- Haskell-Cafe - Haskell.org

Tor project
by Vasili I. Galchin 31 Jul '14

31 Jul '14

Haskell contributions to http://www.torproject.org .. .any thoughts ??

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Intuition to understand poor man's concurrency
by martin 31 Jul '14

31 Jul '14

Hello all, I am trying to understand the ideas of Koen Klaessen, published in Functional Pearls: "A poor man's concurrency" (1993). The code in the paper doesn't compile. E.g. uses "lambda dot" instead of "labmda arrow", i.e. the way the labmda calculus guys write things. Was that ever legal haskell or is this the result of some lhs2lex pretty printer? Anyways, I believe I was able to convert that into modern haskell syntax - at least it compiles. But I have trouble to understand the Monad instance presented there. Could anyobody walk me through the bind function? But even more important: how do you guys develop an intuition about what the bind operator does in a specific monad. I saw a Google tech talk where Douglas Crockford explains mondads in terms of Objects in javascript (https://www.youtube.com/watch?v=b0EF0VTs9Dc) which was certainly enlightening, but I couldn't apply it to this particular modad. I also tried using something like Data Flow Diagrams. This kinda works for simple Mondads such as the state mondad, but I couldn't apply it to the concurrency mondad. In general DFDs are not very good when it comes to higher order functions. In any case here is the code. I made it more verbose than necessary in order to understand it (bind was originally just one line), but to no avail. newtype C m a = C ((a -> Action m) -> Action m) instance Monad m => Monad (C m) where (C m) >>= k = C cont where cont c = m (\a -> let C h = k a in h c) return x = C $ \c -> c x data Action m = Atom (m (Action m)) | Fork (Action m) (Action m) | Stop

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Newtype derivation understanding problem
by Konstantine Rybnikov 31 Jul '14

31 Jul '14

Excuse me if the question is silly. Thought this MonadCatchIO will be removed in next version of snap, current version has it and I can't understand this derivation: [0] L.Lensed seems like just a newtype wrapper around a function [1], and there is no instance of MonadCatchIO for a function. How is it derived? [0]: https://github.com/snapframework/snap/blob/0.13.2.8/src/Snap/Snaplet/Intern… [1]: https://github.com/snapframework/snap/blob/0.13.2.8/src/Snap/Snaplet/Intern…

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Haskell Weekly News: Issue 300
by Daniel Santa Cruz 31 Jul '14

