- Haskell-Cafe - Haskell.org

ANNOUNCE: mcpi-0.0.0.2 (The MineCraft-Pi API in Haskell)
by Doug Burke 21 Feb '13

21 Feb '13

I have released versions 0.0.0.1 and 0.0.0.2 (the latter being documentation improvements but it hasn't yet been built) of mcpi: http://hackage.haskell.org/package/mcpi https://github.com/DougBurke/hmcpi It is a very simple, and very incomplete, interface to MineCraft: Pi edition http://pi.minecraft.net/ Now I just need to find out how to get ghc 7.4.2 on my Raspberry Pi (oops, I mean the Raspberry Pi I told my wife I bought as a Christmas gift for the kids) so that I can use this from ghci. Any comments and improvements are more than welcome, Doug

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Unused definitions across modules in a package
by Felipe Almeida Lessa 21 Feb '13

21 Feb '13

Hey! GHC warns me about unused definitions in a module. However, is it possible to warn me about unused definitions in a package? For example, suppose that module A exports function f and that module B uses A.f (everything on a single package). However, after some time B stops using A.f. Is there a way of receiving a warning saying that "A exports f but no other module uses it."? (I know that this warning wouldn't be useful for libraries, since their whole point is to export things.) Cheers! -- Felipe.

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Fwd: How to input Unicode string in Haskell program?
by Semyon Kholodnov 21 Feb '13

21 Feb '13

---------- Forwarded message ---------- From: Semyon Kholodnov <joker.vd(a)gmail.com> Date: Thu, 21 Feb 2013 16:26:58 +0400 Subject: Re: [Haskell-cafe] How to input Unicode string in Haskell program? To: Alexander V Vershilov <alexander.vershilov(a)gmail.com> I know that this problem doesn't exist on Linux. But I work on Windows. And I use WinGHCi primarily, because it has RTF component in it which shows Unicode. But it turns out WinGHCi merely sends commands and receives results to/from ghci.exe. And it does it in a weird way: it sets ghci's console code pages to current system codepages (ACP), reads results from ghci as ACP, but sends commands to it as UTF8. Which got interpreted as ACP. Now, however, I have a fix for WinGHCi: in StartGHCI.c one should replace SetConsoleOutputCP(GetACP()); SetConsoleCP(GetACP()); with SetConsoleOutputCP(CP_UTF8); SetConsoleCP(CP_UTF8); and in Utf8.c, in UnicodeToLocalCodePage() body, "WideCharToMultiByte( CP_ACP" has to be replaced with "WideCharToMultiByte( CP_UTF8", and in LocalCodePageToUnicode() body, "MultiByteToWideChar( CP_ACP" has to be replaced with "MultiByteToWideChar( CP_UTF8". After recompiling, everything works great: Prelude> x <- getLine résumé 履歴書 резюме Prelude> putStrLn x résumé 履歴書 резюме Prelude> Is there any way to ask for this fix to be included in WinGHCi and Haskell Platform? 2013/2/21, Alexander V Vershilov <alexander.vershilov(a)gmail.com>: > The problem is that Prelude.getLine uses current locale to load characters: > for example if you have utf8 locale, then everything works out of the box: > >> $ runhaskell 1.hs >> résumé 履歴書 резюме >> résumé 履歴書 резюме > > But if you change locale you'll have error: > >> LANG="C" runhaskell 1.hs >> résumé 履歴書 резюме >> 1.hs: <stdin>: hGetLine: invalid argument (invalid byte sequence) > > To force haskell use UTF8 you can load string as byte sequence and convert > it to UTF-8 > charecters for example by > > import qualified Data.ByteString as S > import qualified Data.Text.Encoding as T > > main = do > x <- fmap T.decodeUtf8 S.getLine > > now code will work even with different locale, and you'll load UTF8 from > shell > independenty of user input's there > > -- > Alexander > > > On 21 February 2013 13:58, Semyon Kholodnov <joker.vd(a)gmail.com> wrote: > >> Imagine we have this simple program: >> >> module Main(main) where >> >> main = do >> x <- getLine >> putStrLn x >> >> Now I want to run it somehow, enter "résumé 履歴書 резюме" and see this >> string printed back as "résumé 履歴書 резюме". Now, the first problem is >> that my computer runs Windows, which means that I can't use ghci >> ":main" or result of "ghc main.hs" to enter such an outrageous string >> — Windows console is locked to one specific local code page, and no >> codepage contains Latin-1, Cyrillic and Kanji symbols at the same >> time. >> >> But there is also WinGHCi. So I do ":main", copy-paste this string >> into the window (It works! Because Windows has Unicode for 20 years >> now), but the output is all messed up. In a rather curious way, >> actually: the input string is converted to UTF-8 byte string, and its >> bytes are treated as being characters from my local code page. >> >> So, it appears that I have no way to enter Unicode strings into my >> Haskell programs by hands, I should read them from files. That's sad, >> and I refuse to think I am the first one with such a problem, so I >> assume there is a solution/workaround. Now would someone please tell >> me this solution? Except from "Just stick to 127 letters of ASCII", of >> course. >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> > > > > -- > Alexander >

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PhD Studentship in Nottingham
by Graham Hutton 21 Feb '13

21 Feb '13

Dear all, The School of Computer Science at the University of Nottingham is currently advertising a PhD position on applying functional programming techniques to optimisation problems: http://www.cs.nott.ac.uk/~ajp/PhD-opportunities.htm This would be a great position for someone who is interested in applying FP ideas to real-world problems. The closing date for applications is 28th February 2013. If you have any queries about the position, please contact Andrew Parkes <andrew.parkes(a)nottingham.ac.uk>. Best wishes, Graham -- Prof Graham Hutton Functional Programming Lab School of Computer Science University of Nottingham, UK http://www.cs.nott.ac.uk/~gmh This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please send it back to me, and immediately delete it. Please do not use, copy or disclose the information contained in this message or in any attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. This message has been checked for viruses but the contents of an attachment may still contain software viruses which could damage your computer system: you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

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Embedded haskell?
by Mike Meyer 21 Feb '13

