- Haskell-Cafe - Haskell.org

Haskell Weekly News: Issue 277
by Daniel Santa Cruz 22 Aug '13

22 Aug '13

Welcome to issue 277 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of August 11 to 17, 2013. Quotes of the Week * psygnisfive: Seminearring is the task of giving seminars. * Taneb: You know you've made it when SPJ asks you for help with Haskell. * byorgey: . o O ( "For my next trick, I will CPS-encode this rabbit!" -- edwardk ) * shachaf: I prefer the cogolden rule: When CPS-transforming, doubt. * edwardk: "You see these scars? I got them back in 7.7." * danharaj: parsing is hard, let's go shopping Top Reddit Stories * Mio: A High-Performance Multicore IO Manager for GHC [pdf] Domain: haskell.cs.yale.edu, Score: 147, Comments: 31 On Reddit: [1] http://goo.gl/ZTy1Yl Original: [2] http://goo.gl/6foNWC * Destroying Performance with Strictness Domain: neilmitchell.blogspot.ru, Score: 92, Comments: 45 On Reddit: [3] http://goo.gl/1rkx3a Original: [4] http://goo.gl/rb5UnM * A great series of posts on the power of monoids Domain: self.haskell, Score: 78, Comments: 10 On Reddit: [5] http://goo.gl/TmAAwT Original: [6] http://goo.gl/TmAAwT * Cellular Automata by Edward Kmett Domain: fpcomplete.com, Score: 69, Comments: 7 On Reddit: [7] http://goo.gl/EIFK5A Original: [8] http://goo.gl/03mNhV * Elm 0.9 - New type checker, much better error messages Domain: elm-lang.org, Score: 50, Comments: 14 On Reddit: [9] http://goo.gl/ngSBPX Original: [10] http://goo.gl/4Jp3l6 * Revisiting Matrix Multiplication, Part I by Edward Kmett Domain: fpcomplete.com, Score: 45, Comments: 4 On Reddit: [11] http://goo.gl/gYfm5S Original: [12] http://goo.gl/bg75ZX * Revisiting Matrix Multiplication, Part II by Edward Kmett Domain: fpcomplete.com, Score: 37, Comments: 9 On Reddit: [13] http://goo.gl/qU2r69 Original: [14] http://goo.gl/5cRq72 * Roles: a new feature of GHC (patch GND + type families) Domain: typesandkinds.wordpress.com, Score: 34, Comments: 28 On Reddit: [15] http://goo.gl/HhVOTu Original: [16] http://goo.gl/Twx5YB * Type Inference, Haskell and Dependent Types :: PDF Domain: personal.cis.strath.ac.uk, Score: 33, Comments: 0 On Reddit: [17] http://goo.gl/MbtYGO Original: [18] http://goo.gl/uOx01F * Embedded Languages for High-Performance Computing [Chakravarty, PDF 70 slides] Domain: speakerdeck.com, Score: 31, Comments: 4 On Reddit: [19] http://goo.gl/mb2frN Original: [20] http://goo.gl/EEG0Wk * Graph drawing with gloss Domain: mazzo.li, Score: 30, Comments: 7 On Reddit: [21] http://goo.gl/j0CZmo Original: [22] http://goo.gl/5ZtFuu * Benchmarking sqlite-simple against Python and C Domain: nurpax.github.io, Score: 28, Comments: 9 On Reddit: [23] http://goo.gl/CNyjWa Original: [24] http://goo.gl/gFmWgT * What is "lifting"? Explained on stackoverflow Domain: stackoverflow.com, Score: 25, Comments: 7 On Reddit: [25] http://goo.gl/HzXWb9 Original: [26] http://goo.gl/hQxTce * GHC API: Interpreted, compiled and package modules Domain: parenz.wordpress.com, Score: 23, Comments: 0 On Reddit: [27] http://goo.gl/xUeUYq Original: [28] http://goo.gl/llQsyE * Time to start writing on the new binding: SDL 2.0 release. Domain: lists.libsdl.org, Score: 22, Comments: 15 On Reddit: [29] http://goo.gl/G0ut36 Original: [30] http://goo.gl/MuO1uB * FP Complete's Haskell programming competition now accepting entries Domain: fpcomplete.com, Score: 21, Comments: 7 On Reddit: [31] http://goo.gl/tbEBKG Original: [32] http://goo.gl/ADsrJ * Pretext by experiments and guesses Domain: fpcomplete.com, Score: 18, Comments: 15 On Reddit: [33] http://goo.gl/oVpKIe Original: [34] http://goo.gl/2PFthz Top StackOverflow Questions * Haskell random numbers suddenly start to “converge” after months of running votes: 30, answers: 1 Read on SO: [35] http://goo.gl/oxUAQ5 * Why does ghc generate so many useless-looking nested case expressions? votes: 10, answers: 1 Read on SO: [36] http://goo.gl/6cdTd1 Until next time, [37]+Daniel Santa Cruz References 1. http://haskell.cs.yale.edu/wp-content/uploads/2013/08/hask035-voellmy.pdf 2. http://www.reddit.com/r/haskell/comments/1k6fsl/mio_a_highperformance_multi… 3. http://neilmitchell.blogspot.ru/2013/08/destroying-performance-with-strictn… 4. http://www.reddit.com/r/haskell/comments/1ker84/destroying_performance_with… 5. http://www.reddit.com/r/haskell/comments/1ka4q6/a_great_series_of_posts_on_… 6. http://www.reddit.com/r/haskell/comments/1ka4q6/a_great_series_of_posts_on_… 7. https://www.fpcomplete.com/user/edwardk/cellular-automata 8. http://www.reddit.com/r/haskell/comments/1kj5zh/cellular_automata_by_edward… 9. http://elm-lang.org/blog/announce/version-0.9.elm 10. http://www.reddit.com/r/haskell/comments/1ka0ld/elm_09_new_type_checker_muc… 11. https://www.fpcomplete.com/user/edwardk/revisiting-matrix-multiplication-pa… 12. http://www.reddit.com/r/haskell/comments/1kecqt/revisiting_matrix_multiplic… 13. https://www.fpcomplete.com/user/edwardk/revisiting-matrix-multiplication-pa… 14. http://www.reddit.com/r/haskell/comments/1khu61/revisiting_matrix_multiplic… 15. http://typesandkinds.wordpress.com/2013/08/15/roles-a-new-feature-of-ghc/ 16. http://www.reddit.com/r/haskell/comments/1kgivs/roles_a_new_feature_of_ghc_… 17. https://personal.cis.strath.ac.uk/adam.gundry/thesis/ 18. http://www.reddit.com/r/haskell/comments/1k6g0w/type_inference_haskell_and_… 19. https://speakerdeck.com/mchakravarty/embedded-languages-for-high-performanc… 20. http://www.reddit.com/r/haskell/comments/1kc9j1/embedded_languages_for_high… 21. http://mazzo.li/posts/graph-drawing.html 22. http://www.reddit.com/r/haskell/comments/1kb36j/graph_drawing_with_gloss/ 23. http://nurpax.github.io/posts/2013-08-17-sqlite-simple-benchmarking.html 24. http://www.reddit.com/r/haskell/comments/1kjpwz/benchmarking_sqlitesimple_a… 25. http://stackoverflow.com/questions/2395697/haskell-newbie-question-what-is-… 26. http://www.reddit.com/r/haskell/comments/1k7i4c/what_is_lifting_explained_o… 27. http://parenz.wordpress.com/2013/08/17/ghc-api-interpreted-compiled-and-pac… 28. http://www.reddit.com/r/haskell/comments/1kjl3h/ghc_api_interpreted_compile… 29. http://lists.libsdl.org/pipermail/sdl-libsdl.org/2013-August/089854.html 30. http://www.reddit.com/r/haskell/comments/1k9wh8/time_to_start_writing_on_th… 31. https://www.fpcomplete.com/business/competition/competition-overview 32. http://www.reddit.com/r/haskell/comments/1k8ox7/fp_completes_haskell_progra… 33. https://www.fpcomplete.com/user/tel/pretext-by-experiments-and-guesses 34. http://www.reddit.com/r/haskell/comments/1kkoup/pretext_by_experiments_and_… 35. http://stackoverflow.com/questions/18200283/haskell-random-numbers-suddenly… 36. http://stackoverflow.com/questions/18192184/why-does-ghc-generate-so-many-u… 37. https://plus.google.com/105107667630152149014/about