31 Jul '14

Welcome to issue 300 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers from July 13 to 26, 2014 Looks like we are chuck-full of goodies this time around! Enjoy! Quotes of the Week * Cale: Functions aren't monads, the type constructor (->) e is a monad * glguy: There's no achievement for using all the operators * benmachine: adoption by lots of people may stunt progress of haskell, but it will probably help the progress of people Top Reddit Stories * Somehow, this happened. Haskell Ryan Gosling. Domain: haskellryangosling.tumblr.com, Score: 144, Comments: 31 Original: [1] http://goo.gl/IQ2f3s On Reddit: [2] http://goo.gl/H24aRa * Papers every haskeller should read Domain: self.haskell, Score: 104, Comments: 35 Original: [3] http://goo.gl/R7uGSB On Reddit: [4] http://goo.gl/R7uGSB * Strict Language Pragma Proposal Domain: ghc.haskell.org, Score: 86, Comments: 95 Original: [5] http://goo.gl/Yq28yt On Reddit: [6] http://goo.gl/BlmXpA * Announcing engine-io and socket-io for Haskell Domain: ocharles.org.uk, Score: 81, Comments: 7 Original: [7] http://goo.gl/PGW8Y8 On Reddit: [8] http://goo.gl/IsGRGi * The Haskell Cast #8 - Ollie Charles on 24 Days of Hackage and Nix Domain: haskellcast.com, Score: 70, Comments: 12 Original: [9] http://goo.gl/jBc9bk On Reddit: [10] http://goo.gl/syhwPd * Blazing Fast HTML - Virtual DOM in Elm Domain: elm-lang.org, Score: 60, Comments: 28 Original: [11] http://goo.gl/3luwtw On Reddit: [12] http://goo.gl/88zNkZ * Idris 0.9.14 released, with updated JavaScript backend, quasiquotes, and lots of internal cleanups and improvements Domain: idris-lang.org, Score: 55, Comments: 11 Original: [13] http://goo.gl/wmW3RM On Reddit: [14] http://goo.gl/fN28z6 * hoodle 0.3 released - free pen note taking program Domain: ianwookim.org, Score: 53, Comments: 15 Original: [15] http://goo.gl/jauFC On Reddit: [16] http://goo.gl/upMnf6 * This just in, from my local GHC/Cabal checkout... (re: Cabal Hell) Domain: self.haskell, Score: 52, Comments: 13 Original: [17] http://goo.gl/tD2yOg On Reddit: [18] http://goo.gl/tD2yOg * Haskell for all: Equational reasoning at scale Domain: haskellforall.com, Score: 52, Comments: 29 Original: [19] http://goo.gl/BZtZ4R On Reddit: [20] http://goo.gl/xKxCM3 * Complete roadmap from total novice to Haskell mastery? Domain: self.haskell, Score: 50, Comments: 74 Original: [21] http://goo.gl/5y1lXA On Reddit: [22] http://goo.gl/5y1lXA * Write webservices around databases with 0 boilerplate: announcing servant 0.1 Domain: alpmestan.com, Score: 50, Comments: 16 Original: [23] http://goo.gl/vzz8aT On Reddit: [24] http://goo.gl/qTty6u * Mutable Algorithms in Immutable Languages, Part 3 Domain: tel.github.io, Score: 49, Comments: 2 Original: [25] http://goo.gl/uG5Bds On Reddit: [26] http://goo.gl/qrdk6Y * Best Practices for Avoiding Cabal Hell Domain: softwaresimply.blogspot.com, Score: 47, Comments: 13 Original: [27] http://goo.gl/OqG241 On Reddit: [28] http://goo.gl/fapyz2 * Applicative (Make) vs Monadic (Shake) build systems Domain: neilmitchell.blogspot.co.uk, Score: 47, Comments: 7 Original: [29] http://goo.gl/CPyzpd On Reddit: [30] http://goo.gl/2eW8Ed * Nemnem - Haskell source hyperlinker Domain: robinp.github.io, Score: 41, Comments: 33 Original: [31] http://goo.gl/2msbLR On Reddit: [32] http://goo.gl/R6hBOl * Let me tell you about the types of data Domain: tel.github.io, Score: 41, Comments: 51 Original: [33] http://goo.gl/ZjJelo On Reddit: [34] http://goo.gl/bbmXSW * Lens Tutorial - Introduction (part 1) Domain: blog.jakubarnold.cz, Score: 39, Comments: 11 Original: [35] http://goo.gl/yQRoPY On Reddit: [36] http://goo.gl/fnbZ0F * Help - I wrote some Haskell code, it works, but it's slower than Python. What did I do