21 Feb '13

I've been working with open source rc aircraft transmitter software, and in looking at the shortcomings of one of them, started thinking about embedding a language. There are a number of options that can work here, like FORTH or a basic. But then I realized that Haskell - or similar functional language - could well be a good fit. The software is meant to let the end user express how the various inputs - joysticks, switches, trims, knobs - are mapped to values the radio sends on the various control channels. All the key values are immutable - you either read them from hardware once in the process of building a frame to transmit, or you fill them into a frame and transmit it, then start over for the next frame. You just need to let the end user express the functions to go from one to the other. The other restraint is that you need to be able to change the code in the field, with the only computer available being the embedded one. You might have a touch-screen, or you might just have cursor keys. Either way, actually inputting a program could be interesting. Similarly, the entire system: compiler, interpreter, whatever - needs to run on the embedded computer. A quick google turns up Hume, which seems to be designed for this kind of thing, though not with the "in the field" restrictions. Anyone have any other suggestions of software that might fit here? Experience with any of that software? Other suggestions? Thanks, <mike

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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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generalized, tail-recursive left fold that can finish tne computation prematurely
by Petr Pudlák 18 Feb '13

18 Feb '13

Dear Haskellers, while playing with folds and trying to implement `!!` by folding, I came to the conclusion that: - `foldr` is unsuitable because it counts the elements from the end, while `!!` needs counting from the start (and it's not tail recursive). - `foldl` is also unsuitable, because it always traverses the whole list. I came up with the following tail-recursive generalization of `foldl` that allows exiting the computation prematurely: foldlE :: (a -> c) -> (a -> b -> Either c a) -> Either c a -> [b] -> c foldlE f g = fld where fld (Left c) _ = c fld (Right a) [] = f a fld (Right a) (x:xs) = fld (g a x) xs `foldl` can be defined from it as foldl'' :: (a -> b -> a) -> a -> [b] -> a foldl'' f z = foldlE id ((Right .) . f) (Right z) and `!!` as: -- Checks for a negative index omitted for brevity. index :: Int -> [a] -> a index i = foldlE (error $ "No such index") f (Right i) where f 0 x = Left x f n _ = Right (n - 1) Is something like that already available somewhere? Best regards, Petr Pudlak

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generalized, tail-recursive left fold that can finish tne computation prematurely
by Petr Pudlák 18 Feb '13

18 Feb '13

Dear Haskellers, while playing with folds and trying to implement `!!` by folding, I came to the conclusion that: - `foldr` is unsuitable because it counts the elements from the end, while `!!` needs counting from the start (and it's not tail recursive). - `foldl` is also unsuitable, because it always traverses the whole list. I came up with the following tail-recursive generalization of `foldl` that allows exiting the computation prematurely: foldlE :: (a -> c) -> (a -> b -> Either c a) -> Either c a -> [b] -> c foldlE f g = fld where fld (Left c) _ = c fld (Right a) [] = f a fld (Right a) (x:xs) = fld (g a x) xs `foldl` can be defined from it as foldl'' :: (a -> b -> a) -> a -> [b] -> a foldl'' f z = foldlE id ((Right .) . f) (Right z) and `!!` as: -- Checks for a negative index omitted for brevity. index :: Int -> [a] -> a index i = foldlE (error $ "No such index") f (Right i) where f 0 x = Left x f n _ = Right (n - 1) Is something like that already available somewhere? Best regards, Petr Pudlak

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JSON querying
by Sergey Mironov 18 Feb '13

18 Feb '13

Hi folks. Hackage contains several JSON packages but as far as I see, they all provide 'static' conversion from JSON format to Haskell data type. Is there a method of converting object containing optional filed 'a' to for example Maybe a. Thanks, Sergey

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Re: [Haskell-cafe] ifdef based on which OS you're on
by Doug McIlroy 17 Feb '13

17 Feb '13

With apologies for prolonging a tangential topic, I'd like to sharpen anti-ifdef comments that have been posted already. 1. An ifdef for portability is an admission of nonportability. What it does is point out a nonportability--a useful crutch for maintainers, but a crutch nonetheless. 2. Ifdefs violate program structure. All ifdefs appear at top level. Yet, save for those that control top-level declarations, they embody conditionals at inner levels of structure. Doug McIlroy

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Terminal Win32/Building Haskeline/8.3 file issue
by Matthew Lamari 17 Feb '13

17 Feb '13

Hit this compiling darcs on a new machine. . . . But it may be more generic to Haskell platform itself. Bottom line, it presents a situation where someone could end up dead-in-the-water with respect to using haskell libraries. I burned a lot of time tracking this down, it may discourage users from the platform. The problem manifests itself when your Win32 home directory has a long name (or one with a space) and you're compiling haskeline, linking on the 15th step. Basically, ld.exe croaks when fed one of these names rather than an 8.3 I can provide more specific details on the point of failure on request; but it was a pain to track down. Maybe a change to the linker, or some specific project, may get rid of the 8.3 dependency. A solution that got me out was to enable 8.3 on the volume, and on the user directory. A fix: Logging in as a *different* user: Open a command prompt with Administrator priveleges fsutil 8dot3name query c: (tells you the current status - you probably see a 2 - volume-select of the feature) fsutil 8dot3name set c: 1 (This sets volume c to create 8.3 names - for future files only) fsutil file setshortname "c:\Users\YouHaveA VeryBigName" YOUHAV~1 (This only works if you're not logged in as 'YouHaveA VeryBigName', otherwise the directory will be in-use). You need this step as even though you've changed the policy on 8.3 naming, this directory was probably made during the prior policy) dir /-n (This shows you the 8.3-sensitive names. You see YOUHAV~1 in the list, you got it right) The user may need to uninstall and reinstall haskell-platform, and wipe all Cabal and GHC directories under AppData/Roaming

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MonadReader laws
by Roman Cheplyaka 17 Feb '13

