February 2011 - Haskell-Cafe

Why is there no "splitSeperator" function in Data.List
by Robert Clausecker 14 Feb '11

14 Feb '11

Is there any reason, that one can't find a function that splits a list at a seperator in the standard library? I imagined something like this: splitSeperator :: Eq a => a -> [a] -> [[a]] splitSeperator ',' "foo,bar,baz" --> ["foo","bar","baz"] Or something similar? This is needed so often, even if I can implement it in one line, is there any reason why it's not in the libs? Yours, Robert Clausecker

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Re: [Haskell-cafe] ANN: FunGEn-0.3 simple 2D game engine released
by Simon Michael 14 Feb '11

14 Feb '11

Hi Andre, good to hear from you. On Feb 13, 2011, at 5:27 PM, Andre Wilson Brotto Furtado wrote: > This is great, thanks Simon. I'm currently not involved in any > haskell projects anymore, but please do keep the ball rolling for > FunGEn. > > I'm currenty exploring how domain-specific development and software > product lines can be streamlined for game development. You can check > it out at http://sharpludus.codeplex.com. Volunteers to port such a > work to Haskell are welcome. Cool. Thanks, -Simon

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Vector library
by Pierre-Etienne Meunier 14 Feb '11

14 Feb '11

Hi, This is mostly a question for Roman : how do you use your vector library with multi-dimensional arrays ? I mean, the array library from the standard libraries does something more intelligent than the C-like solution with indirections. For instance, it allows you to program a single function that works for any dimensionality of the array. An example where it is useful ? Right : Some time ago I wrote a polynomial system solver for polynomials in Bernstein form. It needs to perform De Casteljau's algorithm to compute the coefficients of the restriction of its argument to a subinterval of [0,1]. The Ix thing (mainly the bounds function) allows for writing only one version of the function for any number of variables (ok, I needed to write another class to tell me there were n variables were when the index was a n-uple, and how to iterate over my data in some dimension between 0 and n-1). I know I could do it with vectors only by wrapping them in another type carrying the bounds information, but isn't there any more elegant way to do it ? Like an optional thing in the vector type, an alternative one, I don't know. Thanks, Pierre Some code example : instance Restrictable (Double,Double,Double,Double,Double,Double,Double,Double) (Int,Int,Int,Int,Int) where restriction (a,b,c,d,e,f,g,h) lower upper= let ((i0,i1,i2,i3,i4),(j0,j1,j2,j3,j4))=bounds lower dd=rangeSize (i4,j4) n0=rangeSize (i0,j0) n1=rangeSize (i1,j1) n2=rangeSize (i2,j2) n3=rangeSize (i3,j3) restr0=restrict 1 (n1*n2*n3) n0 dd a b coefs restr1=restrict n0 (n2*n3) n1 dd c d restr0 restr2=restrict (n0*n1) n3 n2 dd e f restr1 restr3=restrict (n0*n1*n2) 1 n3 dd g h restr2 in something Where restrict is written like : restrict bef aft nv dd a b coefs= let idx i j k d=((i*nv+j)*aft + k)*dd + d in something

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Haskell AES lazy vs. strict bytestrings
by tsuraan 13 Feb '11

13 Feb '11

By my reading of the documentation of the AES package in hackage (http://hackage.haskell.org/packages/archive/AES/0.2.7/doc/html/Codec-Crypto…) the sizes of the strict bytestring chunks of the lazy bytestring given to crypt must be multiples of 16 bytes or the resulting output will act as though the strict bytestrings were padded to 16 bytes with zeroes. I'm having trouble seeing this behaviour, so I'm wondering if I'm mis-reading the documentation or if I'm not understanding what's happening. Here's a ghci run: Prelude> import Codec.Crypto.AES Prelude Codec.Crypto.AES> let lconcat = Data.ByteString.Lazy.concat Prelude Codec.Crypto.AES> let lpack=Data.ByteString.Lazy.Char8.pack Prelude Codec.Crypto.AES> let strict = Data.ByteString.concat $ Data.ByteString.Lazy.toChunks $ lconcat [lpack "hi", lpack "there", lpack "everybody"] Prelude Codec.Crypto.AES> let lazy = lconcat [lpack "hi", lpack "there", lpack "everybody"] Prelude Codec.Crypto.AES> lazy Chunk "hi" (Chunk "there" (Chunk "everybody" Empty)) Prelude Codec.Crypto.AES> let pw = pack "0123456789abcdef" Prelude Codec.Crypto.AES> let sencode = crypt' CTR pw pw Encrypt strict Prelude Codec.Crypto.AES> let lencode = crypt CTR pw pw Encrypt lazy Prelude Codec.Crypto.AES> Data.ByteString.concat $ Data.ByteString.Lazy.toChunks lencode "\SUB\ESC\n\224{\174\152dv\194j\DC1\ESCf\209\FS" Prelude Codec.Crypto.AES> sencode "\SUB\ESC\n\224{\174\152dv\194j\DC1\ESCf\209\FS" So, I'm not seeing the padding of each strict bytestring of the lazy bytestring in the output encoding. I'd say that's a good thing, but it's not what I expect from the documentation, so I'd just like to know if I'm doing something wrong before I go and depend on lazy bytestrings working like this.

