December 2011 - Haskell-Cafe

More liberal than liberal type synonyms
by Dan Doel 08 Dec '11

08 Dec '11

Greetings, In the process of working on a Haskell-alike language recently, Ed Kmett and I realized that we had (without really thinking about it) implemented type synonyms that are a bit more liberal than GHC's. With LiberalTypeSynonyms enabled, GHC allows: type Foo a b = b -> a type Bar f = f String Int baz :: Bar Foo baz = show because Bar expands to saturate Foo. However, we had also implemented the following, which fails in GHC: type Foo a b = b -> a type Bar f = f (Foo Int) (Foo Int) type Baz f g = f Int -> g Int quux :: Bar Baz quux = id That is: type synonyms are allowed to be partially applied within other type synonyms, as long as similar transitive saturation guarantees are met during their use. I don't know how useful it is, but I was curious if anyone can see anything wrong with allowing this (it seems okay to me after a little thought), and thought I'd float the idea out to the GHC developers, in case they're interested in picking it up. -- Dan

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Converting string to System.Time.ClockTime
by dokondr 08 Dec '11

08 Dec '11

Hi, What would be the simplest way to convert strings like "Wed, 07 Dec 2011 10:09:21 +0000" to System.Time.ClockTime ? Thanks!

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Disjunctive patterns
by Asger Feldthaus 08 Dec '11

08 Dec '11

Haskell doesn't seem to support disjunctive patterns, and I'm having a difficult time writing good Haskell code in situations that would otherwise call for that type of pattern. Suppose for an example I have this data type: data T = Foo Int | Bar Int | Baz In OCaml I can write something like: (* foo : T -> T -> int *) fun foo x y = match (x,y) with | (Foo a | Bar a, Foo b | Bar b) -> a + b | (Baz, Foo a) | (Bar a, Baz) -> -a | (Baz, Bar a) | (Foo a, Baz) -> a | _ -> 0 In Haskell I can't find any equivalent to the disjunctive pattern. If expanded naively, my Haskell version would look like this: foo :: T -> T -> Int foo x y = case (x,y) of (Foo a, Foo b) -> a + b (Foo a, Bar b) -> a + b (Bar a, Foo b) -> a + b (Bar a, Bar b) -> a + b (Baz, Foo a) -> -a (Bar a, Baz) -> -a (Baz, Bar a) -> a (Foo a, Baz) -> a _ -> 0 While my example is still managable in size, this quickly goes out of hand in practice. I've tried using pattern guards but with limited success. For example: foo2 :: T -> T -> Int foo2 x y = case (x,y) of (x,y) | Just a <- open x, Just b <- open y -> a+b (Baz, Foo a) -> -a (Bar a, Baz) -> -a (Baz, Bar a) -> a (Foo a, Baz) -> a _ -> 0 where open (Foo x) = Just x open (Bar x) = Just x open Baz = Nothing I admit it doesn't look that bad in my crafted example, but this approach doesn't seem to well work for me in practice. In any case, it's still far more verbose than the disjunctive pattern version. Nesting the case expressions instead of tuple-matching can reduce some code duplication, but in general it becomes really verbose, and it is easy to make mistakes when you have partially overlapped patterns in the disjunctive-pattern version. Here's the example with nested cases: foo3 :: T -> T -> Int foo3 x y = case x of Foo a -> case y of Foo b -> a+b Bar b -> a+b Baz -> a Bar a -> case y of Foo b -> a+b Bar b -> a+b Baz -> -a Baz -> case y of Foo b -> -b Bar b -> b Baz -> 0 What do people do in this situation - is there a good trick I've overlooked? And is there some reason why Haskell does not support disjunctive patterns? Thanks, Asger

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Haskell Weekly News: Issue 210
by Daniel Santa Cruz 08 Dec '11

