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Haskell Weekly News: Issue 259
by Daniel Santa Cruz 21 Feb '13

21 Feb '13

Welcome to issue 259 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of February 10 to 16, 2013. Quotes of the Week * frerich: i thought endo is where the ewoks live * edwardk: on the other hand i also write excessively polymorphic code Top Reddit Stories * Comonads are objects Domain: haskellforall.com, Score: 105, Comments: 86 On Reddit: [1] http://goo.gl/ssFWD Original: [2] http://goo.gl/ADN3k * GHC HEAD now has OverloadedLists as an extension - allowing you to more easily specify literal Maps, Text, Vector, ByteString etc values. Domain: hackage.haskell.org, Score: 86, Comments: 66 On Reddit: [3] http://goo.gl/tVmBE Original: [4] http://goo.gl/m08GR * GHC HEAD: new parallel I/O manager merged Domain: haskell.org, Score: 67, Comments: 39 On Reddit: [5] http://goo.gl/HX04c Original: [6] http://goo.gl/j8AaN * The Algebra of Algebraic Data Types (Part I) Domain: chris-taylor.github.com, Score: 51, Comments: 23 On Reddit: [7] http://goo.gl/9WjrL Original: [8] http://goo.gl/Jyco0 * deprecation rage :: hpaste Domain: hpaste.org, Score: 49, Comments: 9 On Reddit: [9] http://goo.gl/ycHWD Original: [10] http://goo.gl/KAdsV * Github incorrectly recognizes haskell code as perl thus making language usage studies based on github useless. Domain: github.com, Score: 45, Comments: 45 On Reddit: [11] http://goo.gl/WQN38 Original: [12] http://goo.gl/jfpeH * StackOverflow [haskell] hits 10k questions! Domain: imgur.com, Score: 41, Comments: 31 On Reddit: [13] http://goo.gl/znqwX Original: [14] http://goo.gl/cdm3C * Don Stewart provides a great concise overview of the many areas of Haskell performance analysis Domain: stackoverflow.com, Score: 39, Comments: 1 On Reddit: [15] http://goo.gl/W9AOL Original: [16] http://goo.gl/nC6Mk * The Algebra of Algebraic Data Types, Part 2 Domain: chris-taylor.github.com, Score: 37, Comments: 9 On Reddit: [17] http://goo.gl/VHUjp Original: [18] http://goo.gl/mk95s * Upcoming conduit 1.0 & comparison to pipes & io-streams Domain: yesodweb.com, Score: 32, Comments: 11 On Reddit: [19] http://goo.gl/dbnpi Original: [20] http://goo.gl/8RHfb * Library: A collection of tools for processing PDF files in Haskell Domain: github.com, Score: 31, Comments: 38 On Reddit: [21] http://goo.gl/BtpFw Original: [22] http://goo.gl/loL4n * Introduction to Haskell, Lecture 5 is Live (Creating Data Types) Domain: shuklan.com, Score: 29, Comments: 2 On Reddit: [23] http://goo.gl/h2q3G Original: [24] http://goo.gl/ASx0B * FP Complete Hiring for Haskell evangelist / customer and community relations advocate Domain: haskellers.com, Score: 28, Comments: 0 On Reddit: [25] http://goo.gl/g2nic Original: [26] http://goo.gl/di728 * Quick and Easy DSLs with Writer Endo Domain: ocharles.org.uk, Score: 22, Comments: 17 On Reddit: [27] http://goo.gl/6p43B Original: [28] http://goo.gl/p8aX4 Top StackOverflow Questions * Least-strict (*) votes: 17, answers: 2 Read on SO: [29] http://goo.gl/tHzUf * Referential transparency with polymorphism in Haskell votes: 14, answers: 1 Read on SO: [30] http://goo.gl/PyBgK * Truncating to Word type votes: 11, answers: 1 Read on SO: [31] http://goo.gl/C4wwu * Is there a generalization of these Free-like constructions? votes: 11, answers: 0 Read on SO: [32] http://goo.gl/WLb7S * How fast is Data.Sequence.Seq compared to []? votes: 9, answers: 1 Read on SO: [33] http://goo.gl/d0x2d * Does FreeT keep the equational reasoning benefits of Free? votes: 9, answers: 1 Read on SO: [34] http://goo.gl/7vu37 * Understanding recursion in Haskell votes: 7, answers: 5 Read on SO: [35] http://goo.gl/I2SBM * What laws are the standard Haskell type classes expected to uphold? votes: 7, answers: 2 Read on SO: [36] http://goo.gl/FqQ5J * What does (== “ ”) mean, in Haskell? votes: 7, answers: 4 Read on SO: [37] http://goo.gl/EecMz * Linking separate projects in GHC votes: 7, answers: 1 Read on SO: [38] http://goo.gl/ZjVao * What are structures with “subtraction” but no inverse? votes: 7, answers: 1 Read on SO: [39] http://goo.gl/SaQ66 Until next time, +Daniel Santa Cruz References 1. http://www.haskellforall.com/2013/02/you-could-have-invented-comonads.html 2. http://www.reddit.com/r/haskell/comments/18isiu/comonads_are_objects/ 3. http://hackage.haskell.org/trac/ghc/wiki/OverloadedLists 4. http://www.reddit.com/r/haskell/comments/18ncub/ghc_head_now_has_overloaded… 5. http://www.haskell.org/pipermail/ghc-devs/2013-February/000414.html 6. http://www.reddit.com/r/haskell/comments/18d81l/ghc_head_new_parallel_io_ma… 7. http://chris-taylor.github.com/blog/2013/02/10/the-algebra-of-algebraic-dat… 8. http://www.reddit.com/r/haskell/comments/18902u/the_algebra_of_algebraic_da… 9. http://hpaste.org/82331 10. http://www.reddit.com/r/haskell/comments/18hnh3/deprecation_rage_hpaste/ 11. https://github.com/timthelion?tab=repositories 12. http://www.reddit.com/r/haskell/comments/189cti/github_incorrectly_recogniz… 13. http://imgur.com/Wtx1kT4 14. http://www.reddit.com/r/haskell/comments/18kwe2/stackoverflow_haskell_hits_… 15. http://stackoverflow.com/a/3276557/74497 16. http://www.reddit.com/r/haskell/comments/18jkvw/don_stewart_provides_a_grea… 17. http://chris-taylor.github.com/blog/2013/02/11/the-algebra-of-algebraic-dat… 18. http://www.reddit.com/r/haskell/comments/18bqth/the_algebra_of_algebraic_da… 19. http://www.yesodweb.com/blog/2013/02/upcoming-conduit-1-0 20. http://www.reddit.com/r/haskell/comments/18iskd/upcoming_conduit_10_compari… 21. https://github.com/Yuras/pdf-toolbox 22. http://www.reddit.com/r/haskell/comments/18n0l1/library_a_collection_of_too… 23. http://shuklan.com/haskell/lec05.html 24. http://www.reddit.com/r/haskell/comments/18e0vn/introduction_to_haskell_lec… 25. http://www.haskellers.com/jobs/25 26. http://www.reddit.com/r/haskell/comments/18b37b/fp_complete_hiring_for_hask… 27. http://ocharles.org.uk/blog/posts/2013-02-12-quick-dsls-with-endo-writers.h… 28. http://www.reddit.com/r/haskell/comments/18dhwf/quick_and_easy_dsls_with_wr… 29. http://stackoverflow.com/questions/14906261/least-strict 30. http://stackoverflow.com/questions/14865734/referential-transparency-with-p… 31. http://stackoverflow.com/questions/14820699/truncating-to-word-type 32. http://stackoverflow.com/questions/14852712/is-there-a-generalization-of-th… 33. http://stackoverflow.com/questions/14798613/how-fast-is-data-sequence-seq-c… 34. http://stackoverflow.com/questions/14907510/does-freet-keep-the-equational-… 35. http://stackoverflow.com/questions/14820138/understanding-recursion-in-hask… 36. http://stackoverflow.com/questions/14861379/what-laws-are-the-standard-hask… 37. http://stackoverflow.com/questions/14865442/what-does-mean-in-haskell 38. http://stackoverflow.com/questions/14884561/linking-separate-projects-in-ghc 39. http://stackoverflow.com/questions/14913904/what-are-structures-with-subtra…