wrong? Domain: self.haskell, Score: 39, Comments: 43 Original: [37] http://goo.gl/CG13CR On Reddit: [38] http://goo.gl/CG13CR * IntrinsicSuperclasses for Haskell (new proposal for default superclass instances by Conor McBride) Domain: ghc.haskell.org, Score: 37, Comments: 10 Original: [39] http://goo.gl/vDYUQh On Reddit: [40] http://goo.gl/vZjHXS * hackage-diff: Compare the public API of different versions of a Hackage library Domain: self.haskell, Score: 37, Comments: 32 Original: [41] http://goo.gl/y682CB On Reddit: [42] http://goo.gl/y682CB * How do you avoid the Cabal Hell™? Domain: self.haskell, Score: 36, Comments: 30 Original: [43] http://goo.gl/9uS54H On Reddit: [44] http://goo.gl/9uS54H * Edward Kmett on Hask Domain: youtu.be, Score: 35, Comments: 34 Original: [45] http://goo.gl/1lfywk On Reddit: [46] http://goo.gl/D0AuFQ * Mutable Algorithms in Immutable Languages, Part 2 Domain: tel.github.io, Score: 33, Comments: 1 Original: [47] http://goo.gl/g0ylvT On Reddit: [48] http://goo.gl/jwwr87 * Multi-instance packages status report Domain: permalink.gmane.org, Score: 32, Comments: 7 Original: [49] http://goo.gl/JSPx39 On Reddit: [50] http://goo.gl/2rzPJL * Slides from Conan Elliott's workshop on Denotational Design: from meanings to programs Domain: conal.net, Score: 32, Comments: 6 Original: [51] http://goo.gl/GwO4Fv On Reddit: [52] http://goo.gl/0u9KVl * Reactive-banana anti-tutorial Domain: gelisam.blogspot.ca, Score: 31, Comments: 23 Original: [53] http://goo.gl/D7mv4X On Reddit: [54] http://goo.gl/shMY0N * Intro to Machines & Arrows Part 2: 'Auto' as Category, Applicative & Arrow, ft. locally stateful compositions; Further down the road to an Arrowized FRP implementation. Domain: blog.jle.im, Score: 28, Comments: 9 Original: [55] http://goo.gl/xkwocR On Reddit: [56] http://goo.gl/phqXn8 Top StackOverflow Questions * Examples of histomorphisms in Haskell votes: 16, answers: 2 Read on SO: [57] http://goo.gl/ZmGzI6 * Plan B, or what's the opposite of Maybe's >>=? votes: 15, answers: 2 Read on SO: [58] http://goo.gl/rYWlzQ * Let-renaming function breaks code votes: 14, answers: 2 Read on SO: [59] http://goo.gl/R9CGyL * Pattern matching on rank-2 type votes: 14, answers: 3 Read on SO: [60] http://goo.gl/s0IszK * Haskell algorithm advice and suggestions for alternate solutions votes: 14, answers: 1 Read on SO: [61] http://goo.gl/2l23N8 * Why can't Haskell be tricked into performing IO operations by using strict evaluation? votes: 14, answers: 2 Read on SO: [62] http://goo.gl/Ey9795 * How does lifting (in a functional programming context) relate to category theory? votes: 14, answers: 1 Read on SO: [63] http://goo.gl/esTTbO Until next time, [64]+Daniel Santa Cruz References 1. http://haskellryangosling.tumblr.com/ 2. http://www.reddit.com/r/haskell/comments/2avnc0/somehow_this_happened_haske… 3. http://www.reddit.com/r/haskell/comments/2blsqa/papers_every_haskeller_shou… 4. http://www.reddit.com/r/haskell/comments/2blsqa/papers_every_haskeller_shou… 5. https://ghc.haskell.org/trac/ghc/wiki/StrictPragma 6. http://www.reddit.com/r/haskell/comments/2bi69g/strict_language_pragma_prop… 7. https://ocharles.org.uk/blog/posts/2014-07-13-announcing-socket-io-for-hask… 8. http://www.reddit.com/r/haskell/comments/2aks91/announcing_engineio_and_soc… 9. http://www.haskellcast.com/episode/008-ollie-charles-on-24-days-of-hackage-… 10. http://www.reddit.com/r/haskell/comments/2bbc82/the_haskell_cast_8_ollie_ch… 11. http://elm-lang.org/blog/Blazing-Fast-Html.elm 12. http://www.reddit.com/r/haskell/comments/2bam6p/blazing_fast_html_virtual_d… 13. http://www.idris-lang.org/idris-0-9-14-released/ 14. http://www.reddit.com/r/haskell/comments/2aw6vl/idris_0914_released_with_up… 