January 2013 - Haskell-Cafe

Lens, with ability to create a data-field when it's missing
by Takayuki Muranushi 10 Jan '13

10 Jan '13

Dear All, I really am enjoying the lens (it is tough to learn, though :) Recently, I've been trying to implement a dynamic object --- a collection of methods that you can update at run-time --- using lens [1][2][3]. After several attempts, I think I have located the center of my problem: Lens with the ability "to create a data-field when it's missing." My question: is it possible? There are many excellent examples of partial lenses in the lens library, such as '_head' from Data.List.Lens 'dynamic' from Data.Dynamic.Lens . To me, they seem to share the following set of common behavior (allow me to use inaccurate terms...) - set l a s : if the field is present, insert 'a'; otherwise, do nothing. - over l f s : if the field is present, map 'f' over it. otherwise, do nothing. - preview l s : if the field is present, return 'Just a'. otherwise, return 'Nothing' . Instead, I want the following set of behaviors: - set l a s : if the field is present, insert 'a'; otherwise, *create the field* and insert 'a'. - over l f s : if the field is present, map 'f' over it. otherwise, do nothing. - preview l s : if the field is present, return 'Just a'. otherwise, return 'Nothing' . For example, the current behavior of _head is >>> [] & _head .~ 1 [] I want to implement a variant _head' , such that >>> [] & _head' .~ 1 [1] Let us call this behavior 'Member' for the moment. I have a implementation of 'Member' [4][5] but they are unsatisfactory. [4] involves runtime errors and breaking of lens laws. [5] shows that we can "create a field" only if we treat Dynamic explicitly out of lens context. I don't know how 'Member' will fit in the lens hierarchy http://i.imgur.com/FgfVW.png or even if it fits at all. Member is weaker than Getter and probably is a Fold since you have only partial getting (^?) for Member. Member is weaker than Prism since you cannot construct a whole Object from just one Member. On the other hand, setter side of Member requires (s -> b -> t) interface of a Lens, because the setter need to update the whole Object when the field under concern is missing. That said, let me put this question in another way; " We can construct a (Simple Lens s a) from (getter :: s -> a) and (setter :: s -> a -> s), and we can construct a (Getter s a) from (getter :: s -> a). Then why we cannot construct a (Simple Setter s a) from (setter :: s -> a -> s) ? " Does Member deserve a new node in The Lens Hierarchy tree? Or can we implement it by combinations of existing prisms, folds etc? Or does it fall outside of the lens framework? I appreciate any comments. Takayuki [1] https://github.com/nushio3/practice/tree/master/duck [2] http://d.hatena.ne.jp/nushio/20121226#p2 (Japanese) [3] http://hackage.haskell.org/packages/archive/dynamic-object/0.1.0.1/doc/html… [4] https://github.com/nushio3/practice/blob/master/lens/newfield.hs [5] https://github.com/nushio3/practice/blob/master/lens/object-4.hs -- Takayuki MURANUSHI The Hakubi Center for Advanced Research, Kyoto University http://www.hakubi.kyoto-u.ac.jp/02_mem/h22/muranushi.html