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parser combinator for left-recursive grammars
by oleg＠okmij.org 20 Feb '13

20 Feb '13

It is indeed true that a grammar with left-recursion can be transformed to an equivalent grammar without left recursion -- equivalent in terms of the language recognized -- but _not_ in the parse trees. Linguists in particular care about parses. Therefore, it was linguists who developed the parser combinator library for general grammar with left recursion and eps-loops: Frost, Richard, Hafiz, Rahmatullah, and Callaghan, Paul. Parser Combinators for Ambiguous Left-Recursive Grammars. PADL2008. http://cs.uwindsor.ca/~richard/PUBLICATIONS/PADL_08.pdf I have tried dealing with left-recursive grammars and too wrote a parser combinator library: http://okmij.org/ftp/Haskell/LeftRecursion.hs It can handle eps-cycles, ambiguity and other pathology. Here is a sample bad grammar: S -> S A C | C A -> B | aCa B -> B C -> b | C A For more details, see December 2009 Haskell-Cafe thread Parsec-like parser combinator that handles left recursion?

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What is a Haskell way to implement flags?
by Branimir Maksimovic 20 Feb '13

20 Feb '13

In C usual way is to set some bit in integer variable by shifting or oring,and than check flag integer variable by anding with particular flag value.What is Haskell way? Thanks.

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Free lunch with GADTs
by Tristan Ravitch 19 Feb '13

19 Feb '13

Hi cafe, I'm playing around with GADTs and was hoping they would let me have my cake and eat it too. I am trying to use GADTs to tag a type with some extra information. This information is very useful in a few cases, but the more common case is that I just have a list of items without the tags. Ideally I'll be able to recover the tags from the common case via pattern matching. What I have right now is: {-# LANGUAGE GADTs, ExistentialQuantification, EmptyDataDecls, RankNTypes, LiberalTypeSynonyms #-} data Tag1 data Tag2 data T tag where T1 :: Int -> T Tag1 T2 :: Double -> T Tag2 f1 :: [T t] -> Int f1 = foldr worker 0 where worker (T1 i) a = i+a worker _ a = a f2 :: [T Tag1] -> Int f2 = foldr worker 0 where worker :: T Tag1 -> Int -> Int worker (T1 i) a = a + i In f2 I can work with just values with one tag, but f1 is also important to me (working with the untagged values). f1 type checks but I do not know how to make a list [T t]. If I try the naive thing in ghci, it understandably tells me: > let l = [T1 5, T2 6.0] <interactive>:6:16: Couldn't match type `Tag2' with `Tag1' Expected type: T Tag1 Actual type: T Tag2 In the return type of a call of `T2' In the expression: T2 6.0 In the expression: [T1 5, T2 6.0] Fair enough. Is there any magical way to make this possible? I am hoping to avoid making a separate existential wrapper around my type. I know that approach works, but that would make the more common case much less convenient. I also remember trying to use a type synonym with -XLiberalTypeSynonyms: type SimpleT = forall tag . T tag I managed to basically get that working with another variant of my list processing function, but it required -XImpredicativeTypes, which apparently did very unfortunate things to type inference. Thanks

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layers: A prototypical 2d platform game
by Daniel Trstenjak 19 Feb '13