15. http://ianwookim.org/hoodle/ 16. http://www.reddit.com/r/haskell/comments/2aotln/hoodle_03_released_free_pen… 17. http://www.reddit.com/r/haskell/comments/2b7odl/this_just_in_from_my_local_… 18. http://www.reddit.com/r/haskell/comments/2b7odl/this_just_in_from_my_local_… 19. http://www.haskellforall.com/2014/07/equational-reasoning-at-scale.html 20. http://www.reddit.com/r/haskell/comments/2b7xwx/haskell_for_all_equational_… 21. http://www.reddit.com/r/haskell/comments/2ali12/complete_roadmap_from_total… 22. http://www.reddit.com/r/haskell/comments/2ali12/complete_roadmap_from_total… 23. http://alpmestan.com/posts/2014-07-26-announcing-servant.html 24. http://www.reddit.com/r/haskell/comments/2bs536/write_webservices_around_da… 25. http://tel.github.io/2014/07/15/mutable_algorithms_in_immutable_languages_p… 26. http://www.reddit.com/r/haskell/comments/2ase1q/mutable_algorithms_in_immut… 27. http://softwaresimply.blogspot.com/2014/07/haskell-best-practices-for-avoid… 28. http://www.reddit.com/r/haskell/comments/2ameew/best_practices_for_avoiding… 29. http://neilmitchell.blogspot.co.uk/2014/07/applicative-vs-monadic-build-sys… 30. http://www.reddit.com/r/haskell/comments/2blex3/applicative_make_vs_monadic… 31. http://robinp.github.io/nemnem/ 32. http://www.reddit.com/r/haskell/comments/2ata3n/nemnem_haskell_source_hyper… 33. http://tel.github.io/2014/07/23/types_of_data/ 34. http://www.reddit.com/r/haskell/comments/2bj7it/let_me_tell_you_about_the_t… 35. http://blog.jakubarnold.cz/2014/07/14/lens-tutorial-introduction-part-1.html 36. http://www.reddit.com/r/haskell/comments/2ap02f/lens_tutorial_introduction_… 37. http://www.reddit.com/r/haskell/comments/2brxwp/help_i_wrote_some_haskell_c… 38. http://www.reddit.com/r/haskell/comments/2brxwp/help_i_wrote_some_haskell_c… 39. https://ghc.haskell.org/trac/ghc/wiki/IntrinsicSuperclasses 40. http://www.reddit.com/r/haskell/comments/2avs16/intrinsicsuperclasses_for_h… 41. http://www.reddit.com/r/haskell/comments/2b5jfd/hackagediff_compare_the_pub… 42. http://www.reddit.com/r/haskell/comments/2b5jfd/hackagediff_compare_the_pub… 43. http://www.reddit.com/r/haskell/comments/2al3vx/how_do_you_avoid_the_cabal_… 44. http://www.reddit.com/r/haskell/comments/2al3vx/how_do_you_avoid_the_cabal_… 45. http://youtu.be/Klwkt9oJwg0 46. http://www.reddit.com/r/haskell/comments/2b3rk5/edward_kmett_on_hask/ 47. http://tel.github.io/2014/07/13/mutable_algorithms_in_immutable_languages_p… 48. http://www.reddit.com/r/haskell/comments/2al4cw/mutable_algorithms_in_immut… 49. http://permalink.gmane.org/gmane.comp.lang.haskell.ghc.devel/5413 50. http://www.reddit.com/r/haskell/comments/2bbys6/multiinstance_packages_stat… 51. http://conal.net/talks/lambdajam-2014.pdf 52. http://www.reddit.com/r/haskell/comments/2brioe/slides_from_conan_elliotts_… 53. http://gelisam.blogspot.ca/2014/07/reactive-banana-anti-tutorial.html 54. http://www.reddit.com/r/haskell/comments/2auswf/reactivebanana_antitutorial/ 55. http://blog.jle.im/entry/auto-as-category-applicative-arrow-intro-to-machin… 56. http://www.reddit.com/r/haskell/comments/2aot3i/intro_to_machines_arrows_pa… 57. http://stackoverflow.com/questions/24884475/examples-of-histomorphisms-in-h… 58. http://stackoverflow.com/questions/24902463/plan-b-or-whats-the-opposite-of… 59. http://stackoverflow.com/questions/24724738/let-renaming-function-breaks-co… 60. http://stackoverflow.com/questions/24744294/pattern-matching-on-rank-2-type 61. http://stackoverflow.com/questions/24748750/haskell-algorithm-advice-and-su… 62. http://stackoverflow.com/questions/24775528/why-cant-haskell-be-tricked-int… 63. http://stackoverflow.com/questions/24856963/how-does-lifting-in-a-functiona… 64. https://plus.google.com/105107667630152149014/about

