- Haskell-Cafe - Haskell.org

ANNOUNCE: SmallCheck 0.5
by Roman Cheplyaka 20 Nov '11

20 Nov '11

Following the recent discussion on Haskell-Cafe, I've taken over maintainership of SmallCheck. This is to announce a new minor release, SmallCheck 0.5. It fixes build on GHC 7.2 and makes a few cosmetic changes. http://hackage.haskell.org/package/smallcheck-0.5 In the future there will be more releases that hopefully will make SmallCheck more convenient to use. Feel free to submit bugs, wishes and patches through github: https://github.com/feuerbach/smallcheck -- Roman I. Cheplyaka :: http://ro-che.info/

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Const vs Constant types
by Denis Moskvin 20 Nov '11

20 Nov '11

Is there any rationale for coexistence of newtype Const a b = Const { getConst :: a } from Control.Applicative and newtype Constant a b = Constant { getConstant :: a } from Data.Functor.Constant (transformers package)? Denis Moskvin

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Documenting strictness properties for Data.Map.Strict
by Johan Tibell 19 Nov '11

19 Nov '11

Hi all, Data.Map is getting split into Data.Map.Lazy and Data.Map.Strict (with Data.Map re-exporting the lazy API). I want to better document the strictness properties of the two new modules. Right now the documentation for Data.Map.Strict reads: Strictness properties ===================== * All functions are strict in both key and value arguments. Examples: insertWith (+) k undefined m == undefined delete undefined m == undefined * Keys and values are evaluated to WHNF before they are stored in the map. Examples: map (\ v -> undefined) == undefined mapKeys (\ k -> undefined) == undefined I'm not entirely happy with this formulation. I'm looking for something that's clear (i.e. precise and concise, without leaving out important information), assuming that the reader already knows how lazy evaluation works at a high level. Ideas? Cheers, Johan

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Stream fusion
by Yves Parès 18 Nov '11

18 Nov '11

Hello, While re-reading RealWorldHaskell, chapter 25, I saw that -- unlike I believed -- loop fusion wasn't activated by default under GHC for lists (but that module Data.List.Stream from package stream-fusion could provide it). Is that still the case? If not, then are there some cases of list processing where loop fusion would be a bad thing? (Ergo cases where you should stick to Prelude/Data.List functions and not use Data.List.Stream implementation) Thanks.

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[ANN] transformers-base, transformers-abort, monad-abort-fd
by Mikhail Vorozhtsov 18 Nov '11

18 Nov '11

Hi Cafe. I've been using these three small transformer libraries for awhile, so it's probably time to announce them. transformers-base[1] introduces a generalized version of MonadIO, MonadBase (BaseM in monadLib terms). It's very useful when you are trying to make a stateful API work in both IO and STM (and all transformer stacks on top of them). transformers-abort[2] basically gives you two versions of EitherT, one for errors and one for short-circuiting. Includes instances for semigroupoids and monad-control classes. monad-abort-fd[3] is a typical companion auto-lifter package for transformers-abort. But it also provides a generalized version[4] of Control.Exception which tries to thread effects properly (e.g. finalizers can read (if control didn't escape) and modify the state in StateT). [1] http://hackage.haskell.org/package/transformers-base [2] http://hackage.haskell.org/package/transformers-abort [3] http://hackage.haskell.org/package/monad-abort-fd [4] http://hackage.haskell.org/packages/archive/monad-abort-fd/0.3/doc/html/src…

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Can I have an alias for selector?
by Magicloud Magiclouds 18 Nov '11

18 Nov '11

Hi, For example: "data A = B { c :: Int }". Then I must use it this way "(B 0) { c = 1 }". Anyway I could make it like "(B 0) { d = 1 }"? -- 竹密岂妨流水过山高哪阻野云飞

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Re: [Haskell-cafe] Deduce problem.
by oleg＠okmij.org 17 Nov '11

