April 2012 - Haskell-Cafe

Administrative questions about Haskell GSOC (Google Summer of Code)
by Ryan Newton 19 Apr '12

19 Apr '12

Hello, As someone new to the GSOC system there are some things I'm confused about. There's a bunch of info out there, and I certainly haven't read all of it. Who are the "program administrator" and "organization administrator" for the Haskell organization? Those roles are mentioned in the manual here: http://en.flossmanuals.net/GSoCMentoring/what-is-gsoc/ Presumably those are the people that actually click "accept" on the melange site: https://google-melange.appspot.com/gsoc/homepage/google/gsoc2012 How many slots did we get? Is the system just to accept the top rated student proposals? If I can find the answers to these questions maybe I can update the Wiki for next year so it has a bit more info for mentors and about the process as well as students: http://hackage.haskell.org/trac/summer-of-code/wiki/Soc2012 Thank you, -Ryan

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Rewrite rules for enumFromTo
by Michael Snoyman 19 Apr '12

19 Apr '12

Hi all, Following a little thread on Reddit[1], I'm trying to add rewrite rules to conduit to make some simple usages of `yield` more efficient. I've pushed these changes to a branch on Github[2]. However, I'm not able to fully optimize the following program: import Data.Conduit import qualified Data.Conduit.List as CL main :: IO () main = do x <- mapM_ yield [1..1000] $$ CL.fold (+) 0 print (x :: Int) Ideally, I would like to rewrite the entirety of `mapM_ yield [1..1000]` to `Data.Conduit.List.enumFromTo 1 1000` and thereby avoid the intermediate list. However, whenever I add such a rule, it doesn't fire. Instead, -ddump-rule-firings tells me: Rule fired: Class op enumFromTo Rule fired: mapM_ yield Rule fired: Class op + Rule fired: Class op >>= Rule fired: Class op show Rule fired: eftIntList I'm quite a novice at rewrite rules; can anyone recommend an approach to get my rule to fire first? Thanks, Michael PS: In case you're wondering, the `mapM_ yield` rule turns `mapM_ yield` into `yieldMany`. So ideally, I'd like to have another rule that turns `yieldMany [x..y]` into `Data.Conduit.List.enumFromTo x y`. [1] http://www.reddit.com/r/haskell/comments/sdzmx/many_ways_to_skin_a_conduit/… [2] https://github.com/snoyberg/conduit/tree/rewrite

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Haskell Weekly News: Issue 223
by Daniel Santa Cruz 18 Apr '12