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www.galois.com vs Google??
by Vasili I. Galchin 31 Jul '14

31 Jul '14

Hello Haskellers(and all FPL people), I just ran across the following effort by Google: http://blogs.techworld.com/war-on-error/2014/07/googles-project-zero-flaw-p… It seems to me that www.galois.com(www.janestreetcapital.com) realizes that the solution to software correctness problems doesn't lie with uncontrolled "mutability" ... Question: does Google concur with www.galois.com or google's effort just "more of the same"?? Vasili

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The ML Family workshop: program and the 2nd call for participation
by oleg＠okmij.org 31 Jul '14

31 Jul '14

Higher-order, Typed, Inferred, Strict: ACM SIGPLAN ML Family Workshop Thursday September 4, 2014, Gothenburg, Sweden Call For Participation and Program http://okmij.org/ftp/ML/ML14.html Early registration deadline is August 3. Please register at https://regmaster4.com/2014conf/ICFP14/register.php The workshop is conducted in close cooperation with the OCaml Users and Developers Workshop http://ocaml.org/meetings/ocaml/2014/ taking place on September 5. *** Program with short summaries *** (The online version links to the full 2-page abstracts) * Welcome 09:00 * Session 1: Module Systems 09:10 - 10:00 1ML -- core and modules as one (Or: F-ing first-class modules) Andreas Rossberg We propose a redesign of ML in which modules are first-class values. Functions, functors, and even type constructors are one and the same construct. Likewise, no distinction is made between structures, records, or tuples, including tuples over types. Yet, 1ML does not depend on dependent types, and its type structure is expressible in terms of plain System F-omega, in a minor variation of our F-ing modules approach. Moreover, it supports (incomplete) Hindley/Milner-style type inference. Both is enabled by a simple universe distinction into small and large types. Type-level module aliases: independent and equal Jacques Garrigue; Leo White We promote the use of type-level module aliases, a trivial extension of the ML module system, which helps avoiding code dependencies, and provides an alternative to strengthening for type equalities. * Session 2: Beyond Hindley-Milner 10:30 - 11:20 The Rust Language and Type System (Demo) Felix Klock; Nicholas Matsakis Rust is a new programming language for developing reliable and efficient systems. It is designed to support concurrency and parallelism in building applications and libraries that take full advantage of modern hardware. Rust's static type system is safe and expressive and provides strong guarantees about isolation, concurrency, and memory safety. Rust also offers a clear performance model, making it easier to predict and reason about program efficiency. One important way it accomplishes this is by allowing fine-grained control over memory representations, with direct support for stack allocation and contiguous record storage. The language balances such controls with the absolute requirement for safety: Rust's type system and runtime guarantee the absence of data races, buffer overflows, stack overflows, and accesses to uninitialized or deallocated memory. In this demonstration, we will briefly review the language features that Rust leverages to accomplish the above goals, focusing in particular on Rust's advanced type system, and then show a collection of concrete examples of program subroutines that are efficient, easy for programmers to reason about, and maintain the above safety property. Doing web-based data analytics with F# (Informed Position) Tomas Petricek; Don Syme With type providers that integrate external data directly into the static type system, F# has become a fantastic language for doing data analysis. Rather than looking at F# features in isolation, this paper takes a holistic view and presents the F# approach through a case study of a simple web-based data analytics platform. * Session 3: Verification 11:40 - 12:30 Well-typed generic smart-fuzzing for APIs (Experience report) Thomas Braibant; Jonathan Protzenko; Gabriel Scherer In spite of recent advances in program certification, testing remains a widely-used component of the software development cycle. Various flavors of testing exist: popular ones include unit testing, which consists in manually crafting test cases for specific parts of the code base, as well as quickcheck-style testing, where instances of a type are automatically generated to serve as test inputs. These classical methods of testing can be thought of as internal testing: the test routines access the internal representation of the data structures to be checked. We propose a new method of external testing where the library only deals with a module interface. The data structures are exported as abstract types; the testing framework behaves like regular client code and combines functions exported by the module to build new elements of the various abstract types. Any counter-examples are then presented to the user. Furthermore, we demonstrate that this methodology applies to the smart-fuzzing of security APIs. Improving the CakeML Verified ML Compiler Ramana Kumar; Magnus O. Myreen; Michael Norrish; Scott Owens The CakeML project comprises a mechanised semantics for a subset of Standard ML and a formally verified compiler. We will discuss our plans for improving and applying CakeML in four directions: optimisations, new primitives, a library, and verified applications. * Session 4: Implicits 14:00 - 14:50 Implicits in Practice (Demo) Nada Amin; Tiark Rompf Popularized by Scala, implicits are a versatile language feature that are receiving attention from the wider PL community. This demo will present common use cases and programming patterns with implicits in Scala. Modular implicits Leo White; Frédéric Bour We propose a system for ad-hoc polymorphism in OCaml based on using modules as type-directed implicit parameters. * Session 5: To the bare metal 15:10 - 16:00 Metaprogramming with ML modules in the MirageOS (Experience report) Anil Madhavapeddy; Thomas Gazagnaire; David Scott; Richard Mortier In this talk, we will go through how MirageOS lets the programmer build modular operating system components using a combination of functors and metaprogramming to ensure portability across both Unix and Xen, while preserving a usable developer workflow. Compiling SML# with LLVM: a Challenge of Implementing ML on a Common Compiler Infrastructure Katsuhiro Ueno; Atsushi Ohori We report on an LLVM backend of SML#. This development provides detailed accounts of implementing functional language functionalities in a common compiler infrastructure developed mainly for imperative languages. We also describe techniques to compile SML#'s elaborated features including separate compilation with polymorphism, and SML#'s unboxed data representation. * Session 6: No longer foreign 16:30 - 18:00 A Simple and Practical Linear Algebra Library Interface with Static Size Checking (Experience report) Akinori Abe; Eijiro Sumii While advanced type systems--specifically, dependent types on natural numbers--can statically ensure consistency among the sizes of collections such as lists and arrays, such type systems generally require non-trivial changes to existing languages and application programs, or tricky type-level programming. We have developed a linear algebra library interface that guarantees consistency (with respect to dimensions) of matrix (and vector) operations by using generative phantom types as fresh identifiers for statically checking the equality of sizes (i.e., dimensions). This interface has three attractive features in particular. (i) It can be implemented only using fairly standard ML types and its module system. Indeed, we implemented the interface in OCaml (without significant extensions like GADTs) as a wrapper for an existing library. (ii) For most high-level operations on matrices (e.g., addition and multiplication), the consistency of sizes is verified statically. (Certain low-level operations, like accesses to elements by indices, need dynamic checks.) (iii) Application programs in a traditional linear algebra library can be easily migrated to our interface. Most of the required changes can be made mechanically. To evaluate the usability of our interface, we ported to it a practical machine learning library (OCaml-GPR) from an existing linear algebra library (Lacaml), thereby ensuring the consistency of sizes. SML3d: 3D Graphics for Standard ML (Demo) John Reppy The SML3d system is a collection of libraries designed to support real-time 3D graphics programming in Standard ML (SML). This paper gives an overview of the system and briefly highlights some of the more interesting aspects of its design and implementation. * Poster presentation, at the joint poster session with the OCaml workshop Nullable Type Inference Michel Mauny; Benoit Vaugon We present a type system for nullable types in an ML-like programming language. We start with a simple system, presented as an algorithm, whose interest is to introduce the formalism that we use. We then extend it as system using subtyping constraints, that accepts more programs. We state the usual properties for both systems. This is work in progress. Program Committee Kenichi Asai Ochanomizu University, Japan Matthew Fluet Rochester Institute of Technology, USA Jacques Garrigue Nagoya University, Japan Dave Herman Mozilla, USA Stefan Holdermans Vector Fabrics, Netherlands Oleg Kiselyov (Chair) University of Tsukuba, Japan Keiko Nakata Tallinn University of Technology, Estonia Didier Remy INRIA Paris-Rocquencourt, France Zhong Shao Yale University, USA Hongwei Xi Boston University, USA