19 Feb '13

Hi all, also if there's not that much to see and only a few minutes of gameplay, but after spending quite a few hours writing it, getting a feeling for Haskell and it's usage, perhaps it's in some way useful for someone, even if just for a few minutes of distraction. https://github.com/dan-t/layers Greetings, Daniel

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Subject: ANNOUNCE: grid-3.0.1 (tile maps for board games or maths)
by Amy de Buitléir 19 Feb '13

19 Feb '13

I'm happy to announce a new major release of the grid package: http://hackage.haskell.org/package/grid https://github.com/mhwombat/grid/wiki (wiki) WHAT'S NEW: Functions for reporting the boundary and centre of bounded grid have been added, along with some miscellaneous new utility functions. IMPORTANT: The order of parameters for some functions has changed to make it easier to use them with map and fold operations. ABOUT GRID: Grid provides tools for working with regular arrangements of tiles, such as might be used in a board game or self-organising map (SOM). Grid currently supports triangular, square, and hexagonal tiles, with various 2D and toroidal layouts. If you need a tile shape or layout that isn't currently provided, please let me know. See Math.Geometry.Grid for an example of how to use the package. Suggestions for improvement are welcome.

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FunPtr to C function with #arguments determined at runtime?
by Ryan Newton 19 Feb '13

19 Feb '13

Hello cafe, I've been poking around and I haven't seen this addressed anywhere except obliquely in the end of section 8.5.1 of the report, where it says that variable argument C functions aren't supported: http://www.haskell.org/onlinereport/haskell2010/haskellch8.html The scenario is pretty simple. I generate C code at runtime. I compile it to a .so. I know how many arguments it expects (but only at runtime), and I get a FunPtr back from 'dlsym'. How do I call it? I was hoping there would be some "Raw" lower level FFI layer that I could use to invoke a C function without automagic marshaling and all the other goodness provided by the normal "foreign import" mechanism. Failing that, will I just have to generate complex wrappers on the C side which I call N times to load up N arguments into some stateful container before finally launching the function? Thanks, -Ryan

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adding recursion to a DSL
by fritsch＠joerg.cc 19 Feb '13

19 Feb '13

I have a tiny DSL that actually works quite well. When I say import language.CWMWL main = runCWMWL $ do out (matrixMult, A, 1, row, matrix-row) then runCWMWL is a function that is exported by language.CWMWL. This parses the experession and takes some action. Now, A is the name of the matrix and the third tuple element would represent the numbe of the row. For example 1 to 10000. I want to achieve some sort of "elegant" (means readable code, a good representation) recursion that would let me do something like sequence [ out (matrixMult, A, n, row, matrix-row) | n <- [1..1000] ] but in a nicer manner an without expending this to 10000 lines of code at compile time. How can I best introduce recursion into my DSL or borrow this from the host language Haskell "effectively"? --Joerg

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Aeson + MongoDB, how to effortlessly store and retrieve json?
by Alfredo Di Napoli 19 Feb '13

19 Feb '13

Good evening guys, suppose I write a very simple parser using Aeson with these types and ToJSON / FromJSON instances: https://github.com/cakesolutions/the-pragmatic-haskeller/blob/master/01-jso… https://github.com/cakesolutions/the-pragmatic-haskeller/blob/master/01-jso… My question is simple: is it possible to "automagically" store the haskell data structure produced from the aeson encoding using http://hackage.haskell.org/packages/archive/mongoDB/1.3.2/doc/html/Database… Any tip/suggestion is welcome, Thanks, Alfredo

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~ operator ?
by briand＠aracnet.com 18 Feb '13

18 Feb '13

Hi all, I was creating "bigger" uncurries which I am simply extending from an existing uncurry I found some where, e.g. uncurry4 :: (a -> b -> c -> d -> e) -> ((a, b, c, d) -> e) uncurry4 f ~(a,b,c,d) = f a b c d when I realized, what's the "~" for ? I've only been able to find a partial explanation that it involves preserving laziness, or something, maybe ? I was hoping someone could enlighten me. Thanks Brian

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