18 Apr '12

Welcome to issue 223 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of April 8 to 14, 2012. Quotes of the Week * merijn: I'm tempted to just do unsafePerformIO newEmptyMVar at the top level quicksilver: I can't really be expected to deal with your sinfulness. That's between you and your god. * acowley: Good haskell code is 20 LANGUAGE pragmas, 40 imports, then one line of perl. * deggis: oh. running GHC HEAD. must try that next week if coffee just someday isn't enough * dmwit: hGetContents does not close its handle. dmwit: hGetContents semicloses its handle. * monochrom: isTrue = (unsafeCoerce :: Either a b -> Bool) . (unsafeCoerce :: Maybe c -> Either a b) . (unsafeCoerce :: Bool -> Maybe c) * dmwit: For basically all good properties P, IEEE 754 is not P. * Cale: Little known fact: For any positive integer n, the infinite sequence of Project Euler problems has only finitely many elements whose solution is not divisible by n. Top Reddit Stories * Announcing Yesod 1.0 Domain: yesodweb.com, Score: 101, Comments: 42 On Reddit: [1] http://goo.gl/izD19 Original: [2] http://goo.gl/oTSto * Why reasoning about space usage in Haskell is hard Domain: ro-che.info, Score: 63, Comments: 14 On Reddit: [3] http://goo.gl/yIkI4 Original: [4] http://goo.gl/IAQtu * GHC HEAD now has type level natural number/ literals Domain: github.com, Score: 63, Comments: 18 On Reddit: [5] http://goo.gl/GfGbB Original: [6] http://goo.gl/Lkp6v * Working Together Domain: yesodweb.com, Score: 62, Comments: 31 On Reddit: [7] http://goo.gl/o3G85 Original: [8] http://goo.gl/jBdJA * How less is more : polymorphic types can improve safety even in monomorphic functions Domain: blog.malde.org, Score: 61, Comments: 13 On Reddit: [9] http://goo.gl/fzUlO Original: [10] http://goo.gl/WHE3A * A Hopefully Fair and Useful Comparison of Haskell Web Frameworks Domain: softwaresimply.blogspot.com, Score: 60, Comments: 30 On Reddit: [11] http://goo.gl/WuDuL Original: [12] http://goo.gl/DXBA8 * First impressions of Haskell: A review of my Haskell adventure Domain: binarysculpting.com, Score: 46, Comments: 36 On Reddit: [13] http://goo.gl/gG0fl Original: [14] http://goo.gl/ZqUT3 * A "radix tree" import syntax? Domain: self.haskell, Score: 41, Comments: 26 On Reddit: [15] http://goo.gl/kBOLZ Original: [16] http://goo.gl/kBOLZ * Using ContT to please the eye Domain: self.haskell, Score: 28, Comments: 15 On Reddit: [17] http://goo.gl/hbFrs Original: [18] http://goo.gl/hbFrs * Designing a DCPU-16 emulator in Haskell: on determinism and I/O Domain: jaspervdj.be, Score: 28, Comments: 6 On Reddit: [19] http://goo.gl/KHIgP Original: [20] http://goo.gl/NT89i Top StackOverflow Questions * What are type quantifiers? votes: 15, answers: 3 Read on SO: [21] http://goo.gl/d3hnH * Haskell: How does 'atomicModifyIORef' work? votes: 12, answers: 2 Read on SO: [22] http://goo.gl/3b5dK * Is there a runtime penalty associated with typeclasses? votes: 12, answers: 1 Read on SO: [23] http://goo.gl/Isw7k * Timing out pure functions votes: 11, answers: 1 Read on SO: [24] http://goo.gl/aTzVG * Haskell - simple constructor comparison (?) function votes: 11, answers: 3 Read on SO: [25] http://goo.gl/ZC6CC * Partial Application with Infix Functions votes: 10, answers: 3 Read on SO: [26] http://goo.gl/QvabF * Haskell: Why is there no type mismatch (and why does this compile)? votes: 9, answers: 2 Read on SO: [27] http://goo.gl/1gxFE * “Dependent optional” data in Haskell votes: 9, answers: 4 Read on SO: [28] http://goo.gl/9K5Wx * How can I help SpecConstr in GHC? votes: 8, answers: 2 Read on SO: [29] http://goo.gl/PGa4C * How to get good performance when writing a list of integers from 1 to 10 million to a file? votes: 7, answers: 2 Read on SO: [30] http://goo.gl/Kym6r * Efficiently turn a ByteString into a hex representation votes: 7, answers: 1 Read on SO: [31] http://goo.gl/aq2ct Until next time, Daniel Santa Cruz References 1. http://www.yesodweb.com/blog/2012/04/announcing-yesod-1-0 2. http://www.reddit.com/r/haskell/comments/s0tr2/announcing_yesod_10/ 3. http://ro-che.info/articles/2012-04-08-space-usage-reasoning.html 4. http://www.reddit.com/r/haskell/comments/rzu8a/why_reasoning_about_space_us… 5. https://github.com/ghc/packages-base/blob/b5b5ec18819fb85ebf1bdb6345ac03178… 6. http://www.reddit.com/r/haskell/comments/s0vln/ghc_head_now_has_type_level_… 7. http://www.yesodweb.com/blog/2012/04/working-together 8. http://www.reddit.com/r/haskell/comments/s2ei5/working_together/ 9. http://blog.malde.org/posts/polymorphic-types-are-safer.html 10. http://www.reddit.com/r/haskell/comments/s7q3b/how_less_is_more_polymorphic… 11. http://softwaresimply.blogspot.com/2012/04/hopefully-fair-and-useful-compar… 12. http://www.reddit.com/r/haskell/comments/s3pbm/a_hopefully_fair_and_useful_… 13. http://binarysculpting.com/2012/04/12/a-review-of-my-haskell-adventure/ 14. http://www.reddit.com/r/haskell/comments/s6okb/first_impressions_of_haskell… 15. http://www.reddit.com/r/haskell/comments/s2o8p/a_radix_tree_import_syntax/ 16. http://www.reddit.com/r/haskell/comments/s2o8p/a_radix_tree_import_syntax/ 17. http://www.reddit.com/r/haskell/comments/s49kk/using_contt_to_please_the_ey… 18. http://www.reddit.com/r/haskell/comments/s49kk/using_contt_to_please_the_ey… 19. http://jaspervdj.be/posts/2012-04-12-st-io-dcpu-16.html 20. http://www.reddit.com/r/haskell/comments/s5xqb/designing_a_dcpu16_emulator_… 21. http://stackoverflow.com/questions/10062253/what-are-type-quantifiers 22. http://stackoverflow.com/questions/10102881/haskell-how-does-atomicmodifyio… 23. http://stackoverflow.com/questions/10148897/is-there-a-runtime-penalty-asso… 24. http://stackoverflow.com/questions/10071560/timing-out-pure-functions 25. http://stackoverflow.com/questions/10112733/haskell-simple-constructor-comp… 26. http://stackoverflow.com/questions/10131300/partial-application-with-infix-… 27. http://stackoverflow.com/questions/10091874/haskell-why-is-there-no-type-mi… 28. http://stackoverflow.com/questions/10136222/dependent-optional-data-in-hask… 29. http://stackoverflow.com/questions/10145954/how-can-i-help-specconstr-in-ghc 30. http://stackoverflow.com/questions/10080273/how-to-get-good-performance-whe… 31. http://stackoverflow.com/questions/10099921/efficiently-turn-a-bytestring-i…