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Deduce issue.
by Magicloud Magiclouds 30 Jul '14

30 Jul '14

Hi, I have code like follow. And compiling gave me error as the last part. What should I do? ``` class WidgetClass w data Widget = forall a. WidgetClass a => Widget a instance WidgetClass Widget lookupWin :: (WidgetClass w) => WINDOW -> w -> Maybe w lookupWin xwin widget = if widgetWindowId widget == xwin then Just widget else foldl (\a (Widget b) -> maybe (lookupWin xwin b) Just a ) Nothing $ widgetChildren widget ``` --- ``` Graphics/HUI/Widget.hs:15:23: Could not deduce (w ~ a) from the context (WidgetClass w) bound by the type signature for lookupWin :: WidgetClass w => WINDOW -> w -> Maybe w at Graphics/HUI/Widget.hs:10:14-54 or from (WidgetClass a) bound by a pattern with constructor Widget :: forall a. WidgetClass a => a -> Widget, in a lambda abstraction at Graphics/HUI/Widget.hs:14:19-26 ‘w’ is a rigid type variable bound by the type signature for lookupWin :: WidgetClass w => WINDOW -> w -> Maybe w at Graphics/HUI/Widget.hs:10:14 ‘a’ is a rigid type variable bound by a pattern with constructor Widget :: forall a. WidgetClass a => a -> Widget, in a lambda abstraction at Graphics/HUI/Widget.hs:14:19 Expected type: Maybe w Actual type: Maybe a Relevant bindings include b :: a (bound at Graphics/HUI/Widget.hs:14:26) a :: Maybe w (bound at Graphics/HUI/Widget.hs:14:16) widget :: w (bound at Graphics/HUI/Widget.hs:11:16) lookupWin :: WINDOW -> w -> Maybe w (bound at Graphics/HUI/Widget.hs:11:1) In the first argument of ‘maybe’, namely ‘(lookupWin xwin b)’ In the expression: maybe (lookupWin xwin b) Just a ``` -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

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redundant loads and saves in code generated for recursive functions?
by Jyotirmoy Bhattacharya 30 Jul '14

