- Haskell-Cafe - Haskell.org

A list library without fixpoints and recursive types.
by flicky frans 21 Apr '14

21 Apr '14

Hello. I read some Oleg Kiselyov's papers and tried to write more or less efficient list library with lists represented as their right fold, as mentioned here http://okmij.org/ftp/Haskell/zip-folds.lhs : It would be interesting to see, constructively, how to build the full-fledged (FR)-list library and avoid general recursion. Let the FR-list be the only driver of iterations. Here is the code: http://ideone.com/NKNeWy I don't post it to hackage, since I'm not sure, these lists are really efficient. There are two main ideas: 1) List is represented as an accumulating right fold, and an accumulator passes from left to right. 2) For efficiency purposes elements skipping is incorporated into the cons (($:) in the code) function. So there is only one isSkipped test (d == 0 in the code) for every element, no matter, how many times drop (or tail or dropWhile) was applied. So there are no redundant (<= 0) checks, as described in the zip-folds paper: The function sdrop shows the major deficiency: we're stuck with the (<=0) test for the rest of the stream. In this case, some delimited continuation operators like `control' can help, in limited circumstances. Here are the pluses: 1) No fixpoints and recursive types. 2) These lists are totally isomorphic to ordinary haskell's, even in a strict monad. 3) No redundant (<= 0) checks. There are some not very big problems with this encoding: 1) Code becomes more complicated than with the ordinary right fold. 2) In fscanl and fsndMapAccumL functions skipped elements are traversed twice, because you need to traverse an accumulator through skipped elements too. And the same holds for the fdropWhile function with additional skipping overhead. 3) Since an accumulator passes from left to right, it's not possible to efficiently fuse fscanr and fmapAccumR functions, so they are defined in the terms of ($:) and ffoldr. And one big problem: zipWith and all other functions, that require parallel lists processing, are quadratic. And the same holds for the ftails function and all other functions, that deconstruct a list by repeatedly applying (fhead . ftail) or collect lists, obtained by repeteadly applied ftail function. However I have never seen any other representation, expressible in pure System F with higher-rank types, which allows such deconstruction in a linear time. If someone knows, give me a link, please. Any suggestions are welcome.

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Re: [Haskell-cafe] Syntax proposal for "reverse apply"/"pipeline apply" (flip ($))
by Doug McIlroy 20 Apr '14

20 Apr '14

> '$$' should be strongly considered That breaks time-honored practice in hugs and ghci. Doug McIlroy

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Announcing cabal 1.20
by Johan Tibell 20 Apr '14

20 Apr '14

Hi all, On behalf of all the cabal contributors, I'm proud to announce cabal 1.20. With 404 commits since the 1.18 release, this is a feature packed release. Here's a short summary of the perhaps most notable changes: * Dependency freezing * Parallel cabal build * Flag to temporary ignore upper bounds * Unnecessary re-linking avoidance * Streaming cabal test output * New cabal exec command * Haddock configuration options I did a longer write-up about the new features on my blog: http://blog.johantibell.com/2014/04/announcing-cabal-120.html ## Contributors Here are the contributors for this release, ordered by number of commits: * Mikhail Glushenkov * Johan Tibell * Duncan Coutts * Thomas Tuegel * Ian D. Bollinger * Ben Armston * Niklas Hambüchen * Daniel Trstenjak * Tuncer Ayaz * Herbert Valerio Riedel * Tillmann Rendel * Liyang HU * Dominic Steinitz * Brent Yorgey * Ricky Elrod * Geoff Nixon * Florian Hartwig * Bob Ippolito * Björn Peemöller * Wojciech Danilo * Sergei Trofimovich * Ryan Trinkle * Ryan Newton * Roman Cheplyaka * Peter Selinger * Niklas Haas * Nikita Karetnikov * Nick Smallbone * Mike Craig * Markus Pfeiffer * Luke Iannini * Luite Stegeman * John Lato * Jens Petersen * Jason Dagit * Gabor Pali * Daniel Velkov * Ben Millwood Apologies if someone was left out. Once in a while we have to make commits on behalf of an author. Those authors are not captured above. Enjoy, Johan