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Visi 0.1 is available for OS X
by David Pollak 18 Apr '12

18 Apr '12

Folks, Visi 0.1 is available as an executable for OS X 10.7 ( http://tunaforcats.com/visi_0_1.tar.bz ) The blog post announcement is available at http://blog.goodstuff.im/visi-01-for-os-x/ Visi is written mostly in Haskell (which is totally awesome, btw). I'll be hacking on Visi this weekend at BayHac '12 ( http://www.haskell.org/haskellwiki/BayHac2012) Thanks, David -- Visi.Pro, Cloud Computing for the Rest of Us http://visi.pro Lift, the simply functional web framework http://liftweb.net Follow me: http://twitter.com/dpp Blog: http://goodstuff.im

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Re: [Haskell-cafe] [Haskell] ANN: cabal-install-0.14.0
by Felipe Almeida Lessa 18 Apr '12

18 Apr '12

On Tue, Apr 17, 2012 at 1:59 PM, Andres Löh <andres.loeh(a)googlemail.com> wrote: > * Completely new modular dependency solver (default in most cases) Great! =D -- Felipe.

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ghc/profiling and very big "entries"
by Emmanuel CHANTREAU 18 Apr '12

18 Apr '12

Hello I got this line in the profile output: COST CENTRE MODULE no. entries %time %alloc %time %alloc ticks bytes addDeduction L42s.Maths.Prover.Base 22730 2362045 1.1 4.6 52.3 52.6 81 614808848 And it seems that "addDeduction" has been called 2 362 045 times ? But from other (personal) log files this function should be called less than 10 000 times. I wonder if the laziness of haskell can grow the "entries" field which is defined by: "The number of times this particular point in the call tree was entered.", but what "enter" means for ghc is a question for me. Thank you for answering -- Emmanuel Chantréau http://www.maretmanu.org/emmanuel/

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ANNOUNCE: haskell-src-exts-1.13.2
by Niklas Broberg 18 Apr '12

18 Apr '12

Fellow Haskelleers, I'm pleased to announce the release of haskell-src-exts-1.13.2! * On hackage: http://hackage.haskell.org/package/haskell-src-exts * Via cabal: cabal install haskell-src-exts * Darcs repo: http://code.haskell.org/haskell-src-exts This release contains only a few small tweaks and bug-fixes. Changelog: 1.13.1 --> 1.13.2 =============== * Fix the bug with the precedence of unary prefix minus. Previously it was resolved as binding more tightly than any infix operator, now it is correctly treated as having the same fixity as binary infix minus. 1.13.0 --> 1.13.1 =============== * Allow an optional semi before the closing tag of an element. This achieves a similar effect for XmlSyntax in do blocks as DoAndIfThenElse does for the if construct. No more need to indent the closing tag one step further than the opening tag. * Add a dummy 'noLoc :: SrcLoc' to L.H.E.SrcLoc, to use when generating code. It could definitely be done more elegantly, but not without inducing another major version bump, so later. * Fix a regression from 1.11.x where the parser would crash upon encountering non-simple class/data declaration heads, e.g. 'data A [a]'. Now fails with a parse error as intended. Cheers, /Niklas

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ANNOUNCE: pipes-core 0.1.0
by Paolo Capriotti 17 Apr '12