1 1

Haskell Weekly News: Issue 254
by Daniel Santa Cruz 09 Jan '13

09 Jan '13

Welcome to issue 254 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of December 02, 2012 to January 05, 2013. As some of you might have noticed, there has been an interruption in the scheduled transmission of the newsletter. I have, over the past few weeks, completed a move to NYC! It has been wonderful to meet many in the community at the Haskell NYC meetups. Here is to a great year for our cherised community. Let's get things started out the right way, with a litte "Lambda Style" :-) [1] http://goo.gl/nQYdp Quotes of the Week * GuySteele: Some people prefer not to commingle the functional, lambda-calculus part of a language with the parts that do side effects. It seems they believe in the separation of Church and state. * edwardk: type level ieee floats are a crime against nature. i had to implement them in c++ for template meta programming once. never ever again * acowley: (regarding Proxy) Haven't you ever wanted to put something in a box and never think about it? elliott: children * merijn: xenocons: I spend the entire week grading FP homework, I think it's impossible for you to surprise me :p merijn: That's confusing :< * andr00: Today's software engineering word is "farpotshket." This is a Yiddish word meaning, "broken, because someone tried to fix it." * dmwit: I know, I know. "0, 1, many". But then I also write most of my programs in a language whose type system has 4 levels. Four! Is that even a number?? * aristid: a dubious motivation IMHO edwardk: i've had worse =P * ziarkaen: I'm thinking I can kill two birds with one stone by learning Haskell while producing a website, but all the birds are still alive at the moment. * ion: monad.bind = function (func, args) { return func.apply(undefined, [value].concat(Array.prototype.slice.apply(args || []))); }; is idiomatic JavaScript? * beaky: if stacks are recursion, what are queues? Hafydd: Procrastination. * elliott: I think lens is the first library I've seen that promises letting you write abstract totally-inefficient-looking compositional code and getting reliably good compilation which has actually delivered on that promise. * elliott: I didn't say half the things I said. * shachaf: The two-letter and three-letter identifiers are for edwardk's libraries. The one-letter identifiers are for his actual code. elliott: edwardk has non-library code? * neutrino: in many ways, Haskell is like this primodial soup out of which other languages end up springing * mm_freak: bruce schneier is an endofunctor that turns all automorphisms into identities * roconnor: where are all the category theoriest? why don't they already have all the answers for us? edwardk: roconnor: this is the point in your career where you look around for the cavalry and realize that you're it ;) * cmccann: multi-letter type variables is an experimental new extension * elliott: I can't remember the last time I wrote any Haskell without an evil grin. * neutrino: when you're told you have to use javascript, the viable alternative always remains in seppuku. and it's much more honorable. * shachaf: chrisdone: Please make Author not mandatory. [on hpaste] donri: shachaf: just write "not shachaf" and no one will ever know it's you * tac: If you're not part of the solution, you're part of the precipitate * cmccann: some people blame themselves, some people blame the language, but the people who really know what they're doing blame shachaf. Top Reddit Stories * The end of an era, and the dawn of a new one Domain: haskell.org, Score: 148, Comments: 32 On Reddit: [2] http://goo.gl/fiyVt Original: [3] http://goo.gl/dlxsF * Simon Peyton-Jones invests in FP Complete Domain: fpcomplete.com, Score: 92, Comments: 15 On Reddit: [4] http://goo.gl/nPTk1 Original: [5] http://goo.gl/wzfpP * 24 Days of Hackage - Thank YOU! Domain: ocharles.org.uk, Score: 88, Comments: 12 On Reddit: [6] http://goo.gl/2yodj Original: [7] http://goo.gl/hSsH6 * Heist 0.10...now with a >3000x performance improvement Domain: snapframework.com, Score: 81, Comments: 38 On Reddit: [8] http://goo.gl/p7R7u Original: [9] http://goo.gl/znugn * 24 Days of Hackage: EKG Domain: ocharles.org.uk, Score: 68, Comments: 15 On Reddit: [10] http://goo.gl/ltPb1 Original: [11] http://goo.gl/UeYOA * pipes-3.0: Simpler, unified API Domain: haskellforall.com, Score: 