1 0

Yet Another Forkable Class
by John ExFalso 21 Aug '13

21 Aug '13

TLDR: New forkable monad/transformer suggestion http://pastebin.com/QNUVL12v(hpaste is down) Hi, There are a dozen packages on hackage defining a class for monads that can be forked, however none of these are modular enough to be useful in my opinion. In particular the following are not addressed: 1. Cases when the child thread's monad is different from the parent's 2. Monad transformers (this is somewhat addressed with Control.Monad.Trans.Control) I will try to demonstrate both issues with an example. 1. WebSockets WebSockets is a monad that cannot itself be forked. This is because at any given time there should only be a single thread listening on a websocket. However there is a reasonable monad that can be forked off, namely one that can send to the websocket - one that has access to the Sink. So first off a "Forkable" class should not look like this: class (MonadIO m, MonadIO n) => Forkable m where fork :: m () -> m ThreadId But rather like this: class Forkable m n where fork :: n () -> m ThreadId For our example the instance would be instance (Protocol p) => Forkable (WebSockets p) (ReaderT (Sink p) IO) where fork (ReaderT f) = liftIO . forkIO . f =<< getSink Another example would be a child that should not be able to throw errors as opposed to the parent thread. 2. ReaderT Continuing from the previous example to demonstrate the need to distinguish forkable transformers. Say we have some shared state S that both parent and child should have access to: type Parent p = ReaderT (TVar S) (WebSockets p) type Child p = ReaderT (TVar S) (ReaderT (Sink p) IO) The "forkability" of Child from Parent should be implied, however with Forkable we have to write a separate instance. So what I suggest is a second class: class ForkableT t where forkT :: (Forkable m n) => t n () -> t m ThreadId And then: instance ForkableT (ReaderT r) where forkT (ReaderT f) = ReaderT $ fork . f We can also introduce a default for Forkable that uses a ForkableT instance: class (MonadIO m, MonadIO n) => Forkable m n where fork :: n () -> m ThreadId default fork :: ForkableT t => t n () -> t m ThreadId fork = forkT instance (Forkable m n) => Forkable (ReaderT r m) (ReaderT r n) This means Child is automatically Forkable from Parent, no need to write a specific case for our specific monads (and if we newtype it we can use -XGeneralizedNewtypeDeriving) Note how MonadTransControl already solves the specific problem of lifting a forking operation into ReaderT. However consider ResourceT from Control.Monad.Resource: it is basically a ReaderT, however in order to safely deallocate resources when sharing reference counting is needed. This means a simple lift would not suffice. We can nevertheless provide a default ForkableT based on MonadTransControl: class ForkableT t where forkT :: (Forkable m n) => t n () -> t m ThreadId default forkT :: (MonadTransControl t, Forkable m n) => t n () -> t m ThreadId forkT t = liftWith $ \run -> fork $ run t >> return () Actually resourcet's reference counting resourceForkIO also nicely demonstrates the first problem: type Parent p = ResourceT (WebSockets p) type Child p = ResourceT (ReaderT (Sink p) IO) Note how we cannot use resourceForkIO without touching the underlying monads. What do you think? Is there already an established way of modular forking? I wouldn't like to litter hackage with another unusable Forkable class:)

1 0

ANN: hspec-test-framework - Run test-framework tests with Hspec
by Simon Hengel 21 Aug '13

21 Aug '13

Hi, I just released hspec-test-framework[1] and hspec-test-framework-th[2] to Hackage. They can be used to run test-framework tests with Hspec unmodified. This can also be used to work around test-framework's incompatibility with QuickCheck-2.6 and base-4.7.0 ;) Have a look at the README for usage instructions: https://github.com/sol/hspec-test-framework#readme Cheers, Simon [1] http://hackage.haskell.org/package/hspec-test-framework [2] http://hackage.haskell.org/package/hspec-test-framework-th

4 4

haskore -> lilypond -> typesetting?
by Johannes Waldmann 21 Aug '13

21 Aug '13

I tried using lilypond ( http://www.lilypond.org/ ) for typesetting of sheet music. While the output looks nice, the input language IMHO is quite horrible, because the underlying data/execution model is underspecified. For some parts, it tries to describe the logical structure of the score; but for others, the layout; and in addition it has several non-obvious context-dependencies (but see below), preventing modularity. Is there a better option? E.g., starting from a clear mathematical model, as in Haskore, and use lilypond only as a PDF rendering engine? Do I want hly / hts perhaps? http://rd.slavepianos.org/?t=hly As I see it, the main high-level design problem is that the source language needs partial evaluation annotations for abstractions applications: sometimes they should be expanded (for MIDI rendering, always) and sometimes not (in typesetting, to create repetition marks instead of actually repeating notes). PS: I agree that some of lilypond's context dependencies (relative pitch, implicit note length) do really save large amounts of tedious typing: "c4 e g a c1" is much more economical than "[c 1 qn, e 1 qn, g 1 qn , a 1 qn, c 2 fn]" which I guess is the Haskore equivalent.