17 Apr '12

I'm pleased to announce the release of version 0.1.0 of pipes-core, a library for efficient, safe and compositional IO, similar in scope to iteratee and conduits. http://hackage.haskell.org/package/pipes-core This release fixes a mistake in the previous version which violated the category laws in certain cases. I have now written a fairly explicit proof of the category laws for the current implementation, which I'll try to keep updated as the library evolves: https://github.com/pcapriotti/pipes-core/wiki/Category-laws-for-pipes --- # CHANGELOG for pipes-core: ## Version 0.1.0 - Significant simplification of the internals. - Removed `ensure` primitive. # CHANGELOG for pipes-extra: ## Version 0.1.0 - Improved naming in Control.Pipe.Coroutine. - Replaced ChunkPipe with PutbackPipe. --- BR, Paolo Capriotti

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Osx, brew install of 64 bit ghc
by G 17 Apr '12

17 Apr '12

Got a new ghc via brew 7.0.4 and then recompiled / installed haskell-platform via brew Not getting issue trying to compile using ideah in intellij such as Bad interface file: /usr/local/Cellar/ghc/64-apple-darwin/lib/ghc-7.0.4/base-4.3.1.0/Prelude.hi magic number mismatch: old/corrupt interface file? (wanted 129742, got 33214052) Anyone point me in the right direction as to the issue ? Have tried trashing the project etc with no luck. Thanks

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CFT -- Haskell Implementors' Workshop 2012
by Johan Tibell 17 Apr '12

17 Apr '12

Call for Talks ACM SIGPLAN Haskell Implementors' Workshop http://haskell.org/haskellwiki/HaskellImplementorsWorkshop/2012 Copenhagen, Denmark, September 14th, 2012 The workshop will be held in conjunction with ICFP 2012 http://www.icfpconference.org/icfp2012/ Important dates Proposal Deadline: 10th July 2012 Notification: 27th July 2012 Workshop: 14th September 2012 The Haskell Implementors' Workshop is to be held alongside ICFP 2012 this year in Copenhagen, Denmark. There will be no proceedings; it is an informal gathering of people involved in the design and development of Haskell implementations, tools, libraries, and supporting infrastructure. This relatively new workshop reflects the growth of the user community: there is a clear need for a well-supported tool chain for the development, distribution, deployment, and configuration of Haskell software. The aim is for this workshop to give the people involved with building the infrastructure behind this ecosystem an opportunity to bat around ideas, share experiences, and ask for feedback from fellow experts. We intend the workshop to have an informal and interactive feel, with a flexible timetable and plenty of room for ad-hoc discussion, demos, and impromptu short talks. Scope and target audience ------------------------- It is important to distinguish the Haskell Implementors' Workshop from the Haskell Symposium which is also co-located with ICFP 2012. The Haskell Symposium is for the publication of Haskell-related research. In contrast, the Haskell Implementors' Workshop will have no proceedings -- although we will aim to make talk videos, slides and presented data available with the consent of the speakers. In the Haskell Implementors' Workshop, we hope to study the underlying technology. We want to bring together anyone interested in the nitty-gritty details behind turning plain-text source code into a deployed product. Having said that, members of the wider Haskell community are more than welcome to attend the workshop -- we need your feedback to keep the Haskell ecosystem thriving. The scope covers any of the following topics. There may be some topics that people feel we've missed, so by all means submit a proposal even if it doesn't fit exactly into one of these buckets: * Compilation techniques * Language features and extensions * Type system implementation * Concurrency and parallelism: language design and implementation * Performance, optimisation and benchmarking * Virtual machines and run-time systems * Libraries and tools for development or deployment Talks ----- At this stage we would like to invite proposals from potential speakers for a relatively short talk. We are aiming for 20 minute talks with 10 minutes for questions and changeovers. We want to hear from people writing compilers, tools, or libraries, people with cool ideas for directions in which we should take the platform, proposals for new features to be implemented, and half-baked crazy ideas. Please submit a talk title and abstract of no more than 200 words to: johan.tibell(a)gmail.com We will also have a lightning talks session which will be organised on the day. These talks will be 2-10 minutes, depending on available time. Suggested topics for lightning talks are to present a single idea, a work-in-progress project, a problem to intrigue and perplex Haskell implementors, or simply to ask for feedback and collaborators. Organisers ---------- * Lennart Augustsson (Standard Chartered Bank) * Manuel M T Chakravarty (University of New South Wales) * Gregory Collins - co-chair (Google) * Simon Marlow (Microsoft Research) * David Terei (Stanford University) * Johan Tibell - co-chair (Google)

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