30 Jul '14

Dear All, I am new to Haskell so please forgive me if I am asking about something already well-understood. I was trying to understand the performance of my Haskell program compiled with the LLVM backend. I used -ddump-llvm to dump the LLVM assembly and then ran llc -O3 on the resulting file to look at the native assembly. One of the generated function starts off with s5BH_info: # @s5BH_info # BB#0: subq $208, %rsp movq %r13, 200(%rsp) movq %rbp, 192(%rsp) movq %r12, 184(%rsp) movq %rbx, 176(%rsp) movq %r14, 168(%rsp) movq %rsi, 160(%rsp) movq %rdi, 152(%rsp) movq %r8, 144(%rsp) movq %r9, 136(%rsp) movq %r15, 128(%rsp) movss %xmm1, 124(%rsp) movss %xmm2, 120(%rsp) movss %xmm3, 116(%rsp) movss %xmm4, 112(%rsp) movsd %xmm5, 104(%rsp) movsd %xmm6, 96(%rsp) At some point down the line the function makes a tail call to itself and this is the code generated movq %r14, 168(%rsp) movq 200(%rsp), %r13 movq 192(%rsp), %rbp movq 184(%rsp), %r12 movq 176(%rsp), %rbx movq 128(%rsp), %r15 movsd 104(%rsp), %xmm5 addq $208, %rsp jmp s5BH_info So it looks like some values are being moved from registers to the stack only to be immediately moved from the stack to the register on entry to the function. It should be possible to eliminate both the load and the stores. Is this behaviour due to LLVM or GHC? If it is GHC, it this an optimization a newcomer can attempt to implement or are there deep issues here? Jyotirmoy Bhattacharya

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How to update a cabal dependency automatically
by ye yan 30 Jul '14

30 Jul '14

I have been working on a haskell scotty project recently. I have just added configuration support for the project, which added an extra dependency of Control.Lens. My question is: is there a way update .cabal file automatically, just like cabal init but without recreate .cabal file? Thanks in advance.

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early adopters rejoyce! Haskell Platform 2014.2.0.0 Release Candidate 2
by Mark Lentczner 29 Jul '14