64, Comments: 71 On Reddit: [12] http://goo.gl/ZSVtT Original: [13] http://goo.gl/6Yt01 * Hoodle : Pen note-taking program written in haskell Domain: youtube.com, Score: 64, Comments: 21 On Reddit: [14] http://goo.gl/aJ9G9 Original: [15] http://goo.gl/GsZ85 * 24 Days of Hackage: lens Domain: ocharles.org.uk, Score: 62, Comments: 46 On Reddit: [16] http://goo.gl/71HbV Original: [17] http://goo.gl/L78Hp * Cartesian Closed Comic #19: Facebook Domain: ro-che.info, Score: 61, Comments: 8 On Reddit: [18] http://goo.gl/LKqzz Original: [19] http://goo.gl/tSrjJ * Announcing: Snap Framework v0.10 Domain: snapframework.com, Score: 61, Comments: 11 On Reddit: [20] http://goo.gl/TibIw Original: [21] http://goo.gl/Nm98e * Surprises of the Haskell module system (part 1) Domain: ro-che.info, Score: 57, Comments: 11 On Reddit: [22] http://goo.gl/tOiiK Original: [23] http://goo.gl/vLEZP * 24 Days of Hackage: configurator Domain: ocharles.org.uk, Score: 53, Comments: 7 On Reddit: [24] http://goo.gl/jFKt2 Original: [25] http://goo.gl/nu2zj * Fay :: Haskell -> JavaScript Domain: fay-lang.org, Score: 52, Comments: 14 On Reddit: [26] http://goo.gl/wAH7C Original: [27] http://goo.gl/uitoD * Announcing: Clay, a CSS preprocessor as a Haskell EDSL. Domain: sebastiaanvisser.github.com, Score: 52, Comments: 53 On Reddit: [28] http://goo.gl/1ej1n Original: [29] http://goo.gl/bWfU2 * 24 Days of Hackage: base Domain: ocharles.org.uk, Score: 51, Comments: 10 On Reddit: [30] http://goo.gl/e5gLp Original: [31] http://goo.gl/7MY8o * Haskell for all: The Continuation Monad Domain: haskellforall.com, Score: 51, Comments: 42 On Reddit: [32] http://goo.gl/YKDOo Original: [33] http://goo.gl/zJ42x * ANNNOUNCE: lens 3.7 released Domain: haskell.org, Score: 50, Comments: 9 On Reddit: [34] http://goo.gl/FecK0 Original: [35] http://goo.gl/n4tR0 * 24 Days of Hackage: optparse-applicative Domain: ocharles.org.uk, Score: 50, Comments: 21 On Reddit: [36] http://goo.gl/JyFVh Original: [37] http://goo.gl/PP4CX * 24 Days of Hackage: snap Domain: ocharles.org.uk, Score: 50, Comments: 35 On Reddit: [38] http://goo.gl/9Wp3q Original: [39] http://goo.gl/WlHlU * What doesn't Haskell do well? Domain: self.haskell, Score: 47, Comments: 153 On Reddit: [40] http://goo.gl/VRJMn Original: [41] http://goo.gl/VRJMn * 24 Days of Hackage: criterion Domain: ocharles.org.uk, Score: 46, Comments: 10 On Reddit: [42] http://goo.gl/wYrdm Original: [43] http://goo.gl/rYT9C * Some specific thoughts on why using Haskell makes you a better programmer. Domain: dubhrosa.blogspot.co.uk, Score: 46, Comments: 30 On Reddit: [44] http://goo.gl/dN1Zd Original: [45] http://goo.gl/DDNuQ * Growing Haskell Platform Domain: mtnviewmark.wordpress.com, Score: 43, Comments: 16 On Reddit: [46] http://goo.gl/jNefn Original: [47] http://goo.gl/a5udB * 24 Days of Hackage: text Domain: ocharles.org.uk, Score: 43, Comments: 21 On Reddit: [48] http://goo.gl/3VmmH Original: [49] http://goo.gl/13MOc Top StackOverflow Questions * What's the absurd function in Data.Void useful for? votes: 34, answers: 5 Read on SO: [50] http://goo.gl/ny4gh * Benefit of avoiding multiple list traversals votes: 27, answers: 4 Read on SO: [51] http://goo.gl/Z6DNg * Images in Haddock documentation votes: 22, answers: 2 Read on SO: [52] http://goo.gl/7tFgE * Idiomatic option pricing and risk using Repa parallel arrays votes: 22, answers: 1 Read on SO: [53] http://goo.gl/Ih9cn * How much memory does a thunk use? votes: 21, answers: 2 Read on SO: [54] http://goo.gl/VkdIX * Can I constrain a type family? votes: 20, answers: 2 Read on SO: [55] http://goo.gl/0kBEU * How to write platform-independent code in Haskell (ghc) votes: 18, answers: 1 Read on SO: [56] http://goo.gl/13bEB * Play a wav file with Haskell votes: 17, answers: 3 Read on SO: [57] http://goo.gl/sKpgf * Why are difference lists more efficient than regular concatenation? votes: 16, answers: 2 Read on SO: [58] http://goo.gl/GCVpe * GUI tools that are actively developed and well documented for Haskell votes: 15, answers: 2 Read on SO: [59] http://goo.gl/M2L0l * What are the adjoint functor pairs corresponding to common monads in Haskell? votes: 15, answers: 2 Read on SO: [60] http://goo.gl/UnUik Until next time, +Daniel Santa Cruz https://plus.google.com/105107667630152149014/about References 1. http://www.youtube.com/watch?v=Ci48kqp11F8 