17 Feb '13

Has anyone formulated a reasonable set of laws for MonadReader? The instances for continuation-based monad transformers contradict any intuition about local and ask. E.g. for LogicT: > flip runReader 1 $ observeAllT $ local (const 2) ask [1] > flip runReader 1 $ observeAllT $ local (const 2) mzero <|> ask [2] I wonder whether there are any real use cases for such instances (and, consequently, whether they should be removed). Roman

1 0

[ANNOUNCE] London Haskell user group - Being Lazy - Wed 27-Feb-13
by Derek Wright 17 Feb '13

17 Feb '13

Wednesday 27-Feb-2013 6-45pm (prompt) City University, College Building, St John Street, London. EC1V 4PB http://www.meetup.com/London-HUG/events/103545552/ Being Lazy Speaker: Peter Marks This talk will explore lazy evaluation in Haskell. After a brief introduction to non-strictness, and establishing a basic conceptual model of evaluation in Haskell, the primary focus will be on how we can utilize laziness to allow us to write programs differently to how we would in an eagerly evaluated language. The session will be rich on code with a worked example of a complete program. We'll also touch on some of the pitfalls of laziness that we run into in real world situations. Peter Marks has been developing software for over thirty years and has been using Haskell commercially for four years. He currently leads the FPF team at Barclays and is an active member of the London Haskell community, running the Hoodlums meetup group. N.B. The start time is slightly later than previously, to make sure everybody has arrived by the time the talk starts. Doors open 6-30pm, try not to be late!

1 0

ANNOUNCE: hackage-proxy 0.1.0.0
by Michael Snoyman 17 Feb '13

17 Feb '13

I'd like to announce the first release of a new tool called hackage-proxy. The purpose is to provide a local proxy for a Hackage server, which somehow modifies files in transport. The motivating case for this was getting more meaningful error output from Stackage when compiling against GHC HEAD. When compiling against actual Hackage, cabal will simply refuse to try to build packages which have upper version bounds such as "base < 4.7". This introduces a big dilemma: * Package authors do not want to bump the version bounds on their packages until they've tested against that version. * It's very difficult to do meaningful tests of GHC until packages on Hackage have been updated. Hopefully this package can help resolve the dilemma. Instead of requiring authors to upload new versions of their packages in order to test them, this proxy will modify the cabal files it downloads and strip off the version bounds of specified packages. Then, you can test with a newer version of GHC and find actual compilation errors instead of version bound constraints. ## Example Usage 1. cabal install hackage-proxy 2. Run hackage-proxy. By default, it will use the official Hackage server as the source, drop bounds on the packages base, process, directory, template-haskell, and Cabal, and serve from port 4200. All of this can be modified via command-line options. 3. Edit your ~/.cabal/config file. Comment out the hackage.haskell.orglines, and add in something like the following: remote-repo: hackage-proxy:http://localhost:4200 4. cabal update 5. cabal install your-package-list-here I think this can be a very valuable tool for anyone wanting to test out newer versions of GHC. In addition, as part of my normal Stackage work, I'm now collecting fairly detailed error logs of a number of packages. If this would be useful for the GHC team or anyone else, let me know and I can try and provide the logs somehow.

1 0

Netwire, keyboard events, and games
by Patrick Hurst 17 Feb '13

17 Feb '13

I'm using netwire to build a game; one of the things the player can do is move around using WASD. I want to use key *events* as the 'basis' of my wires, not the entire state vector of the keyboard so that, for example, a press event on D increments the velocity by (200, 0) and a release event decrements it by that much. However, this has the problem that I can't actually get the velocity unless I have an event to feed into the wires, which means I can only update the screen when the user presses or releases a key. While this might make for interesting gameplay, it's not what I want. is the right thing to do to make the input something like a Maybe KeyEvent instead, and pass in Nothing whenever I want to get output without an event for, e.g., a render?

3 3

What magic has the new IO manager done to improve performance ?
by yi huang 16 Feb '13

16 Feb '13

I' m curious about the design and trade offs behind the new IO manager. I see two changes from the code: 1. Run IO manager thread on each capability. 2. Use ONESHOT flag to save a system call. Is there other interesting things to know? Is it possible to use epoll's ET mode to save even more system calls? -- http://yi-programmer.com/

2 1

Text.JSON and utf8
by Martin Hilbig 16 Feb '13

16 Feb '13

hi, tl;dr: i propose this patch to Text/JSON/String.hs and would like to know why it is needed: @@ -375,7 +375,7 @@ where go s1 = case s1 of - (x :xs) | x < '\x20' || x > '\x7e' -> '\\' : encControl x (go xs) + (x :xs) | x < '\x20' -> '\\' : encControl x (go xs) ('"' :xs) -> '\\' : '"' : go xs ('\\':xs) -> '\\' : '\\' : go xs (x :xs) -> x : go xs i recently stumbled upon CouchDB telling me i'm sending invalid json. i basically read lines from a utf8 file with german umlauts and send them to CouchDB using Text.JSON and Database.CouchDB. $ file lines.txt lines.txt: UTF-8 Unicode text lets take 'ö' as an example. i use LANG=de_DE.utf8 ghci tells > 'ö' '\246' > putChar '\246' ö > putChar 'ö' ö > :m + Text.JSON Database.CouchDB > runCouchDB' $ newNamedDoc (db "foo") (doc "bar") (showJSON $ toJSObject [("test","ö")]) *** Exception: HTTP/1.1 400 Bad Request Server: CouchDB/1.2.1 (Erlang OTP/R15B03) Date: Mon, 11 Feb 2013 13:24:49 GMT Content-Type: text/plain; charset=utf-8 Content-Length: 48 Cache-Control: must-revalidate couchdb log says: Invalid JSON: {{error,{10,"lexical error: invalid bytes in UTF8 string.\n"}},<<"{\"test\":\"<F6>\"}">>} this is indeed hex ö: > :m + Numeric > putChar $ toEnum $ fst $ head $ readHex "f6" ö if i apply the above patch and reinstall JSON and CouchDB the doc creation works: > runCouchDB' $ newNamedDoc (db "db") (doc "foo") (showJSON $ toJSObject [("test", "ö")]) Right someRev but i dont get back the ö i expected: > Just (_,_,x) <-runCouchDB' $ getDoc (db "foo") (doc "bar") :: IO (Maybe (Doc,Rev,JSObject String)) > let Ok y = valFromObj "test" =<< readJSON x :: Result String > y "\195\188" > putStrLn y Ã¼ apperently with curl everything works fine: $ curl localhost:5984/db/foo -XPUT -d '{"test": "ö"}' {"ok":true,"id":"foo","rev":"someOtherRev"} $ curl localhost:5984/db/foo {"_id":"bars","_rev":"someOtherRev","test":"ö"} so how can i get my precious ö back? what am i doing wrong or does Text.JSON need another patch? another question: why does encControl in Text/JSON/String.hs handle the cases x < '\x100' and x < '\x1000' even though they can never be reached with the old predicate in encJSString (x < '\x20') finally: is '\x7e' the right literal for the job? thanks for reading have fun martin