17 Nov '11

Multi-parameter type classes are more flexible. Here is how you can write your old code: > {-# LANGUAGE MultiParamTypeClasses, FlexibleInstances #-} > > class (ClassA a, ClassB b) => ClassC a b where > from :: a -> [b] > to :: a -> [b] > > data H = H > > class ClassA a where toInt :: a -> Int > class ClassB b where fromInt :: Int -> b > > instance ClassB H where fromInt _ = H > > data Test = Test { m :: H } > instance ClassA Test where toInt _ = 0 > > instance ClassC Test H where > from = (:[]) . m > to = (:[]) . m The constraints in the ClassC a b declaration specify that in all instances of ClassC, the type a must be in ClassA and the type b must be in ClassB. This is the case for the "ClassC Test H" instance. You can also specify that for some particular 'a' the function 'from' can produce the value of the type [b] for any b in ClassB. The caller will determine which b it wants. This is similar to your original intention, as I understand. > instance ClassA Int where toInt = id > > instance ClassB b => ClassC Int b where > from x = [fromInt x] > > t1:: [H] > t1 = from (5::Int)

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ANNOUNCE: graphviz-2999.12.0.4
by Ivan Lazar Miljenovic 17 Nov '11

17 Nov '11

I've uploaded version 2999.12.0.4. This release fixes the problem described by Max Rabkin below (who also determined how to fix: I *always* get tripped up by asTypeOf :s). It also has improved documentation to help people that are wanting to construct Dot graphs explicitly, and cleans up error messages produced when provided Dot code isn't parseable (along with hints for common solutions). On 11 November 2011 02:19, Max Rabkin <max.rabkin(a)gmail.com> wrote: > My understanding of the documentation for Data.GraphViz.dotizeGraph > and graphToGraph is that they should add position attributes to a > graph. But they always seem to return graphs with empty attribute > lists. What am I doing wrong in the following tiny example? > >> dotizeGraph nonClusteredParams (insNode (0, "Blah") $ empty :: Gr String ()) > > 0:([],"Blah")->[] > > I expected the first list to contain at least a Pos attribute. > > I have written a wrapper around /usr/share/dot which shows the program > is outputting positional information. > > --Max > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

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Haskell Weekly News: Issue 208
by Daniel Santa Cruz 17 Nov '11