29 Jul '14

The long anticipated Haskell Platform 2014.2 release, including GHC 7.8.3, and numerous updated packages, is almost here! We have created "Release Candidate 2" installers for OS X and Windows, and believe, barring show stopper issues, creepers exploding, or unexpected trips to the nether, these will likely be blessed as the final by the end of the week. If you would like to be an early adopter, please try 'em out... - source tarball: haskell-platform-2014.2.0.0-RC2.tar.gz <http://www.ozonehouse.com/mark/platform/haskell-platform-2014.2.0.0-RC2.tar…> - source repo: haskell/haskell-platform at 2014.2.0.0-RC2 <https://github.com/haskell/haskell-platform/tree/2014.2.0.0-RC2> - windows 32bit: hskellPlatform-2014.2.0.0-i386-RC2-setup.exe <http://www.ozonehouse.com/mark/platform/hskellPlatform-2014.2.0.0-i386-RC2-…> - windows 64bit:hskellPlatform-2014.2.0.0-x86_64-RC2-setup.exe <http://www.ozonehouse.com/mark/platform/hskellPlatform-2014.2.0.0-x86_64-RC…> - os x 64bit: Haskell Platform 2014.2.0.0 64bit RC2.signed.pkg <http://www.ozonehouse.com/mark/platform/Haskell%20Platform%202014.2.0.0%206…> - travis-ci build: haskell/haskell-platform <https://travis-ci.org/haskell/haskell-platform> *Short Notes for RC2, read these if nothing else:* *General* - includes GHC 7.8.3 - includes long awaited updates to OpenGL & GLUT packages - most other packages brought up to date (or nearly so) - website is still woefully out of date... will work on it this week *Windows* *Extra thanks to Randy Polen for burning the midnight-oil to get ths out* - removed unneeded python (et al) files from the GHC bindist for 64-bit Windows (referenced in GHC ticket #9014 <https://ghc.haskell.org/trac/ghc/ticket/9014>) - added HTML "view source" pages for the GHC packages that was missing from the GHC bindist for both 32- and 64-bit Windows. *Mac OS X* *If you installed RC1 or older versions of the platform, you can remove them first with the command *sudo uninstall-hs only 7.8.3 --remove *Run it without *--remove* to see what it will do first, it if you like. You can also just install this one right on top of RC1. If you have older 7.8.3 installs from other distributions (bindist, built from src, etc...) you may want to manually remove them first... YMMV.**. Yoda says: "When doubt you have, the $PATH examine, you must!"* - works on 10.6! and 10.7, 10.8, and 10.9; with gcc or clang based Xcodes - works on 10.10!!! (Yosemite, developer preview 4 release) - works with clang out of the box, and haddock build issues with clang, fixed. *N.B.: If you have hand installed cabal 1.20, please be sure it uses Cabal 1.20.0.2 to fix the haddock issue. The platform on OS X includes cabal 1.18, using 1.18.1.4, which fixes the problem.* *Source tarball* - uses new build machinery... all the kinks may not be worked out, and not all build featuers (notably a --prefix command) are implemented yet. This doesn't affect the content of the Platform, though. - generic *nix builds, and specific distribution builds are forth coming (though feel free to chip in and help!) *Longer Notes for RC2:* *General* - Built with the new shake based build system - cgi package not included as it doesn't build under 7.8, and no word from the maintainer in quite some time - hscolour is included as a tool, mostly as it is used to build the platform itself on win and mac... but technically it isn't part of the platform - the haskell-platform package itself is not in these releases... it never contained anything other than dependencies. *Windows* - The Haskell Platform now provides a native Windows 64-bit installation (haskell-platform issue #54 <https://github.com/haskell/haskell-platform/issues/54>) - All included packages built without --enable-split-objs (GHC 7.8 FAQ <https://ghc.haskell.org/trac/ghc/wiki/GHC-7.8-FAQ#Cabal>) - All included packages built without --enable-shared (GHC ticket #8228 <https://ghc.haskell.org/trac/ghc/ticket/8228>) - If other Haskell Platform installations are detected during the installation of Haskell Platform 2014.2.0.0, a warning is displayed to the user that this is not recommended since problems will arise due to how the PATH is used in many cases to find ancillary build tools - Using ghci to build an executable that links against a DLL may result in numerous warnings about symbols (GHC ticket #9297 <https://ghc.haskell.org/trac/ghc/ticket/9297>) - The Haskell Platform for Windows, both 32-bit and 64-bit, now includes an updated version of the OpenGL Utility Toolkit (GLUT) from the FreeGLUT project, utilizing the pre-built distribution from http://www.transmissionzero.co.uk/software/freeglut-devel/ with the MinGW build (freeglut-MinGW-2.8.1-1.mp.zip). (haskell-platform issue #81 <https://github.com/haskell/haskell-platform/issues/81>) *Mac OS X* - file layout on the Mac has improved. In particular, executables are now installed directly in $prefix/bin dirs, rather than within the package dir - Distributed with a build of 7.8.3 that differs from the released bindist in two ways: a) it was built split-objs for smaller resulting executables, b) it includes Cabal-1.18.1.4 which fixes a particularly nasty problem with haddock, -XCPP, and clang based systems. This ghc-7.8.3 bindist is available as well: - ghc-7.8.3-x86_64-apple-darwin-r3.tar.bz2 <http://www.ozonehouse.com/mark/platform/ghc-7.8.3-x86_64-apple-darwin-r3.ta…> - haddocks finally cross link between packages correctly - includes a new experimental activate-hs command that can switch between multiple installed versions of the platform - includes a slightly updated uninstall-hs command - the cabal command is wrapped to provide a smoother file layout on the Mac... the wrapping only updates the ~/.cabal/config file the first time you run it.. please pay attention to its output - if you have a custom config file, you'll want to update it, as Cabal's defaults have changed *Timetable* - End of this week (from my vacation, I'll point out), we'll declare success and rename the files. — Mark *SHA-256 sums :* 62f39246ad95dd2aed6ece5138f6297f945d2b450f215d074820294310e0c48a Haskell Platform 2014.2.0.0 64bit RC2.signed.pkg 7c7d3585e89e1407461efea29dcaa9628c3be3c47d93a13b5a4978046375e4fd haskell-platform-2014.2.0.0-RC2.tar.gz 6eedd76aafb266d9a09baff80cd2973498ab59195c771f7cd64425d40be29c49 hskellPlatform-2014.2.0.0-i386-RC2-setup.exe b22115ed84d1f7e747d7f0b47e32e1489e4a24613d69c91df4ae32052f88b130 hskellPlatform-2014.2.0.0-x86_64-RC2-setup.exe

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