08 Dec '11

Welcome to issue 210 of the HWN, a newsletter covering developments in the Haskell community. This release covers the week of November 27 to December 3, 2011. You can find the HTML version at: http://contemplatecode.blogspot.com/2011/12/haskell-weekly-news-issue-210.h… Announcements Heinrich Hordegen announced a tentative 2012 meeting schedule for Munich Haskell group. [1] http://goo.gl/JrnP6 New and Updated Projects * data-timeout (Mikhail Vorozhtsov) Allows one to specify time units for timeouts and convert between them. [2] http://goo.gl/fbfGM * Netwire (Ertugrul Soylemez; 3.0.0) Major API change. [3] http://goo.gl/8hPwS * monad-control (Bas van Dijk; 0.3) New and improved API. [4] http://goo.gl/so3s9 Quotes of the Week * Jafet: They're not nasty; their niceness is just distributed over more people * copumpkin: U NEED MOANAD * xplat: i have a feeling that trying to write an interesting dependently-typed program in Shen at any time in the near future would be like trying to host a holiday dinner starting with an empty lot, 2x4s, a stack of window panes, some cables and wiring boxes, sheets of drywall, disconnected appliances, roof shingles, rolls of insulation and bags of concrete * Axman6: {-# PRAGMA ThisShitNeedsMoarFast #-} Top Reddit Stories * Name Your Type Variables! Domain: amateurtopologist.com, Score: 71, Comments: 38 On Reddit: [5] http://goo.gl/I2uny Original: [6] http://goo.gl/yRbmO * Insane Haskell Professor on Lambda Calculus. Wait for it... Domain: youtube.com, Score: 66, Comments: 17 On Reddit: [7] http://goo.gl/uDQJS Original: [8] http://goo.gl/IauHk * criterion 0.6 produces shiny new performance reports Domain: bos.github.com, Score: 63, Comments: 8 On Reddit: [9] http://goo.gl/F2K76 Original: [10] http://goo.gl/x720h * [commit: base] master: Add traceStack :: String -> a -> a Domain: haskell.org, Score: 48, Comments: 5 On Reddit: [11] http://goo.gl/HCbf2 Original: [12] http://goo.gl/JzO3O * Elm: functional reactive web-programming (compiles to html, css, js). Just added reactive values, basic canvas support, and new interactive examples. Domain: elm-lang.org, Score: 45, Comments: 19 On Reddit: [13] http://goo.gl/mKS9a Original: [14] http://goo.gl/ASt27 * Snap 0.7 released Domain: snapframework.com, Score: 41, Comments: 1 On Reddit: [15] http://goo.gl/R3A8Y Original: [16] http://goo.gl/SVgXy * Haskell Communities and Activities Report - November 2011 edition Domain: haskell.org, Score: 35, Comments: 0 On Reddit: [17] http://goo.gl/X82AK Original: [18] http://goo.gl/CGKAr * "fix' f = f (fix' f)" vs. "fix f = let x = f x in x" Domain: groups.google.com, Score: 33, Comments: 15 On Reddit: [19] http://goo.gl/vadtO Original: [20] http://goo.gl/SQSFL * Announcing cabal-src: solve (some of) cabal dependency hell Domain: yesodweb.com, Score: 32, Comments: 15 On Reddit: [21] http://goo.gl/PIB4S Original: [22] http://goo.gl/00LZz * aeson 0.4: easier, faster, nicer Domain: github.com, Score: 31, Comments: 18 On Reddit: [23] http://goo.gl/G1J5A Original: [24] http://goo.gl/DQ4bc * Package authors: Please use Packdeps! Domain: yesodweb.com, Score: 24, Comments: 10 On Reddit: [25] http://goo.gl/bLYvt Original: [26] http://goo.gl/fNh9n * Slides from my guest lecture at Stanford Domain: blog.johantibell.com, Score: 22, Comments: 14 On Reddit: [27] http://goo.gl/ITFkN Original: [28] http://goo.gl/4w1tK Top StackOverflow Questions * How to write a Haskell function that takes a variadic function as an argument votes: 28, answers: 1 Read on SO: [29] http://goo.gl/JqZ3H * Functional lenses votes: 18, answers: 2 Read on SO: [30] http://goo.gl/Qn2Ff * Haskell : An example