3 2

What magic has the new parallel io manager done to improve performance?
by yi huang 16 Feb '13

16 Feb '13

I'm curious about the design and trade offs in IO manager, I can point out two changes that should boost performance: 1. Run an IO manager thread on each capability. 2. Use ONESHOT flag to save a system call. I wonder is there anything else? It must be interesting. -- http://yi-programmer.com/

1 1

how to release (on hackage) a package that needs foreign libs?
by Johannes Waldmann 16 Feb '13

16 Feb '13

What is the recommended approach for publishing (on hackage) a package that depends on external libraries? In my case, the current version of http://hackage.haskell.org/package/satchmo (sources at https://github.com/jwaldmann/satchmo) depends on minisat-haskell-bindings (in Haskell), minisat-c-bindings (in C), and minisat (C++), all from https://github.com/niklasso/ My guess is that this would require that the external libraries are packaged (deb, rpm?), then hackage gods install them, and only then, my package will be built (and docs will be shown)? - J.W.

1 0

optimization of recursive functions
by Andrew Polonsky 16 Feb '13

16 Feb '13

Hello, Is there any difference in efficiency between these two functions, when compiled with all optimizations? map f [] = [] map f (a:as) = f a : map f as and map f x = map' x where map' [] = [] map' (a:as) = f a : map' as Thanks, Andrew

3 3

ANN: rss2irc-1.0, announces RSS/Atom feed updates to IRC
by Simon Michael 15 Feb '13

15 Feb '13

I'm pleased to announce a new release of rss2irc, the software behind hackagebot on #haskell. rss2irc is an IRC bot that polls an RSS or Atom feed and announces updates to an IRC channel, with options for customizing output and behavior. It aims to be an easy-to-use, reliable, well-behaved bot. Release notes: 1.0 (2013/2/15) New: * more robust item detection and duplicate announcement protection, with simpler options * easier irc address syntax, drop -p/--port option * can poll urls with semicolon parameter separator (eg darcsweb's) * can poll https feeds * can poll from stdin (-) * can poll a file containing multiple copies of a feed (eg for testing) * can announce item urls containing percent * `--cache-control` option sets a HTTP Cache-Control header * `--use-actions` announces with CTCP ACTIONs (like the /me command) Fixed: * updated for GHC 7.6 & current libs * initialises http properly on microsoft windows * builds threaded and optimised by default * thread and error handling is more robust, eg don't ignore exceptions in the irc writer thread * no longer adds stray "upload:" to IRC messages * renamed --dupe-descriptions to `--allow-duplicates` * dropped --debug flag * new item detection and announcing is more robust * announcements on console are clearer * a simulated irc connection is not logged unless --debug-irc is used Best, -Simon

1 0

(+) on two lists ?
by sheng chen 15 Feb '13

15 Feb '13

Hi, I was puzzled by the following little program. sum' [] = [] sum' (x:xs) = x + sum' xs I thought the GHC type checker will report a type error. However, the type checker accepts this program and gives the type Num [a] => [[a]] -> [a] When I add type annotation to the program sum' :: Num [a] => [[a]] -> [a] sum' [] = [] sum' (x:xs) = x + sum' xs The GHC asks me to add FlexibleContexts language extension. I would appreciate explanation on this issue. Thanks, Sheng

5 4

How to quantify the overhead?
by Daryoush Mehrtash 15 Feb '13

15 Feb '13

Before I write the code I like to be able to quantify my expected result. But I am having hard time quantifying my expected result for alternative approaches in Haskell. I would appreciate any comments from experienced Haskell-ers on this problem: Suppose I have a big list of integers and I like to find the first two that add up to a number, say 10. One approach is to put the numbers in the map as I read them and each step look for the 10-x of the number before going on to the next value in the list. Alternatively, I am trying to see if I it make sense to covert the list to a series of computations that each is looking for the 10-x in the rest of the list (running them in breath first search). I like to find out sanity of using the Applicative/Alternative class's (<|>) operator to set up the computation. So each element of the list (say x) is recursively converted to a computation to SearchFor (10 - x) that is (<|>)-ed with all previous computation until one of the computations returns the pair that add up to 10 or all computation reach end of the list. Does the approach make any sense? What kind of overhead should I expect if I convert the numbers to a computation on the list? In one case I would have a number in a map and in the other I would have a number in a computation over a list. How do I quantify the overheads? Thanks, Daryoush

2 1

python under attack. ??Haskell??
by Rustom Mody 15 Feb '13

15 Feb '13

Please see http://pyfound.blogspot.in/2013/02/python-trademark-at-risk-in-europe-we.ht… I cannot say that I understand whats really going on The one thing that I get is that there was some minor neglect a decade or more ago. <http://blog.languager.org>Maybe there are things that Haskell needs to beware of?