2. http://www.haskell.org/pipermail/haskell-cafe/2012-December/105041.html 3. http://www.reddit.com/r/haskell/comments/14bvtz/the_end_of_an_era_and_the_d… 4. http://fpcomplete.com/why-im-investing-in-fp-complete/ 5. http://www.reddit.com/r/haskell/comments/15wc7s/simon_peytonjones_invests_i… 6. http://ocharles.org.uk/blog/posts/2012-12-24-24-days-of-hackage.html 7. http://www.reddit.com/r/haskell/comments/15e0nz/24_days_of_hackage_thank_yo… 8. http://snapframework.com/blog/2012/12/9/heist-0.10-released 9. http://www.reddit.com/r/haskell/comments/14kjct/heist_010now_with_a_3000x_p… 10. http://ocharles.org.uk/blog/posts/2012-12-11-24-day-of-hackage-ekg.html 11. http://www.reddit.com/r/haskell/comments/14ovdp/24_days_of_hackage_ekg/ 12. http://www.haskellforall.com/2012/12/pipes-30-simpler-unified-api.html 13. http://www.reddit.com/r/haskell/comments/14qe8b/pipes30_simpler_unified_api/ 14. http://www.youtube.com/watch?v=Z2wzpyxsVSU 15. http://www.reddit.com/r/haskell/comments/14seiv/hoodle_pen_notetaking_progr… 16. http://ocharles.org.uk/blog/posts/2012-12-09-24-days-of-hackage-lens.html 17. http://www.reddit.com/r/haskell/comments/14kbbi/24_days_of_hackage_lens/ 18. http://ro-che.info/ccc/19.html 19. http://www.reddit.com/r/haskell/comments/145cra/cartesian_closed_comic_19_f… 20. http://snapframework.com/blog/2012/12/10/snap-0.10-released 21. http://www.reddit.com/r/haskell/comments/14lrlr/announcing_snap_framework_v… 22. http://ro-che.info/articles/2012-12-25-haskell-module-system-p1.html 23. http://www.reddit.com/r/haskell/comments/15fnrx/surprises_of_the_haskell_mo… 24. http://ocharles.org.uk/blog/posts/2012-12-21-24-days-of-hackage-configurato… 25. http://www.reddit.com/r/haskell/comments/158svo/24_days_of_hackage_configur… 26. http://fay-lang.org/ 27. http://www.reddit.com/r/haskell/comments/14dxru/fay_haskell_javascript/ 28. http://sebastiaanvisser.github.com/clay/ 29. http://www.reddit.com/r/haskell/comments/15aoif/announcing_clay_a_css_prepr… 30. http://ocharles.org.uk/blog/posts/2012-12-23-24-days-of-hackage-base.html 31. http://www.reddit.com/r/haskell/comments/15c0jm/24_days_of_hackage_base/ 32. http://www.haskellforall.com/2012/12/the-continuation-monad.html 33. http://www.reddit.com/r/haskell/comments/15ogxe/haskell_for_all_the_continu… 34. http://www.haskell.org/pipermail/haskell/2012-December/023598.html 35. http://www.reddit.com/r/haskell/comments/14fw7l/annnounce_lens_37_released/ 36. http://ocharles.org.uk/blog/posts/2012-12-17-24-days-of-hackage-optparse-ap… 37. http://www.reddit.com/r/haskell/comments/150iem/24_days_of_hackage_optparse… 38. http://ocharles.org.uk/blog/posts/2012-12-19-24-days-of-hackage-snap.html 39. http://www.reddit.com/r/haskell/comments/154i72/24_days_of_hackage_snap/ 40. file://localhost/r/haskell/comments/14nj0r/what_doesnt_haskell_do_well/ 41. http://www.reddit.com/r/haskell/comments/14nj0r/what_doesnt_haskell_do_well/ 42. http://ocharles.org.uk/blog/posts/2012-12-18-24-days-of-hackage-criterion.h… 43. http://www.reddit.com/r/haskell/comments/152yv3/24_days_of_hackage_criterio… 44. http://dubhrosa.blogspot.co.uk/2012/12/lessons-learning-haskell.html 45. http://www.reddit.com/r/haskell/comments/15ch1k/some_specific_thoughts_on_w… 46. http://mtnviewmark.wordpress.com/2012/12/06/growing/ 47. http://www.reddit.com/r/haskell/comments/14fmw8/growing_haskell_platform/ 48. http://ocharles.org.uk/blog/posts/2012-12-12-24-days-of-hackage-text.html 49. http://www.reddit.com/r/haskell/comments/14quxz/24_days_of_hackage_text/ 50. http://stackoverflow.com/questions/14131856/whats-the-absurd-function-in-da… 51. http://stackoverflow.com/questions/13686926/benefit-of-avoiding-multiple-li… 52. http://stackoverflow.com/questions/14017112/images-in-haddock-documentation 53. http://stackoverflow.com/questions/14082158/idiomatic-option-pricing-and-ri… 54. http://stackoverflow.com/questions/13982863/how-much-memory-does-a-thunk-use 55. http://stackoverflow.com/questions/14133121/can-i-constrain-a-type-family 56. http://stackoverflow.com/questions/13799199/how-to-write-platform-independe… 57. http://stackoverflow.com/questions/14005592/play-a-wav-file-with-haskell 58. http://stackoverflow.com/questions/13879260/why-are-difference-lists-more-e… 59. http://stackoverflow.com/questions/13887213/gui-tools-that-are-actively-dev… 60. http://stackoverflow.com/questions/13937289/what-are-the-adjoint-functor-pa…