of a Foldable which is not a Functor? votes: 17, answers: 2 Read on SO: [31] http://goo.gl/42lSo * How does IncoherentInstances work? votes: 17, answers: 1 Read on SO: [32] http://goo.gl/O1Ha4 * Minimum specification for Haskell library type classes? votes: 16, answers: 2 Read on SO: [33] http://goo.gl/L7mO6 * Expressive and composable error types votes: 16, answers: 1 Read on SO: [34] http://goo.gl/vkoZb * Distributed Haskell state of the art in 2011? votes: 16, answers: 2 Read on SO: [35] http://goo.gl/mx0kd * Is it possible to implement liftM2 in Scala? votes: 15, answers: 1 Read on SO: [36] http://goo.gl/p6TDl * Are Haskell FlexibleInstances a stable extension to the language? votes: 15, answers: 1 Read on SO: [37] http://goo.gl/sQc37 * Is Milner let polymorphism a rank 2 feature? votes: 13, answers: 4 Read on SO: [38] http://goo.gl/kOcnW Until next time, Daniel Santa Cruz References 1. http://comments.gmane.org/gmane.comp.lang.haskell.cafe/94240 2. http://comments.gmane.org/gmane.comp.lang.haskell.libraries/16691 3. http://comments.gmane.org/gmane.comp.lang.haskell.cafe/94245 4. http://comments.gmane.org/gmane.comp.lang.haskell.general/19038 5. http://www.amateurtopologist.com/blog/2011/10/11/name-your-type-variables/ 6. http://www.reddit.com/r/haskell/comments/mxz3k/name_your_type_variables/ 7. http://www.youtube.com/watch?v=5TMojC0Nb6A 8. http://www.reddit.com/r/haskell/comments/mw094/insane_haskell_professor_on_… 9. http://bos.github.com/criterion/ 10. http://www.reddit.com/r/haskell/comments/mx8vw/criterion_06_produces_shiny_… 11. http://www.haskell.org/pipermail/cvs-libraries/2011-November/014698.html 12. http://www.reddit.com/r/haskell/comments/muq40/commit_base_master_add_trace… 13. http://elm-lang.org/Elm.elm 14. http://www.reddit.com/r/haskell/comments/mwbvj/elm_functional_reactive_webp… 15. http://snapframework.com/blog/2011/12/01/snap-0.7-released 16. http://www.reddit.com/r/haskell/comments/mwx4t/snap_07_released/ 17. http://www.haskell.org/communities/11-2011/html/report.html 18. http://www.reddit.com/r/haskell/comments/mzbia/haskell_communities_and_acti… 19. http://groups.google.com/group/stanford-11au-cs240h/browse_thread/thread/41… 20. http://www.reddit.com/r/haskell/comments/mt7nt/fix_f_f_fix_f_vs_fix_f_let_x… 21. http://www.yesodweb.com/blog/2011/11/cabal-src 22. http://www.reddit.com/r/haskell/comments/mrz9p/announcing_cabalsrc_solve_so… 23. https://github.com/bos/aeson/blob/0.4.0.0/release-notes.markdown 24. http://www.reddit.com/r/haskell/comments/mvtmk/aeson_04_easier_faster_nicer/ 25. http://www.yesodweb.com/blog/2011/12/please-use-packdeps 26. http://www.reddit.com/r/haskell/comments/mvp4w/package_authors_please_use_p… 27. http://blog.johantibell.com/2011/11/slides-from-my-guest-lecture-at.html 28. http://www.reddit.com/r/haskell/comments/mvoqd/slides_from_my_guest_lecture… 29. http://stackoverflow.com/questions/8353845/how-to-write-a-haskell-function-… 30. http://stackoverflow.com/questions/8307370/functional-lenses 31. http://stackoverflow.com/questions/8359115/haskell-an-example-of-a-foldable… 32. http://stackoverflow.com/questions/8371499/how-does-incoherentinstances-work 33. http://stackoverflow.com/questions/8320039/minimum-specification-for-haskel… 34. http://stackoverflow.com/questions/8320855/expressive-and-composable-error-… 35. http://stackoverflow.com/questions/8362998/distributed-haskell-state-of-the… 36. http://stackoverflow.com/questions/8366518/is-it-possible-to-implement-lift… 37. http://stackoverflow.com/questions/8367423/are-haskell-flexibleinstances-a-… 38. http://stackoverflow.com/questions/8296695/is-milner-let-polymorphism-a-ran…