1 0

performance question
by Nicolas Bock 15 Feb '13

15 Feb '13

Hi list, I wrote a script that reads matrix elements from standard input, parses the input using a regular expression, and then bins the matrix elements by magnitude. I wrote the same script in python (just to be sure :) ) and find that the python version vastly outperforms the Haskell script. To be concrete: $ time ./createMatrixDump.py -N 128 | ./printMatrixDecay real 0m2.655s user 0m2.677s sys 0m0.095s $ time ./createMatrixDump.py -N 128 | ./printMatrixDecay.py - real 0m0.445s user 0m0.615s sys 0m0.032s The Haskell script was compiled with "ghc --make printMatrixDecay.hs". Could you have a look at the script and give me some pointers as to where I could improve it, both in terms of performance and also generally, as I am very new to Haskell. Thanks already, nick

14 34

When the gauge depends on what it measures
by Alexander Bernauer 14 Feb '13

14 Feb '13

Hi, I am currently working on benchmarking the pretty library. Pretty itself has no non-trivial dependencies. But criterion and language-c, which I use to perform the benchmarks, directly and/or indirectly depend on pretty. Everything is configured in one Cabal file via library and benchmark sections. Now, whenever I modify pretty and want to rebuild the bench executable, things become messy. Either I rebuild criterion et al. as well, or the bench executable ends up with two different pretty version built in - if this is even possible. Until now I could not find a robust workflow for this scenario. At some point pretty "shadows" itself and the build fails (c.f. my previous caffee post). What is the right way to handle this scenario? Confused greetings, Alex

4 3

Why isn't "Program Derivation" a first class citizen?
by Nehal Patel 14 Feb '13

14 Feb '13

A few months ago I took the Haskell plunge, and all goes well... -- but I really want to understand the paradigms as fully as possible, and as it stands, I find myself with three or four questions for which I've yet to find suitable answers. I've picked one to ask the cafe -- like my other questions, it's somewhat amorphous and ill posed -- so much thanks in advance for any thoughts and comments! ---- Why isn't "Program Derivation" a first class citizen? --- (Hopefully the term "program derivation" is commonly understood? I mean it in the sense usually used to describe the main motif of Bird's "The Algebra of Programming." Others seem to use it as well...) For me, it has come to mean solving problems in roughly the following way 1) Defining the important functions and data types in a pedantic way so that the semantics are clear as possible to a human, but possibly "inefficient" (I use quotes because one of my other questions is about whether it is really possible to reason effectively about program performance in ghc/Haskell…) 2) Work out some proofs of various properties of your functions and data types 3) Use the results from step 2 to provide an alternate implementation with provably same semantics but possibly much better performance. To me it seems that so much of Haskell's design is dedicated to making steps 1-3 possible, and those steps represent for me (and I imagine many others) the thing that makes Haskell (and it cousins) so beautiful. And so my question is, that in 2013, why isn't this process a full fledged part of the language? I imagine I just don't know what I'm talking about, so correct me if I'm wrong, but this is how I understand the workflow used in practice with program derivation: 1) state the problem pedantically in code, 2) work out a bunch of proofs with pen and paper, 3) and then translate that back into code, perhaps leaving you with function_v1, function_v2, function_v3, etc -- that seems fine for 1998, but why is it still being done this way? What I'm asking about might sound related to theorem provers, -- but if so ,I feel like what I'm thinking about is not so much the very difficult problem of automated proofs or even proof assistants, but the somewhat simpler task of proof verification. Concretely, here's a possibility of how I imagine the workflow could be: ++ in code, pedantically setup the problem. ++ in code, state a theorem, and prove it -- this would require a revision to the language (Haskell 201x) and perhaps look like Isabella's ISAR -- a -structured- proof syntax that is easy for humans to construct and understand -- in particular it would possible to easily reuse previous theorems and leave out various steps. At compile time, the compiler would check that the proof was correct, or perhaps complain that it can't see how I went from step 3 to step 4, in which case I might have to add in another step (3b) to help the compiler verify the proof. ++ afterwards, I would use my new theorems to create semantically identical variants of my original functions (again this process would be integrated into the language) While I feel like theorem provers offer some aspects of this workflow (in particular with the ability to export scala or haskell code when you are done), I feel that in practice it is only useful for modeling fairly technically things like protocols and crypto stuff -- whereas if this existed within haskell proper it would find much broader use and have greater impact. I haven't fully fleshed out all the various steps of what exactly it would mean to have program derivation be a first class citizen, but I'll pause here and followup if a conversation ensues. To me, it seems that something like this should be possible -- am i being naive? does it already exist? have people tried and given up? is it just around the corner? can you help me make sense of all of this? thanks! nehal

7 6

ANNOUNCE: graphviz-2999.16.0.0
by Ivan Lazar Miljenovic 14 Feb '13

14 Feb '13

I'm pleased to announce that I've finally tracked down all the pesky little bugs that prevented me from releasing version 2999.16.0.0 of my graphviz library, that helps you use the Graphviz suite of tools to visualise graphs. The important changes are: * Add support for Graphviz-2.30.0: - New attributes: + `Area` + `Head_LP` + `LayerListSep` + `LayerSelect` + `Tail_LP` + `XLP` - `BgColor`, `Color` and `FillColor` now take a list of colors with optional weightings. - Layer handling now supports layer sub-ranges. - Added the voronoi-based option to `Overlap`. - Added the `Striped` and `Wedged` styles. * Updates to attributes and their values: - The following attributes have had their values changed to better reflect what they represent: + `FontPath` takes a `Path` value. + `Layout` takes a `GraphvizCommand` (and thus `GraphvizCommand` has been moved to `Data.GraphViz.Attributes.Complete`). - Added `MDS` to `Model` (which had somehow been overlooked). - Various attributes now have defaults added/updated/removed if wrong. - Removed the following deprecated attributes: + `ShapeFile` + `Z` * Now any value that has a defined default can be parsed when the Dot code just has `attribute=""` (which `dot -Tcanon` is fond of doing to "reset" attributes). * `GraphvizCommand` now defines `Sfdp` (which had somehow been overlooked up till now). * The `canonicalise` and related functions have been re-written; whilst there might be some cases where their behaviour does not fully match how `dot -Tcanon` and `tred` behave (due to the interaction of various attributes), the new implementation is much more sane. * Use temporary files rather than pipes when running dot, etc. Makes it more portable, and also avoids issues where dot, etc. use 100% CPU when a graph is passed in via stdin. Original patch by Henning Thielemann. -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com http://IvanMiljenovic.wordpress.com