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Minimal Haskell Platform & Syntax proposal for "reverse apply"/"pipeline apply" (flip ($))
by Mike Meyer 19 Apr '14

19 Apr '14

On Fri, Apr 18, 2014 at 5:31 AM, Johan Tibell <johan.tibell(a)gmail.com>wrote: > We use this "don't reopen issues unless circumstances have changed" policy > for HP package proposals*. I think it's a good policy, as it saves valuable > community energy to discuss new things. If things have truly changed, we > can of course have the discussion again. > Where can I find that information? I went looking through the HP pages on Haskell.org, and couldn't turn up anything on contributions, or suggestions, or similar things. I found one link to a github repository, but it didn't have a README :-(. > * We took the policy from Python's PEP proposal policy. > While I'm relatively new to the Haskell community, I've been part of the Python community since I had to explain to my clients what Python was. One of the things that's always impressed me about that community is it's devotion to maintaining the zen of the language. I've watched it grow the official (as in "import this") zen, and the PEP process, and watched various PEPS go through the process. I've even written a few myself, though I only submitted one, which was rejected. Watching the discussion of the syntax proposal reminds me of watching some of the discussions about Python language changes - though if we have a BDFL, I haven't heard of them. That's also how discussions about the Python standard library looked (they're part of the PEP process now). If we want the HP to be generally useful - not just something to make beginners lives easier or provide hard-to-use tools - it needs to be dealt with with equal care. I'd like to contribute to that process. So, questions that I couldn't find answers to: Is there a Haskell equivalent to a Python PEP? Should there be? Is there a place to discus HP issues, other than here? If I want to contribute to the development of the HP, where should I start? Thanks, Mike

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Re: [Haskell-cafe] Syntax proposal for "reverse apply"/"pipeline apply" (flip ($))
by Hans Höglund 18 Apr '14

18 Apr '14

There is also is my humble attempt to standardize the (&) formulation: http://hackage.haskell.org/package/reverse-apply As you can see, these definitions now mirror those in 'lens' exactly. I see no reason why this definition should not move to base. IMHO, the Diagrams definition is a very specific one based on the needs of that EDSL, while the lens formulation is the expected one. Mvh, Hans On 17 apr 2014, at 14:00, haskell-cafe-request(a)haskell.org wrote: > Message: 1 > Date: Wed, 16 Apr 2014 16:49:31 -0700 > From: Dan Burton <danburton.email(a)gmail.com> > To: Alexey Muranov <alexey.muranov(a)gmail.com> > Cc: haskell-cafe <haskell-cafe(a)haskell.org> > Subject: Re: [Haskell-cafe] Syntax proposal for "reverse > apply"/"pipeline apply" (flip ($)) > Message-ID: > <CALSygwf0wdBbrfQeED_raWjNfttYaX7pwDTzmBTTHaUqEkX5rg(a)mail.gmail.com> > Content-Type: text/plain; charset="utf-8" > > In the "lens" package, this is (&) at infixl 1. In the "diagrams" package, > this is (#) at infixl 8. You're certainly not the first to want this, but > nobody can ever agree what it should be called or what fixity it should > have. You can always just define it yourself. > > -- Dan Burton > > > On Wed, Apr 16, 2014 at 3:25 PM, Alexey Muranov <alexey.muranov(a)gmail.com>wrote: > >> Hello, >> >> i am completely new to Haskell, but i am somewhat fascinated by >> lambda-calculus and programming. >> >> For whatever it is worth, i would like to propose for discussion a syntax >> for "(flip ($))" operation in Haskell. >> >> I think that a good syntax would be "|^", for example: >> >> square x = x * x >> y = 3 |^ square -- y == 9 >> >> >> Explanation: >> >> * i would have suggested just ^, but it would conflict with number >> exponentiation, >> >> * it is rather common in mathematics to write function application in >> exponential notation: x ^ f instead of f(x), especially if f is an >> automorphism of some structure, >> >> * (flip ($)) is exactly the exponentiation of Church numerals, >> >> * in "The calculi of lambda-conversion", Alonzo Church uses the >> "shorthand" notation "[N^M]" for "(MN)", where M and N are lambda-terms. >> >> * I am probably not the only person missing the ability to apply functions >> from the right: >> >> http://stackoverflow.com/questions/1457140/haskell-composition-vs-fs-pipe-f… >> >> Well, other notations i've thought of are "\^" and "~$". >> >> Alexey. >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >

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arbtt 0.8 released
by Joachim Breitner 18 Apr '14

18 Apr '14

Dear arbtt users, I just uploaded arbtt 0.8 to Hackage. See http://arbtt.nomeata.de/doc/users_guide/release-notes.html#release-notes-0.8 for a list of changes. This release is the result of slightly above 100 commits, closing 8 bugs and took a bit more than a year. I hope you like it! Greetings, Joachim -- Joachim “nomeata” Breitner mail(a)joachim-breitner.de • http://www.joachim-breitner.de/ Jabber: nomeata(a)joachim-breitner.de • GPG-Key: 0xF0FBF51F Debian Developer: nomeata(a)debian.org

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Hackage libraries that make use of GADTs
by George Karachalias 18 Apr '14

18 Apr '14

Hello, Could you tell me some Hackage Libraries that make use (heavy or not) of GADTs? George -- things you own end up owning you

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Stupid newbie question of the day: why is newMVar in the IO monad?
by Brian Hurt 17 Apr '14

17 Apr '14

So, I've hit a problem, and I'm wondering why this is. I want to write a function which returns a global monotonically incrementing int. This *should* be easy- just put a MVar in a global name, then update it as necessary: globalCounter :: MVar Integer globalCounter = undefined genId :: IO Integer genId = modifyMVar (\i -> (i+1, i)) globalCounter The problem with this is defining globalCounter- and that is because newMVar returns IO (MVar a), and not just MVar a. Now, I can go: globalCounter = unsafePerformIO $ newMVar 0 but I hate using unsafePerformIO. And I don't want to pass around the reference itself- I need to be in the IO monad with a StateT transform on top for other reasons, I don't want to complicate things. And even if I were, I would just pass the counter around instead of the reference. But it just feels like Haskell is being gratuitously difficult here. It's not just the name, it's the fact that creating the MVar is *explicitly* modifying the state of the world, which implies there is something more going on here than just allocating some memory. As an example of what I mean, newSTRef returns ST s (STRef s a). This is explicitly saying that the created STRef is only visible in the given thread. This is necessary for the implementation of the STRef, which is a mutable variable with no transactional guarantees. If it were visible from another thread, then it could be accessed from the other thread, creating a potential race condition. That I understand. But that isn't the case here- MVar's are explicitly designed to be accessed from multiple threads. So then I thought that it was something specific with MVars- maybe they need to do an OS call to set them up or something. OK, so let's try some alternatives. Like STM. Nope. newTVar has return type STM (TVar a) and newTMVar returns STM (TMVar a). Throw these into atomically, and I'm right back to where I started. newIORef returns IO (IORef a). And that's just a pointer store (I thought). It's easier for me to believe that I'm missing something here, rather than that Haskell is just being gratuitously difficult. But I honestly don't see what it is I'm missing. Help? Brian

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[ANN] 3T Software Labs MongoDB tools for Haskell programmers.
by Graham Thomson 17 Apr '14

