April 2014 - Haskell-Cafe

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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Please test: Cabal/cabal-install 1.20 RC2
by Johan Tibell 17 Apr '14

17 Apr '14

Hi all, The second, and hopefully last, release candidate for Cabal and cabal-install 1.20 is ready. If no issues are discovered, I plan to release this on Sunday, April 20th. You can help test by installing the release candidate and use it for your day-to-day work: cabal install http://johantibell.com/files/Cabal-1.20.0.0-rc2.tar.gz http://johantibell.com/files/cabal-install-1.20.0.0-rc2.tar.gz Please report any issues at https://github.com/haskell/cabal/issues -- Johan

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Re: [Haskell-cafe] Minimal Haskell Platform
by Travis Cardwell 17 Apr '14

17 Apr '14

On 2014年04月15日 14:02, Simon Hengel wrote: >> It is interesting that cabal does not (try to) install happy as a >> prerequisite for haskell-src-exts, which lists happy as a >> Build-Tools dependency [2]. It is also interesting that `cabal >> install happy` requires itself, despite indication in the >> documentation [3] that it is a valid way to install the package... >> >> Does this indicate that Cabal expects Haskell Platform? > > No, installing build tools is just a missing feature of `cabal-install` > (AFAIK). I chatted with Duncan Coutts in #haskell, and he confirmed that it is indeed just a missing feature. I found the issue here: https://github.com/haskell/cabal/issues/220 I was unable to find an issue regarding the `cabal install happy` failure, so I created one: https://github.com/haskell/cabal/issues/1777 >> Should Cabal be changed to (support minimal installations and) >> manage Build-Tools dependencies? > > Yes. In #haskell, Duncan Coutts said: > as for the global vs. minimal, yes I see it's an issue. One sometimes > wants to start a sandbox with a very minimal set. > > the issue with making a minimal sandbox is simply knowing which > packages should be in it, and we currently just base them all off of > the global one. > > but that's not the only solution, it'd also be possible to copy a > subset, if we know what subset to copy > > and I have a somewhat cunning plan along these lines (related to some > other ghc-pkg/cabal improvement work) which might make that rather > easier > > what I want is for ghc itself to come with multiple profiles, with one > being the minimum (base + rts + deps), and that could be used as a basis > for new envs With such a feature, it sounds like we can get the best of both worlds: * a feature-rich Haskell Platform to support beginners * minimal sandboxes for advanced users > I think most Haskell developers use a reasonably recent version of GHC > (that would currently still be 7.6.3 for me) and the latest version of > cabal-install. Everything else can be installed with cabal-install. In my experience, the alex and happy binaries must also be installed (for a number of packages to be installable via cabal-install). > Where would something like the HP actually make sense? For stuff that > has external dependencies that are not easily installable with > cabal-install (like curses bindings, SSL support, etc.). We have none > of this in the HP. So I think currently we just have additional costs, > but no benefits (+ we harm innovation by arbitrarily "endorsing" random > packages). I understand this point of view, but allow me to offer an opposing one. By putting packages with external dependencies into Haskell Platform, we often increase the dependencies of Haskell Platform itself. For example, Haskell Platform currently includes Graphics packages; installing Haskell Platform on a server entails installing a number of OpenGL libraries that are never used. Cheers, Travis

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Autocomplete command line options with GHC 7.8
by Jan Stolarek 16 Apr '14

16 Apr '14

Hi all, GHC 7.8 adds --show-options flag that prints all supported command line flags on standard output. This can be used to enable autocompletion of command line options for ghc in shells that support autocompletion. If you're using bash add this snippet to your ~/.bashrc file: <<<<<<<<<<<<< START # Autocomplete GHC commands _ghc() { local envs=`ghc --show-options` # get the word currently being completed local cur=${COMP_WORDS[$COMP_CWORD]} # the resulting completions should be put into this array COMPREPLY=( $( compgen -W "$envs" -- $cur ) ) } complete -F _ghc -o default ghc <<<<<<<<<<<< END Enjoy. Janek PS. I also added a wiki page: https://ghc.haskell.org/trac/ghc/wiki/AutocompleteGHCFlags Feel free to add instructions for other shells.

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Re: [Haskell-cafe] Directed rounding in haskell ?
by Barak A. Pearlmutter 16 Apr '14

16 Apr '14

One issue that arises with rounding modes in a pure lazy language like Haskell is that it's pretty hard to nail down the semantics. The "round towards negative infinity" mode is a dynamic hardware feature, available by setting a rounding mode flag in the FPU. If you execute some hunk of code with that flag having different values, you will in general get different results. So if you try to do this in Haskell, you have to decide how to expose it without violating purity. This would break things: roundWith :: RoundMode -> ( () -> a) -> a consider let x = 0.1 + 0.2 in roundWith NegInfty (\_ -> x) vs roundWith NegInfty (\_ -> 0.1 + 0.2) So you need to make different numeric types. This is what Ed Kmett's "rounding" cabal package does, albeit in a much more general way where the new floating types are parameterized by both rounding mode and precision. The bottom line is that IEEE Floating Point numbers are actually a monad. There is NaN, which makes them a Maybe, or given that there are many NaNs perhaps an Either. And there is the rounding mode, which is sort of a Reader. As an aside, changing the rounding mode flags can be surprisingly slow on modern CPUs. One trick for doing interval arithmetic efficiently is to set the flags once, say to TowardInf. Then x*y computes the product of x and y rounding towards positive infinity while -((-x)*y) computes the same product rounding towards negative infinity without fiddling with the rounding mode. So for adding intervals, instead of Interval x0 x1 + Interval y0 y1 = Interval (x0+y0) (x1+y1) Interval x0 x1 * Interval y0 y1 | x0 >= 0 && y0 >= 0 = Interval (x0*y0) (x1*y1) -- etc but with some added hair to fiddle with the rounding modes, one leaves the rounding mode fixed and writes Interval x0 x1 + Interval y0 y1 = Interval (-(-x0-y0)) (x1+y1) Interval x0 x1 * Interval y0 y1 | x0 >= 0 && y0 >= 0 = Interval (-((-x0)*y0)) (x1*y1) -- etc As a final throwaway, allowing (Interval x0 x1) where x0>x1, i.e., improper intervals, gives a very pleasant system with a surprisingly nice and rich semantics. For example, Interval 1 2 - Interval 2 1 = Interval 0 0 which makes for a group rather than just a semigroup. The system is known variously as modal intervals, Kaucher intervals, directed interval arithmetic, or generalized interval arithmetic, and can be used to automatically prove interesting arithmetic properties full of ∀s and ∃s. -- Barak A. Pearlmutter Hamilton Institute & Dept Comp Sci, NUI Maynooth, Co. Kildare, Ireland http://www.bcl.hamilton.ie/~barak/

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