1 0

Haskell Weekly News: Issue 258
by Daniel Santa Cruz 14 Feb '13

14 Feb '13

Welcome to issue 258 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of February 03 to 09, 2013. Quotes of the Week * shachaf: The trouble with the lens rabbit hole is that there are a few of us here at the bottom, digging. * monochrom: "a monad is like drinking water from a bottle without human mouth touching bottle mouth" Top Reddit Stories * GHCi 7.4.2 is finally working on ARM Domain: luzhuomi.blogspot.com, Score: 49, Comments: 15 On Reddit: [1] http://goo.gl/yBh9o Original: [2] http://goo.gl/Kgx9R * Implementation of a Java Just In Time Compiler in Haskell Domain: blog.wien.tomnetworks.com, Score: 41, Comments: 15 On Reddit: [3] http://goo.gl/BjpX3 Original: [4] http://goo.gl/f1Qrh * I/O is pure Domain: chris-taylor.github.com, Score: 32, Comments: 41 On Reddit: [5] http://goo.gl/pYMXz Original: [6] http://goo.gl/9xeKd * Introduction to Haskell, Lecture 4 is Live (Pattern matching and Guards) Domain: shuklan.com, Score: 28, Comments: 7 On Reddit: [7] http://goo.gl/wvFO6 Original: [8] http://goo.gl/sKF5C * What makes comonads important? Domain: self.haskell, Score: 20, Comments: 41 On Reddit: [9] http://goo.gl/CqS7Z Original: [10] http://goo.gl/CqS7Z * OdHac, the Haskell Hackathon in Odessa Domain: haskell.org, Score: 18, Comments: 2 On Reddit: [11] http://goo.gl/ZumYI Original: [12] http://goo.gl/Pfuaq * NYC Haskell Meetup Video: Building a Link Shortener with Haskell and Snap Domain: vimeo.com, Score: 18, Comments: 3 On Reddit: [13] http://goo.gl/j42PG Original: [14] http://goo.gl/RRgsl * Evaluation-State Assertions in Haskell Domain: joachim-breitner.de, Score: 18, Comments: 6 On Reddit: [15] http://goo.gl/lMMT1 Original: [16] http://goo.gl/7eLI5 * A post about computational classical type theory Domain: queuea9.wordpress.com, Score: 17, Comments: 14 On Reddit: [17] http://goo.gl/r8DOO Original: [18] http://goo.gl/UIxIY * parellella has a forum board for haskell. anyone working in this direction? Domain: forums.parallella.org, Score: 13, Comments: 5 On Reddit: [19] http://goo.gl/BSfRd Original: [20] http://goo.gl/qLnG0 * Haskell Lectures - University of Virginia (Nishant Shukla) Domain: shuklan.com, Score: 13, Comments: 3 On Reddit: [21] http://goo.gl/CxTPI Original: [22] http://goo.gl/JkhXz * NYC Haskell Meetup Video: Coding and Reasoning with Purity, Strong Types and Monads Domain: vimeo.com, Score: 13, Comments: 2 On Reddit: [23] http://goo.gl/MmGZb Original: [24] http://goo.gl/Vj63w * Introducing fixed-points via Haskell Domain: apfelmus.nfshost.com, Score: 9, Comments: 34 On Reddit: [25] http://goo.gl/UuCc1 Original: [26] http://goo.gl/SCVJc Top StackOverflow Questions * Is spoon unsafe in Haskell? votes: 19, answers: 2 Read on SO: [27] http://goo.gl/6RDku * What general structure does this type have? votes: 17, answers: 4 Read on SO: [28] http://goo.gl/2GV0V * Why aren't Nums Ords in Haskell? votes: 14, answers: 2 Read on SO: [29] http://goo.gl/8Hrkn * Why does cabal install reinstall packages already in .cabal/lib votes: 12, answers: 1 Read on SO: [30] http://goo.gl/yMXCh * Free Monad of a Monad votes: 12, answers: 2 Read on SO: [31] http://goo.gl/kpV2q * Haskell pattern match “diverge” and ⊥ votes: 11, answers: 2 Read on SO: [32] http://goo.gl/VeRUJ * How can one make a private copy of Hackage votes: 11, answers: 2 Read on SO: [33] http://goo.gl/ZNAwL * Recursion Schemes in Agda votes: 9, answers: 1 Read on SO: [34] http://goo.gl/xtmqr Until next time, +Daniel Santa Cruz References 1. http://luzhuomi.blogspot.com/2013/02/ghci-742-is-finally-working-on-arm.html 2. http://www.reddit.com/r/haskell/comments/17zxgj/ghci_742_is_finally_working… 3. http://blog.wien.tomnetworks.com/2013/02/06/thesis/ 4. http://www.reddit.com/r/haskell/comments/17zqgr/implementation_of_a_java_ju… 5. http://chris-taylor.github.com/ 6. http://www.reddit.com/r/haskell/comments/1874at/io_is_pure/ 7. http://shuklan.com/haskell/lec04.html 8. http://www.reddit.com/r/haskell/comments/17y18q/introduction_to_haskell_lec… 9. http://www.reddit.com/r/haskell/comments/185k0s/what_makes_comonads_importa… 10. http://www.reddit.com/r/haskell/comments/185k0s/what_makes_comonads_importa… 11. http://www.haskell.org/pipermail/haskell-cafe/2013-February/106216.html 12. http://www.reddit.com/r/haskell/comments/17xxyf/odhac_the_haskell_hackathon… 13. http://vimeo.com/59109358 14. http://www.reddit.com/r/haskell/comments/185la5/nyc_haskell_meetup_video_bu… 15. https://www.joachim-breitner.de/blog/archives/590-Evaluation-State-Assertio… 16. http://www.reddit.com/r/haskell/comments/1870oe/evaluationstate_assertions_… 17. http://queuea9.wordpress.com/2012/11/05/canonize-this/ 18. http://www.reddit.com/r/haskell/comments/180uqv/a_post_about_computational_… 19. http://forums.parallella.org/viewforum.php?f=21 20. http://www.reddit.com/r/haskell/comments/180muq/parellella_has_a_forum_boar… 21. http://shuklan.com/haskell/index.html 22. http://www.reddit.com/r/haskell/comments/184r9v/haskell_lectures_university… 23. http://vimeo.com/59215336 24. http://www.reddit.com/r/haskell/comments/185li3/nyc_haskell_meetup_video_co… 25. http://apfelmus.nfshost.com/blog/2010/07/02-fixed-points-video/player.html 26. http://www.reddit.com/r/haskell/comments/186bi1/introducing_fixedpoints_via… 27. http://stackoverflow.com/questions/14710928/is-spoon-unsafe-in-haskell 28. http://stackoverflow.com/questions/14715811/what-general-structure-does-thi… 29. http://stackoverflow.com/questions/14690608/why-arent-nums-ords-in-haskell 30. http://stackoverflow.com/questions/14699269/why-does-cabal-install-reinstal… 31. http://stackoverflow.com/questions/14740213/free-monad-of-a-monad 32. http://stackoverflow.com/questions/14698414/haskell-pattern-match-diverge-a… 33. http://stackoverflow.com/questions/14758423/how-can-one-make-a-private-copy… 34. http://stackoverflow.com/questions/14699334/recursion-schemes-in-agda 35. https://plus.google.com/105107667630152149014/about