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terminateProcess leaves zombie processes around
by Dan Rosén 07 Dec '11

07 Dec '11

Hi Haskell-Cafe, I'm using Haskell to run a lot of instances of an Automated Thorem Prover, eprover. I have pasted a smaller version of my program at http://hpaste.org/54954. It runs eprover sequentially on all input files, with a timeout of 100ms. Unfortunately, it leaves a lot of zombie processes around, probably due to the fact that terminateProcess fails to terminate them, even though eprover terminates on SIGTERM. I have tried to use System.Timeout.timeout around readProcess, but without surprise it did not work. Another way of doing it is to use the timeout from gnu-coreutils, but the timeout resolution is in seconds (same with eprover's flag --cpu-limit). Any ideas how I could write my code to get a clean termination of this process? Best, Dan Rosén

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PhD position at Ghent University
by Tom Schrijvers 07 Dec '11

07 Dec '11

The Programming Languages Group of Ghent University invites applicants for a PhD position. This position centers around the modular treatment of side-effects in purely functional programs and models. It is part of the project "Modular handling of effects in purely functional programs and models" under the direction of Tom Schrijvers. The successful applicant has a master degree in Computer Science or equivalent. Ideally, she/he will also have a strong, documented interest in doing research. Strong problem-solving and programming skills are essential. Prior knowledge of purely functional programming (e.g., Haskell) and models of side-effects (e.g., monads) is an advantage. The PhD position is for 4 years. It starts between January and October 2012. The position is a fully-funded post. The salary is compatible with other Belgian PhD rates and among the better ones in Europe and abroad. For more information, see: http://users.ugent.be/~tschrijv/phdposition2.html -- prof. dr. ir. Tom Schrijvers Programming Languages Group Department of Applied Mathematics and Computer Science University of Ghent Krijgslaan 281 S9 9000 Gent Belgium Phone: +32 9 264 4805 http://users.ugent.be/~tschrijv/

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Neither of the sparks are converted into OS threads but parallelization happens surprisingly!!!
by Burak Ekici 07 Dec '11

07 Dec '11

Dear List, I have a point misunderstood! I am using second generation strategies of Haskell to parallelize some number of algorithms in Haskell. The missing point here is that: In some cases, neither of created sparks are converted into OS threads, but parallelization happens surprisingly, when I use threadscope to see what had happened during the evaluation. What happened to me was that 2 sparks were created one of which was pruned, none of which was converted into OS threads and the last one was missing, however, parallelism happened!!! I wonder what is the reason for this? Could someone aware, please shed light on the issue? Thanks in advance. Saluti, Burak.

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Haskell at University in Munich
by Bartosz Wójcik 06 Dec '11

06 Dec '11

Hello Haskell Cafe, I would be grateful for any information regarding Haskell (or at least Functional Programming) lectures at Universities near to Munich, Germany (Master or Bachelor). Unconfirmed information I've got regarding LMU and TUM are not promising. If Munich and 200 km circle do not provide with any offer, perhaps you may know what is available in Europe, limiting language of study to [German, English, Polish]? Best regards, Bartek

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Re: [Haskell-cafe] Haskell at University in Munich
by Janis Voigtländer 06 Dec '11

06 Dec '11

Am 05.12.2011 12:00, schrieb <bartek(a)sudety.it>: > Hello Haskell Cafe, > > I would be grateful for any information regarding Haskell (or at least > Functional Programming) lectures at Universities near to Munich, Germany > (Master or Bachelor). Unconfirmed information I've got regarding LMU and TUM > are not promising. Is Munich really that bad? FP'ers at LMU and TUM, please speak up. (I know that at least research-wise there are quite a few around there, though I don't know what they do in teaching.) > If Munich and 200 km circle do not provide with any offer, perhaps Less than 200 km from Munich: http://db.inf.uni-tuebingen.de/teaching/ws1112/afp Also, Germany extends farther than 200 km beyond Munich. :-) Say, Bonn: http://www.iai.uni-bonn.de/~jv/teaching/dp11/ http://www.iai.uni-bonn.de/~jv/teaching/ffp/ > you may know what is available in Europe, limiting language of study > to [German, English,Polish]? The thread Sean pointed to may provide information on that. Obvious candidates are Utrecht and Chalmers, both teaching in English I suppose, at least at Master level. Best, Janis. -- Jun.-Prof. Dr. Janis Voigtländer http://www.iai.uni-bonn.de/~jv/ mailto:jv@iai.uni-bonn.de

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Haddock and the qualified imports
by Yves Parès 06 Dec '11

06 Dec '11

Hi, I noticed some time ago the fact that qualified imports doesn't affect the generated documentation. It's kind of clumsy in case of libraries that define a lot of synonyms (vector and bytestring come in my mind first). For instance, in the package utf8-string: http://hackage.haskell.org/packages/archive/utf8-string/0.3.7/doc/html/Data… Here, the documentation doesn't say that the utf8-string/Data.ByteString.UTF8.ByteString datatype used all along comes in fact from Data.ByteString. If it were instead a new implementation of ByteString (as for bytestring/Data.ByteString.Char8.ByteString) the documentation would look exactly the same, so to disambiguate to reader has to crawl through the code to get to the initial definition. (clicking on a 'ByteString' doesn't even redirect you to the original bytestring/Data.ByteString page) It should be written that this 'ByteString' is not a newly defined type but instead a re-exportation. It's even worse when you see the doc of a module that uses in the meantime lazy and strict ByteStrings, or normal and unboxed/storable/<insert flavour here> vector: you have to hover the type name to see which haddock page it points to. In that case, a solution might be to indicate on top of the doc page that it uses another module as a qualified import, and to keep the prefixes in the function signatures.

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