February 2013 - Haskell-Cafe

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 19 Feb '13

19 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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