1 0

searching the attic for lambada
by John Lask 13 Feb '13

13 Feb '13

I'm interested in resurrecting the idl generator from lambada: http://www.dcs.gla.ac.uk/mail-www/haskell/msg02391.html is the code out there in anyone's attic?

2 1

ghc-mod and cabal targets
by Francesco Mazzoli 13 Feb '13

13 Feb '13

Hi, I have been using ghc-mod for a long time and I can’t give it up now, but I have an annoying problem. When I have a cabal file with a library and an executable depending on the library (for example here <https://github.com/bitonic/kant/blob/master/kant.cabal>), ghc-mod is not happy, complaining in every file that Error:<command line>: cannot satisfy -package kant Where ‘kant’ is the package that the cabal file defines with the library that the executable needs. Note that if I issue ‘cabal configure; cabal build’, everything goes smoothly. Did anybody stumble on the same problem? Thanks, Francesco

4 21

Structured Graphs
by John Sharley 13 Feb '13

13 Feb '13

What are the prospects for Haskell supporting Structured Graphs as defined here? http://www.cs.utexas.edu/~wcook/Drafts/2012/graphs.pdf Is there an interest by developers of GHC in doing this? -John

2 1

Problem installing cabal-dev
by David Turner 13 Feb '13

13 Feb '13

Hi, From a clean install of Haskell Platform 2012.4.0.0 (on Windows) I have issued just: > cabal update > cabal install cabal-install > cabal install cabal-dev The last command fails with: Resolving dependencies... In order, the following would be installed: tar-0.3.2.0 (new package) transformers-0.2.2.0 (new version) mtl-2.0.1.0 (new version) parsec-3.1.3 (reinstall) changes: mtl-2.1.2 -> 2.0.1.0 network-2.3.2.0 (new version) HTTP-4000.2.8 (new version) cabal-dev-0.9.1 (new package) cabal: The following packages are likely to be broken by the reinstalls: network-2.3.1.0 haskell-platform-2012.4.0.0 cgi-3001.1.7.4 HTTP-4000.2.5 Use --force-reinstalls if you want to install anyway. I *think* the problem is that cabal-dev has these dependencies: mtl >= 1.1 && < 2.1, transformers >= 0.2 && < 0.3, where the latest version of mtl and transformers are 2.1.2 and 0.3 respectively. At least, if I download the latest cabal-dev package and relax those upper bounds I can get it to install. The irony is, of course, that I'm trying to install cabal-dev to get me out of a totally unrelated dependencies hole! What should I do now? Cheers, -- David Turner Senior Consultant Tracsis PLC Tracsis PLC is a company registered in England and Wales with number 5019106.

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How far compilers are allowed to go with optimizations?
by Jan Stolarek 13 Feb '13

13 Feb '13

Hi all, some time ago me and my friend had a discussion about optimizations performed by GHC. We wrote a bunch of nested list comprehensions that selected objects based on their properties (e.g. finding products with a price higher than 50). Then we rewrote our queries in a more efficient form by using tree-based maps as indices over the data. My friend knows how to turn nested list comprehensions into queries that use maps and claims this could be automated. He argued that it would be fine if compiler did such an optimization automatically, since we can guarantee that both versions have exactly the same semantics and if they have the same semantics we are free to use faster version. From a functional point of view this is a good point, but nevertheless I objected to his opinion, claiming that if compiler performed such a high-level optimization - replace underlying data structure with a different one and turn one algorithm into a completely different one - programmer wouldn't be able to reason about space behaviour of a program. I concluded that such a solution should not be built into a compiler but instead turned into an EDSL. I would like to hear your opinion on this. How far a compiler can go with transforming code? I don't want to concentrate on discussing whether such an optimization is possible to implement from a technical point of view. What I'm interested in is whether such kind of high-level optimization could be accepted. Janek

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[Announcement] hArduino: Control your Arduino board from Haskell
by Levent Erkok 12 Feb '13

12 Feb '13

I'm happy to announce hArduino: a library that allows Arduino boards to be controlled directly from Haskell. hArduino uses the Firmata protocol ( http://firmata.org) to communicate with and control Arduino boards, making it possible to implement many controller projects directly in Haskell. Home page: http://leventerkok.github.com/hArduino/ Hackage: http://hackage.haskell.org/package/hArduino Some example programs: Blink the led: http://hackage.haskell.org/packages/archive/hArduino/latest/doc/html/System… Digital counter: http://hackage.haskell.org/packages/archive/hArduino/latest/doc/html/System… Control an LCD: http://hackage.haskell.org/packages/archive/hArduino/latest/doc/html/System… Short (4.5 mins) youtube video of the blink example: http://www.youtube.com/watch?v=PPa3im44t2g hArduino is work-in-progress: Patches, bug-reports, and feedback is most welcome. -Levent.