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Re: [Haskell-cafe] : Gathering ideas for the Google Summer of Code
by Alexander Solla 13 Feb '11

13 Feb '11

> Last year we worked hard to come up with a good list of projects for the > 2010 Google Summer of Code. A list drafted at ZuriHac is here: > > > http://donsbot.wordpress.com/2010/04/01/the-8-most-important-haskell-org-gs… > > And we ended up with quite a few of those funded, > > > http://donsbot.wordpress.com/2010/04/26/the-7-haskell-projects-in-the-googl… > > Now is the time to think hard about student-sized projects that would be > valuable this year. > > A great place to collect these ideas, and discuss them, is the Haskell > Proposals Reddit, > > http://www.reddit.com/r/haskell_proposals/ > > -- Don > I don't have a reddit account, so I'll post here. One thing I would love to see is a "free theorem deriver" integrated into GHC. It would be lovely to have, say, a type class called "Free" or "Glue" which takes in two two types and whose function represents a glue from the first to the second. It would be nice if those injections were derived, say, by Djinn. Consider my comments (which I am reposting here, in abridged form) from http://www.palgorithm.co.uk/2010/05/why-isnt-haskell-popular-in-industry/ : I just ignore the “line noise”. Line noise is nearly always glue code. I don’t want that named. I just want it done. Yes, it can be hard to write from scratch. But any sane Haskell development environment uses an interactive environment like GHCi. If you use the type checker to guide you, writing the glue becomes trivial. Encode logic in data types, and let the glue write itself. If glue code type checks, it works, guaranteed. These are free theorems. Focus on the important things: the stuff you’re gluing together. ... class Glue a b where glue :: a -> b and hide all the line noise in there. And then your client code will look like blah :: StateT (MaybeT IO) (Integer) (Maybe Integer) blah = glue . parseInt . glue . getLine – or blah = glue $ parseInt $ glue $ getLine This is how “line noise” glue code looks to me. Except line noise is more maintainable. *fnord* Note that these glue functions would each require their own distinct type class instance. The nicer the client code looks, the more complexity you have to shovel into the Glue instances. But you are merely transferring complexity — not creating or destroying it. Complexity is created by data type definitions being glued. There is the potential for a combinatorial explosion, since you can have unused Glue instances, whereas you cannot have unused “line noise” glue code. Mathematicians don’t care about mathematical symbols. They care about the things the symbols join, because the semantics of joining “things” is dependent on — and usually determined by — the things being joined.

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ANN: FunGEn-0.3 simple 2D game engine released
by Simon Michael 13 Feb '11

13 Feb '11

All - inspired by #haskell-game, I'm pleased to announce that FunGEn has been revived as a community project. This makes Andre Furtado's 2002 work available to the new generation of haskell game developers. :) FunGEn (Functional Game Engine) is a platform-independent, BSD- licensed, easy-to-install 2D game engine currently based on OpenGL and GLUT. As of 2011 it is the only general-purpose game engine, and the easiest way to throw together simple 2D games, in Haskell. On the downside, I'm told GLUT can't handle simultaneous keypresses, but the included examples are quite playable. If Andre reads this and would like to get involved, or send corrections to my doc updates, that would be great. In any case, I hope to see your forks and patches making this better. This is also a chance to test darcsden.com's ability to serve as a lighter github. Release: http://hackage.haskell.org/package/FunGEn Code and docs: http://darcsden.com/simon/fungen Original home, more docs: http://www.cin.ufpe.br/~haskell/fungen Best, -Simon On Feb 10, 2011, at 12:40 PM, Simon Michael wrote: > Hi Andre, wman.. you guys haven't been responding, but FYI #haskell- > game IRC channel has been revived and I have just been updating > FunGEn's status on the wiki a bit: > > http://www.haskell.org/haskellwiki/Applications_and_libraries/Games#Game_En… > > http://www.haskell.org/haskellwiki/Game_Development > > If you're available/interested to join us on the channel, or update > the hackage package, you'd be welcome! Otherwise perhaps we'll take > a shot at it some time in future. > > Best, > -Simon