17 Nov '11

Welcome to issue 208 of the HWN, a newsletter covering developments in the Haskell community. This release covers the week of November 6 to 12, 2011. You can find the HTML version at: http://contemplatecode.blogspot.com/2011/11/haskell-weekly-news-issue-208.h… New and Updated Projects * GHC (Ian Lynagh; 7.2.2) bugfix release for the 7.2 branch. [1] http://goo.gl/eB4X6 * CoreErlang (Alex Kropivny; 0.0.2) New maintainer, plus some bug fixes. [2] http://goo.gl/TMUWs * virthualenv (Bartek ƒwik≈owski) A tool inspired by Python's virtualenv. [3] http://goo.gl/3Jvc0 * transformers-base, transformers-abort, monad-abort-fd (Mikhail Vorozhtsov). [4] http://goo.gl/DRNgt Quotes of the Week * Nimatek: People shouldn't be able to add two and two in Haskell without knowing monoids! * shachaf: Sometimes things are complicated because the domain is complicated. Other times things are complicated because edwardk. * shachaf: It's-a me, MonadIO! * elliott: a typeclass is nothing without semantics * Jafet: I didn't know what your question was so I answered the question that I liked * steakknife: Skip multidimensional types and go straight types that can express general relativity. * ddarius: "use the right platitude for the job" * balor: hmm....do not try to compile a file with a 30mb string in it Top Reddit Stories * Quick poll: upvote if you think GHC should enable -O by default, downvote if you think -O0 should continue to be the default Domain: self.haskell, Score: 68, Comments: 46 On Reddit: [5] http://goo.gl/q43Ur Original: [6] http://goo.gl/q43Ur * What's going on with "id id id ... id 0"? Domain: self.haskell, Score: 46, Comments: 25 On Reddit: [7] http://goo.gl/a57l0 Original: [8] http://goo.gl/a57l0 * HaskellDB: A long tutorial Domain: chrisdone.com, Score: 45, Comments: 6 On Reddit: [9] http://goo.gl/FYBiU Original: [10] http://goo.gl/794G2 * The Agda mailing list has the best questions [somewhat off topic] Domain: lists.chalmers.se, Score: 41, Comments: 1 On Reddit: [11] http://goo.gl/ec5gB Original: [12] http://goo.gl/ODpKV * Virtual Haskell Environment Domain: paczesiowa.blogspot.com, Score: 40, Comments: 33 On Reddit: [13] http://goo.gl/17qDB Original: [14] http://goo.gl/V7onW * Why doesn't Haskell's Prelude.read return a Maybe? Domain: stackoverflow.com, Score: 39, Comments: 44 On Reddit: [15] http://goo.gl/vceL7 Original: [16] http://goo.gl/ETu1j * First release of Kansas Lava Domain: ittc.ku.edu, Score: 32, Comments: 1 On Reddit: [17] http://goo.gl/bj8vw Original: [18] http://goo.gl/nhAho * A major new release of the statistics library Domain: serpentine.com, Score: 28, Comments: 2 On Reddit: [19] http://goo.gl/dYwQd Original: [20] http://goo.gl/7QkKY * TRYDSH - Test Your Code Domain: dbwiscam.informatik.uni-tuebingen.de, Score: 27, Comments: 3 On Reddit: [21] http://goo.gl/qbZyA Original: [22] http://goo.gl/IiRf6 * ScalableServer-0.1 - Providing an easy API for attoparsec/enumerator/blaze based TCP services Domain: github.com, Score: 20, Comments: 14 On Reddit: [23] http://goo.gl/aXqQ7 Original: [24] http://goo.gl/Z1zcm * Python 3 Implementation in Haskell [x-post from programming.reddit.com] Domain: reddit.com, Score: 19, Comments: 0 On Reddit: [25] http://goo.gl/lDJBr Original: [26] http://goo.gl/UrWld * let's derive Eq.(==) -- reflexivity not guaranteed Domain: self.haskell, Score: 16, Comments: 11 On Reddit: [27] http://goo.gl/1ANU9 Original: [28] http://goo.gl/1ANU9 * Haskell and High-Performance Computing (HPC) Domain: self.haskell, Score: 16, Comments: 7 On Reddit: [29] http://goo.gl/AZQPv Original: [30] http://goo.gl/AZQPv Top StackOverflow Questions * Why doesn't Haskell's Prelude.read return a Maybe? votes: 29, answers: 4 Read on SO: [31] http://goo.gl/ETu1j * Choosing between a class and a record votes: 27, answers: 3 Read on SO: [32] http://goo.gl/Dn5FT * Function using foldr is too eager votes: 9, answers: 3 Read on SO: [33] http://goo.gl/jUJxG * Odd results from monad transformer benchmark. A bug? votes: 9, answers: 2 Read on SO: [34] http://goo.gl/qlvDj * Binary instance for an existential votes: 8, answers: 3 Read on SO: [35] http://goo.gl/cHzhb * GHC Install Without Root votes: 7, answers: 2 Read on SO: [36] http://goo.gl/mJnx5 * Fixed point combinator in Haskell votes: 7, answers: 5 Read on SO: [37] http://goo.gl/K4VcI * What's the best way to write facebook apps in haskell? votes: 6, answers: 1 Read on SO: [38] http://goo.gl/HeXOp * Creating a list by cumulatively adding elements of another list: Haskell votes: 6, answers: 4 Read on SO: [39] http://goo.gl/neP6s * ghci environment dump votes: 6, answers: 2 Read on SO: [40] http://goo.gl/EnKpp Until next time, Daniel Santa Cruz References 1. http://permalink.gmane.org/gmane.comp.lang.haskell.general/19017 2. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/93712 3. http://permalink.gmane.org/gmane.comp.lang.haskell.general/19011 4. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/93802 5. http://www.reddit.com/r/haskell/comments/m5yhx/quick_poll_upvote_if_you_thi… 6. http://www.reddit.com/r/haskell/comments/m5yhx/quick_poll_upvote_if_you_thi… 7. http://www.reddit.com/r/haskell/comments/m7uph/whats_going_on_with_id_id_id… 8. http://www.reddit.com/r/haskell/comments/m7uph/whats_going_on_with_id_id_id… 9. http://chrisdone.com/posts/2011-11-06-haskelldb-tutorial.html 10. http://www.reddit.com/r/haskell/comments/m2df2/haskelldb_a_long_tutorial/ 11. https://lists.chalmers.se/pipermail/agda/2011/003493.html 12. http://www.reddit.com/r/haskell/comments/m8iwo/the_agda_mailing_list_has_th… 13. http://paczesiowa.blogspot.com/2011/11/virtual-haskell-environment.html 14. http://www.reddit.com/r/haskell/comments/m39v5/virtual_haskell_environment/ 15. http://stackoverflow.com/questions/8066850/why-doesnt-haskells-prelude-read… 16. http://www.reddit.com/r/haskell/comments/m7agm/why_doesnt_haskells_preluder… 17. http://ittc.ku.edu/csdl/fpg/Tools/KansasLava 18. http://www.reddit.com/r/haskell/comments/m3nss/first_release_of_kansas_lava/ 19. http://www.serpentine.com/blog/2011/11/10/a-major-new-release-of-the-haskel… 20. http://www.reddit.com/r/haskell/comments/m8cox/a_major_new_release_of_the_s… 21. http://dbwiscam.informatik.uni-tuebingen.de/trydsh/ 22. http://www.reddit.com/r/haskell/comments/m9s37/trydsh_test_your_code/ 23. https://github.com/jamwt/haskell-scalable-server 24. http://www.reddit.com/r/haskell/comments/m4cz0/scalableserver01_providing_a… 25. http://www.reddit.com/r/programming/comments/m4rbs/python_3_implementation_… 26. http://www.reddit.com/r/haskell/comments/m5l1u/python_3_implementation_in_h… 27. http://www.reddit.com/r/haskell/comments/m9975/lets_derive_eq_reflexivity_n… 28. http://www.reddit.com/r/haskell/comments/m9975/lets_derive_eq_reflexivity_n… 29. http://www.reddit.com/r/haskell/comments/m9s9p/haskell_and_highperformance_… 30. http://www.reddit.com/r/haskell/comments/m9s9p/haskell_and_highperformance_… 31. http://stackoverflow.com/questions/8066850/why-doesnt-haskells-prelude-read… 32. http://stackoverflow.com/questions/8106764/choosing-between-a-class-and-a-r… 33. http://stackoverflow.com/questions/8032228/function-using-foldr-is-too-eager 34. http://stackoverflow.com/questions/8092787/odd-results-from-monad-transform… 35. http://stackoverflow.com/questions/8101067/binary-instance-for-an-existenti… 36. http://stackoverflow.com/questions/8084832/ghc-install-without-root 37. http://stackoverflow.com/questions/8099349/fixed-point-combinator-in-haskell 38. http://stackoverflow.com/questions/8052177/whats-the-best-way-to-write-face… 39. http://stackoverflow.com/questions/8059057/creating-a-list-by-cumulatively-… 40. http://stackoverflow.com/questions/8067368/ghci-environment-dump

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doctest: Interpreter exited with an error: ExitFailure 127
by informationen 17 Nov '11

17 Nov '11

Hi, i have upgraded to doctest version 0.4.1. Now when i try to run the example from the webpage, i get: doctest: Interpreter exited with an error: ExitFailure 127 What's wrong here and how can i fix it? Kind regards Chris This is the content of Fib.hs: module Fib where -- | Compute Fibonacci numbers -- -- Examples: -- -- >>> fib 10 -- 55 -- -- >>> fib 5 -- 5 fib 0 = 0 fib 1 = 1 fib n = fib (n - 1) + fib (n - 2) main = do print $ fib 4

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