17 Apr '14

Hello fair Haskell hackers! I’d like to announce the availability of 3T Software Labs MongoDB tools to the readers of this list. To forward-thinking Haskell adopters and MongoDB users alike, I’d like to extend a special 60-day trial of all the apps in our suite at http://3tsoftwarelabs.com/evaluation-edition using the code ‘HSKL’. My apologies for the spam if you’re not a MongoDB user. If you are, I hope you enjoy our tools and find them useful. Like many of the pioneering Haskell projects described on this list, we’ve just started our adventure, and would love to hear any feedback you have. Thanks a lot, Graham 3T Software Labs

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Haskell Weekly News: Issue 291
by Daniel Santa Cruz 17 Apr '14

17 Apr '14

Welcome to issue 291 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers from April 6 to 12, 2014 Quotes of the Week * geekosaur: IO, IO, it's off to (>>=) I go * edwardk: waiting for sclv to start going around door to door passing out little copies of the homotopy type theory book, and asking people if they have yet had the geometric realization that the simplicial sets are homotopically equivalent to their Lord and Savior. * carter: i'll leave you in the gentle hands of the rest of the channel Top Reddit Stories * GHC 7.8.1 Released Domain: haskell.org, Score: 226, Comments: 106 On Reddit: [1] http://goo.gl/pz4nE Original: [2] http://goo.gl/SjQ3Qv * What To Expect When You're a Platform Library Maintainer Domain: permalink.gmane.org, Score: 56, Comments: 4 On Reddit: [3] http://goo.gl/bD6VVf Original: [4] http://goo.gl/WDvy53 * Why are we relying on numeric identifiers for dependencies when we have so much richer information available? Domain: self.haskell, Score: 44, Comments: 18 On Reddit: [5] http://goo.gl/Z3gBEq Original: [6] http://goo.gl/Z3gBEq * How To Read "Pearls of Functional Algorithm Design" Domain: atamo.com, Score: 41, Comments: 8 On Reddit: [7] http://goo.gl/j4mXVz Original: [8] http://goo.gl/q41KEF * BayHac '14 Sign-Up and List of Classes Domain: self.haskell, Score: 40, Comments: 11 On Reddit: [9] http://goo.gl/xterjO Original: [10] http://goo.gl/xterjO * Is GHC the defacto standard? Domain: self.haskell, Score: 40, Comments: 39 On Reddit: [11] http://goo.gl/eiVjGM Original: [12] http://goo.gl/eiVjGM * Battleships web game written in Haskell/Yesod Domain: www-pg.iai.uni-bonn.de, Score: 34, Comments: 15 On Reddit: [13] http://goo.gl/hl2NC2 Original: [14] http://goo.gl/Wt5jqi * Heartbleed aftermath: should Haskell web frameworks allow using a pure Haskell SSL library instead of OpenSSL? Domain: self.haskell, Score: 32, Comments: 54 On Reddit: [15] http://goo.gl/ycHKNZ Original: [16] http://goo.gl/ycHKNZ * BudHac 2014 Domain: haskell.org, Score: 28, Comments: 3 On Reddit: [17] http://goo.gl/iObjrD Original: [18] http://goo.gl/KRJL2x * Hacking Haskell in nightclubs - video feature on Algorave Domain: motherboard.vice.com, Score: 27, Comments: 2 On Reddit: [19] http://goo.gl/LLG6qR Original: [20] http://goo.gl/PlzTTa * A brief introduction to ConstraintKinds extension (Wolfgang Jeltsch) Domain: jeltsch.wordpress.com, Score: 27, Comments: 4 On Reddit: [21] http://goo.gl/RadGh7 Original: [22] http://goo.gl/CZIfU8 * Generating Mazes with Inductive Graphs Domain: jelv.is, Score: 26, Comments: 11 On Reddit: [23] http://goo.gl/fufmgp Original: [24] http://goo.gl/FxAeGV Top StackOverflow Questions * Why