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arrow notation
by Ross Paterson 12 Feb '13

12 Feb '13

I'd like to hear from anyone who's using arrow notation as supported by GHC, because I'm planning a couple of changes to parts of it.

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[Announcement] HsLlvm v0.1: Using Hoopl to optimize LLVM Code
by Ning Wang 12 Feb '13

12 Feb '13

Hi all, It is my dream for years to be able to manipulate LLVM in type safe language. I has explored the possibility of reimplementing LLVM in Haskell with Hoopl and made a small step to realize this dream. I am writing to announce the release of HsLlvm v0.1, which is a pure Haskell implementation (Not LLVM C++ binding) of LLVM. It is available through the link below. https://github.com/mlite/HsLlvm Hightlight of this release: - Support LLVM 3.1 syntax - LLVM normalization - Dominator, mem2reg, DCE passes Project goals: - port all LLVM optimizations - provide functionalities to generate and manipulate LLVM code. Since LLVM is a complex IR and there are many sophisticated optimizations, this job won't be easy. Your contribution would be of value to eventually achieve this goal. Cheers, Ning

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Fwd: Google Summer of Code 2013
by Johan Tibell 11 Feb '13

11 Feb '13

Hi all, Summer of code has always been a good way for us to get some important work, that no one has time to do, done. I encourage everyone to come up with good summer of code projects so we have a good number when the time for students to apply comes around. Empirically projects that focus on existing infrastructure (e.g. Cabal) work well. They limit the scope enough for students to 1) make something that's impacts lots of people and 2) doesn't give them too much rope*. * Building something good from scratch requires lots of experience, something most students don't have, almost per definition. -- Johan ---------- Forwarded message ---------- From: Carol Smith <carols(a)google.com> Date: Mon, Feb 11, 2013 at 11:02 AM Subject: Google Summer of Code 2013 To: Google Summer of Code Announce < google-summer-of-code-announce(a)googlegroups.com> Hi all, We're pleased to announce that Google Summer of Code will be happening for its ninth year this year. Please check out the blog post [1] about the program and read the FAQs [2] and Timeline [3] on Melange for more information. [1] - http://google-opensource.blogspot.com/2013/02/flip-bits-not-burgers-google-… [2] - http://www.google-melange.com/gsoc/document/show/gsoc_program/google/gsoc20… [3] - http://www.google-melange.com/gsoc/events/google/gsoc2013 Cheers, Carol -- You received this message because you are subscribed to the Google Groups "Google Summer of Code Announce" group. To unsubscribe from this group and stop receiving emails from it, send an email to google-summer-of-code-announce+unsubscribe(a)googlegroups.com. To post to this group, send email to google-summer-of-code-announce(a)googlegroups.com. Visit this group at http://groups.google.com/group/google-summer-of-code-announce?hl=en. For more options, visit https://groups.google.com/groups/opt_out.

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"Error: junk" while building GHC
by Corentin Dupont 11 Feb '13

11 Feb '13

Hi all, I'm trying to build GHC HEAD snapshot 7.7.20130210, I have the following error: "inplace/bin/ghc-stage1" -optc-Ilibraries/ghc-prim/. -optc-I'/home/cdupont/Haskell/ghc-7.7.20130210/rts/dist/build' -optc-I'/home/cdupont/Haskell/ghc-7.7.20130210/includes' -optc-I'/home/cdupont/Haskell/ghc-7.7.20130210/includes/dist-derivedconstants/header' -fPIC -dynamic -H32m -O -package-name ghc-prim-0.3.1.0 -hide-all-packages -i -ilibraries/ghc-prim/. -ilibraries/ghc-prim/dist-install/build -ilibraries/ghc-prim/dist-install/build/autogen -Ilibraries/ghc-prim/dist-install/build -Ilibraries/ghc-prim/dist-install/build/autogen -Ilibraries/ghc-prim/. -optP-include -optPlibraries/ghc-prim/dist-install/build/autogen/cabal_macros.h -package rts-1.0 -package-name ghc-prim -XHaskell98 -XCPP -XMagicHash -XForeignFunctionInterface -XUnliftedFFITypes -XUnboxedTuples -XEmptyDataDecls -XNoImplicitPrelude -O2 -no-user-package-db -rtsopts -c libraries/ghc-prim/cbits/debug.c -o libraries/ghc-prim/dist-install/build/cbits/debug.dyn_o debug.c: Assembler messages: debug.c:12:0: Error: junk `.get_pc_thunk.bx' after expression debug.c:41:0: Error: junk `.get_pc_thunk.bx' after expression debug.c:63:0: Error: junk at end of line, first unrecognized character is `1' debug.c:64:0: Error: junk at end of line, first unrecognized character is `1' debug.c:65:0: Error: Missing symbol name in directive debug.c:65:0: Error: junk at end of line, first unrecognized character is `1' debug.c:66:0: Error: Missing symbol name in directive debug.c:66:0: Error: junk at end of line, first unrecognized character is `.' debug.c:67:0: Error: junk at end of line, first unrecognized character is `1' The file debug.c is not even 12 lines long (and it looks correct)... Any ideas? Thanks, Corentin

1 0

xml conduit
by grant 11 Feb '13

11 Feb '13

Hi, Is there a nice way to update xml. I want to be able to use xml-conduit to find a location in the xml and then add/update that node. eg xpath from //d/e/f and then change the content at 'f' or add a new node <a> ... <d> <e> <f>some data to change </f> </e> </d> ... </a> Thanks for any help, Grant

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Control.Monad provisional?
by Petr Pudlák 11 Feb '13

11 Feb '13

Dear Haskellers, Looking at Control.Monad: http://hackage.haskell.org/packages/archive/base/4.6.0.0/doc/html/Control-M… I see: "Stability provisional". I checked some older versions and it's the same. This feel somewhat unsettling - if Control.Monad is provisional, do we have any "stable" packages at all? Best regards, Petr Pudlak

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