1 0

haskell.cs.yale.edu error
by Greg Fitzgerald 09 Jan '13

09 Jan '13

I was hoping to check on the status of Yampa, but: http://haskell.cs.yale.edu/ [an error occurred while processing this directive] You don't have permission to access the requested directory. There is either no index document or the directory is read-protected. [an error occurred while processing this directive] Do you know who maintains this website? Thanks, Greg

1 0

HaskellDB-HDBC-PostgreSQL installation problem
by Johannes.Reiher 09 Jan '13

09 Jan '13

Hello Community, I have a problem installing the HaskellDB-HDBC-PostgreSQL package with cabal. I'm using Haskell Platform in Windows and I tried to install this package, but there was the error "could not find pq library". So I tried to install libpq wich includes pq but there was another error: "this package needs a unix installation". Is there any way to install pq on Windows or get the HaskellDB-HDBC-PostgreSQL in another way? Thanks for your advice Johannes Reiher

5 4

Undo records
by Casey Basichis 09 Jan '13

09 Jan '13

Hi, I am still getting a hang of Haskell. Sorry if the answer is obvious. What sorts of packages and approaches should I be looking at if I was looking to store something like an Undo stack into a database. Each table would refer to a function. Each records input and outputs would specify both a table ID and record ID. The records would also have a data and a Process ID to associate all functions to a specific process and give them an order. No records are ever deleted. Rolling something back is instead a process of recreating a new, modified graph by taking the old graph from the database. I should note that while I can generate some of the boiler parts from template haskell in advance I'm ultimately using a stage 1 compiler with no GHCI o template haskell. Thanks, Casey -- Casey James Basichis Composer - Cartoon Network http://www.caseyjamesbasichis.com caseybasichis(a)gmail.com 310.387.7540

2 2

Seeking feedback for a tutorial paper draft about GADTs
by Anton Dergunov 08 Jan '13

08 Jan '13

Hi Haskellers, I have written a draft of an introductory-level tutorial paper about GADTs in Haskell (for submittion to proceedings of the recent LASER summer school) and I would like to seek initial feedback about its content: what information is probably missing? are there any subtle mistakes? The main idea of this article was to serve as a starting point for learning GADTs (I was missing this kind of information myself some time ago), so I have collected several examples demonstrating common use cases. -- Best Regards, Anton Dergunov