3 2

What am I missing? Cycle in type synonym declarations
by David Fox 21 Aug '13

21 Aug '13

This file gives me the error "Cycle in type synonym declarations" Can anyone tell me why? I'm just trying to write a function to create a type that is a FooT with the type parameter fixed. {-# LANGUAGE TemplateHaskell #-} import Language.Haskell.TH (Q, Dec, TypeQ) data FooT a = FooT a foo :: TypeQ -> Q [Dec] foo t = [d| type Bar = FooT $t |]

4 4

abs minBound < (0 :: Int) && negate minBound == (minBound :: Int)
by Nicolas Frisby 21 Aug '13

21 Aug '13

The docs at http://www.haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:g… give a NB mentioning that (abs minBound == minBound) is possible for fixed-width types. This holds, for example, at Int. It is also the case that (negate minBound == minBound). Two questions: 1) This behavior surprised me. Does it surprise enough people to include a warning in the Haddock for abs and negate? IE Here. http://www.haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#t:N… 2) Is this a common behavior in other languages? My tinkering with gcc suggests it does not support the value -2^63, but instead bottoms out at (-2^63+1). Thanks.

7 11

ANN: Google co-sponsoring ZuriHac 2013
by Bas van Dijk 21 Aug '13

21 Aug '13

Dear Haskellers, I would like to remind you that the Zurich FP Afternoon (with a keynote by Simon Marlow) is taking place next week (13:00, Thursday, 29 August) and is directly followed by the ZuriHac 2013 Haskell Hackathon [1]. There are still some places available at both events -- you're welcome to register at: http://bit.ly/ZuriHac2013Reg Google Switzerland will co-sponsor the FP Afternoon and ZuriHac 2013 together with ETH Zurich and Erudify. Their sponsorship means we can give each attendee a free t-shirt (picture on the wikipage [2]) and free food and drinks during the FP Afternoon. Google is also offering a quiz during the FP Afternoon with prizes including a Galaxy S4 mini. I hope to see you there! Bas [1] http://www.haskell.org/haskellwiki/ZuriHac2013 [2] http://www.haskell.org/haskellwiki/ZuriHac2013#News

1 0

Ideas on a fast and tidy CSV library
by Justin Paston-Cooper 21 Aug '13

21 Aug '13

Dear All, Recently I have been doing a lot of CSV processing. I initially tried to use the Data.Csv (cassava) library provided on Hackage, but I found this to still be too slow for my needs. In the meantime I have reverted to hacking something together in C, but I have been left wondering whether a tidy solution might be possible to implement in Haskell. I would like to build a library that satisfies the following: 1) Run a function <<f :: a_1 -> ... -> a_n -> m (Maybe (b_1, ..., b_n))>>, with <<m>> some monad and the <<a>>s and <<b>>s being input and output. 2) Be able to specify a maximum record string length and output record string length, so that the string buffers used for reading and outputting lines can be reused, preventing the need for allocating new strings for each record. 3) Allocate only once, the memory where the parsed input values, and output values are put. 4) The library's main function should take some kind of data structure describing the types of the function, the function itself and the filenames of input and output (could also be stdin/stdout). I am not sure yet what would be that best value of <<m>>. I would like to most importantly efficiently, and if possible, purely allow changes in state to a number of variables, such as an aggregation over a certain field in the input. I do not currently have knowledge of the FFI, and how it might be used in this case. I would appreciate any suggestions as to where I should look further. Regards, Justin Paston-Cooper

3 6

PPC binaries for GHC 7.x?
by Rogan Creswick 21 Aug '13

21 Aug '13

Does anyone have PPC binaries for GHC 7.x? I've been trying to help a PPC user compile a large haskell application, and it (and it's dependencies) require a newer ghc; the latest ppc binaries we've found are for 6.10, and we have been unable to compile a never ghc from source (6.12 /almost/ worked, but eventually failed with a linker error that appears to be unsolvable in the near-term). Thanks! Rogan

2 2

GHC and backwards compatibility
by Henning Thielemann 21 Aug '13

21 Aug '13

If you use $ cabal install --constraint="array installed" then cabal-install is forced to use the installed version of array. If a package conflicts with this version, then it will report the conflicting packages.

1 0