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Gathering ideas for the Google Summer of Code
by Don Stewart 13 Feb '11

13 Feb '11

Last year we worked hard to come up with a good list of projects for the 2010 Google Summer of Code. A list drafted at ZuriHac is here: http://donsbot.wordpress.com/2010/04/01/the-8-most-important-haskell-org-gs… And we ended up with quite a few of those funded, http://donsbot.wordpress.com/2010/04/26/the-7-haskell-projects-in-the-googl… Now is the time to think hard about student-sized projects that would be valuable this year. A great place to collect these ideas, and discuss them, is the Haskell Proposals Reddit, http://www.reddit.com/r/haskell_proposals/ -- Don

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Is Show special? Of course not but...
by Cristiano Paris 12 Feb '11

12 Feb '11

Hi cafè, given the following toy code: --------------- module Main where class Dumb p where dumb :: p -> String newtype Branded a b = Branded b unbrand :: Branded a b -> b unbrand (Branded x) = x wrong :: Dumb a => b -> Branded a b wrong = Branded right :: Show a => b -> Branded a b right = Branded --------------- Why: --------------- quarry:Haskell paris$ ghci -O1 GHCi, version 6.12.3: http://www.haskell.org/ghc/ :? for help *Main> unbrand $ right True True *Main> unbrand $ right "Foo" "Foo" --------------- but: --------------- *Main> unbrand $ wrong True <interactive>:1:10: Ambiguous type variable `a' in the constraint: `Dumb a' arising from a use of `wrong' at <interactive>:1:10-19 Probable fix: add a type signature that fixes these type variable(s) --------------- ? Maybe it's a dumb question but... thank you for any explanation... -- Cristiano GPG Key: 4096R/C17E53C6 2010-02-22 Fingerprint = 4575 4FB5 DC8E 7641 D3D8 8EBE DF59 B4E9 C17E 53C6

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GHC and MinGW
by José Romildo Malaquias 12 Feb '11

12 Feb '11

Hello. How do I make ghc use my installation of MinGW on Windows? I have ghc-7.0.1 installed and the latest MinGW packages. I want ghc to automatically find libraries installed by the MinGW installer. Currently I am getting the error with my application: Linking Gui.exe ... C:\Program Files\Haskell\iconv-0.4.1.0\ghc-7.0.1/libHSiconv-0.4.1.0.a(hsiconv.o):hsiconv.c:(.text+0x8): undefined reference to `libiconv_open' where libiconv has been installed with the command mingw-get install mingw32-libiconv and the library is available at c:/devel/MinGW/lib/ Romildo

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ANNOUNCE: Save the date! Haskell hackathon in Philadelphia, July 29-31
by Brent Yorgey 12 Feb '11

12 Feb '11

I am very pleased to announce that Hac phi 3, third in an annual series of Haskell hackathons hosted at the University of Pennsylvania in Philadelphia, will take place from July 29-31. More details will be forthcoming, but put it on your calendar now. The past two years have been a blast -- you won't want to miss this one! Q: Do I have to already be a Haskell expert to attend? A: No! All are welcome. In fact, sitting in a room with a bunch of enthusiastic Haskell hackers is a great way to learn. Past attendees have included seasoned Haskell gurus and raw beginners alike. Q: What does one do at a Haskell hackathon? A: The goal is to create a fun, interactive, collaborative environment for writing Haskell code. You can work on a project of your own, or help someone else with theirs. Q: What will the cost be? A: We're still working out the details, but as in past years, registration will be free, and we hope to be able to provide some of the meals. You'll be on your own for transportation and lodging. The Hac Phi team: Chris Casinghino (ccasin) Daniel Wagner (dmwit) Brent Yorgey (byorgey)

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