MonadPlus and not Monad + Monoid? votes: 18, answers: 2 Read on SO: [25] http://goo.gl/J02Msv * Can a pure function have free variables? votes: 11, answers: 3 Read on SO: [26] http://goo.gl/iyJsn9 * FRP - Event streams and Signals - what is lost in using just signals? votes: 10, answers: 4 Read on SO: [27] http://goo.gl/cXwKKf * Is there a way to make GHC provide the type class constraints of typed holes? votes: 9, answers: 0 Read on SO: [28] http://goo.gl/LYxEvm * Where are the magic rules for GHC assert? votes: 8, answers: 1 Read on SO: [29] http://goo.gl/JL8OLZ * Using Haskell ranges: Why would mapping a floating point function across a range cause it to return an extra element? votes: 7, answers: 1 Read on SO: [30] http://goo.gl/mtBCBa * Why Haskell's Data.List.deleteBy takes in input a comparison function (a -> a -> Bool) and a value instead of a predicate (a -> Bool)? votes: 6, answers: 1 Read on SO: [31] http://goo.gl/Wd9evy Until next time, [32]+Daniel Santa Cruz References 1. http://www.haskell.org/pipermail/haskell/2014-April/024137.html 2. http://www.reddit.com/r/haskell/comments/22lw1b/ghc_781_released/ 3. http://permalink.gmane.org/gmane.comp.lang.haskell.libraries/21681 4. http://www.reddit.com/r/haskell/comments/22oott/what_to_expect_when_youre_a… 5. http://www.reddit.com/r/haskell/comments/22o4i1/why_are_we_relying_on_numer… 6. http://www.reddit.com/r/haskell/comments/22o4i1/why_are_we_relying_on_numer… 7. http://www.atamo.com/blog/how-to-read-pearls-by-richard-bird-1/ 8. http://www.reddit.com/r/haskell/comments/22kavg/how_to_read_pearls_of_funct… 9. http://www.reddit.com/r/haskell/comments/22d1wu/bayhac_14_signup_and_list_o… 10. http://www.reddit.com/r/haskell/comments/22d1wu/bayhac_14_signup_and_list_o… 11. http://www.reddit.com/r/haskell/comments/22r569/is_ghc_the_defacto_standard/ 12. http://www.reddit.com/r/haskell/comments/22r569/is_ghc_the_defacto_standard/ 13. http://www-pg.iai.uni-bonn.de/battleships 14. http://www.reddit.com/r/haskell/comments/22f683/battleships_web_game_writte… 15. http://www.reddit.com/r/haskell/comments/22udou/heartbleed_aftermath_should… 16. http://www.reddit.com/r/haskell/comments/22udou/heartbleed_aftermath_should… 17. http://www.haskell.org/haskellwiki/BudapestHackathon2014 18. http://www.reddit.com/r/haskell/comments/22euqt/budhac_2014/ 19. http://motherboard.vice.com/nl/read/algorave-coden-in-de-club 20. http://www.reddit.com/r/haskell/comments/22ou9y/hacking_haskell_in_nightclu… 21. http://jeltsch.wordpress.com/2013/02/14/the-constraint-kind/ 22. http://www.reddit.com/r/haskell/comments/22tfgl/a_brief_introduction_to_con… 23. http://jelv.is/blog/Generating-Mazes-with-Inductive-Graphs 24. http://www.reddit.com/r/haskell/comments/22ngz0/generating_mazes_with_induc… 25. http://stackoverflow.com/questions/23023961/why-monadplus-and-not-monad-mon… 26. http://stackoverflow.com/questions/22914878/can-a-pure-function-have-free-v… 27. http://stackoverflow.com/questions/22989253/frp-event-streams-and-signals-w… 28. http://stackoverflow.com/questions/23028124/is-there-a-way-to-make-ghc-prov… 29. http://stackoverflow.com/questions/22996814/where-are-the-magic-rules-for-g… 30. http://stackoverflow.com/questions/22901633/using-haskell-ranges-why-would-… 31. http://stackoverflow.com/questions/23000665/why-haskells-data-list-deleteby… 32. https://plus.google.com/105107667630152149014/about

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