1 0

Difference Lists versus Accumulators
by Edsko de Vries 08 Jan '13

08 Jan '13

Hey all, The connection between difference lists and accumulators is probably well known, but I only recently realized it myself and a quick Google search didn't find turn up any page where this was explicitly stated, so I thought this observation might be useful to some. Every beginner Haskell student learns that this definition of "reverse" has O(n^2) complexity: reverse [] = [] reverse (x : xs) = reverse xs ++ [x] But what happens when we return a difference list instead, replacing [] with id, (++) with (.) and [x] with (x :)? reverse' [] = id reverse' (x : xs) = reverse xs . (x :) This definition of reverse' has type reverse' :: [a] -> [a] -> [a] Let's inline the second argument: reverse' [] acc = acc reverse' (x : xs) acc = reverse' xs (x : acc) And we have recovered the standard definition using an accumulator! I thought that was cute :) And may help understanding why difference lists are useful. Edsko

3 2

What happened to first-class modules?
by Ismael Figueroa Palet 08 Jan '13

08 Jan '13

I just read this page http://www.haskell.org/haskellwiki/First-class_module. It seems there was not much no ongoing work on this topic... does somebody know what happened to first-class modules? what are the actual research papers about this topic? Thanks -- Ismael

1 0

help diagnosing space leak with IORef/STRef, just incrementing a million times.
by Thomas Hartman 07 Jan '13

07 Jan '13

I have a space leak in a function that increments a number inside IORef or STRef (either lazy or strict). Is this the kind of problem that can be diagnosed and fixed using the approach described in RWH, with space profiling? Otherwise what's the right approach to diagnose and fix? I wrote a non-list-using, and tail call-using, iterateMonadic funcation that does the same thing as replicateM, in case the problem was due to iterateM using a list structure, but iterateMonadic and replicateM had same memory leak. I can run this problem with +RTS -K100M -RTS but this aditional memory shouldn't be needed. {-# LANGUAGE RankNTypes, BangPatterns #-} import Data.List import Control.Monad.State.Strict import Data.Array.MArray import Data.Array.IO import Data.Array.ST import Control.Monad.ST import Control.Applicative import Data.IORef import Data.STRef.Strict import Data.STRef import GHC.ST import Control.Applicative ((<$>)) main = do -- putStrLn "ioIters, iterateMonadic: " -- print =<< ioIters iters iterateMonadic -- putStrLn "ioIters, replicateM_: " -- print =<< ioIters iters replicateM_ -- putStrLn "lazySTIters, iterateMonadic: " -- print $ lazySTIters iters iterateMonadic -- putStrLn "lazySTIters, replicateM_: " -- print $ lazySTIters iters replicateM_ putStrLn "strictSTIters, iterateMonadic: " print $ strictSTIters iters iterateMonadic -- putStrLn "strictSTIters, replicateM_: " -- print $ strictSTIters iters replicateM_ type Iter = (Monad m) => Int -> m () -> m () iterateMonadic :: (Monad m) => Int -> m a -> m a iterateMonadic n mx = do let loop 0 acc = acc loop n acc = loop (n-1) (acc >> mx ) loop n mx iters = 10^7 ioIters :: Int -> Iter -> IO Int ioIters numIters iteratorM = do tmp <- newIORef 0 iteratorM numIters ( modifyIORef tmp (+1) ) readIORef tmp lazySTIters :: Int -> Iter -> Int lazySTIters numIters iteratorM = runST $ do tmp <- Data.STRef.newSTRef 0 iteratorM numIters ( Data.STRef.modifySTRef tmp (+1) ) Data.STRef.readSTRef tmp strictSTIters :: Int -> Iter -> Int strictSTIters numIters iteratorM = runST $ do tmp <- Data.STRef.Strict.newSTRef 0 iteratorM numIters ( Data.STRef.Strict.modifySTRef tmp incr' ) Data.STRef.Strict.readSTRef tmp incr' (!x) = x + 1

3 2

help diagnosing space leak with IORef/STRef, just incrementing a million times.
by Thomas Bereknyei 07 Jan '13

07 Jan '13

I have had issues like this with modifySTRef before. Try to make a strict version modifySTRef' If memory serves, something like this worked for me. modifySTRef' r f = do a <- readSTRef r writeSTRef r $! f a

1 0