Kristensen 19 Jan '11

19 Jan '11

Just two small comments: The e-mail address planet(a)community.haskell.org mentioned at http://planet.haskell.org/ bounces ("connect to community.haskell.org[72.249.126.23]: Connection timed out"). Also, registering on this mailing list to report the problem, I realized that the mailman images at pages such as http://www.haskell.org/mailman/subscribe/haskell-cafe are broken too. Something you might want to take a look at! Warm regards, Jon Kristensen

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