April 2010 - Haskell-Cafe

How do I use ByteString?
by Günther Schmidt 06 Apr '10

06 Apr '10

Hi all, I've never found an easy way to deal with ByteStrings. I'm using the RSA library and it en- and decodes Data.ByteString.Lazy.ByteString. I initially start with Strings, ie. [Char], but there is no function to convert the 2 back and forth. There is however a function which takes [Word8] to BytesString and back. It all wouldn't be so confusing if there weren't several versions of ByteString in several modules to choose from. And a number of libraries requiring different types of ByteString. I am sure the designers of the bytestring package had good reason for this design, is there also a webpage which explains which one to use and under what circumstances? Günther

6 5

FRP, arrows and loops
by Maciej Piechotka 06 Apr '10

06 Apr '10

Hello. I'm trying to understand the FRP (by implementing FRP system on my own) and I think I'm slowly getting it. 1. How to interpret ArrowLoop? I have two possible implementations: type RunSF a = a Dynamic () data SF a b c = SF (a (Dynamic, b, RunSF, Set Unique) (c, Set Unique, SF a b c)) (...) instance ArrowLoop (SF a) where loop (SF f) = loop' f undefined where loop' g d = proc (dyn, b, r, s) -> do ((c, d'), s, g') <- g <- (dyn, (b, d), r, s) returnA -< (c, s, loop' g' d') instance ArrowLoop a => ArrowLoop (SF a) where loop (SF f) = SF $! proc (d, b, r, s) -> do rec ((c, d), s, f') <- f -< (d, (b, d), r, s) returnA -< (c, s, loop f') Since the first is not unlike ArrayCircuit from arrays I guess second one but I'm not quite sure. 2. Why there is no ArrowIO in arrows? I.e. class Arrow a => ArrowIO a where liftAIO :: Kleisli IO b c -> a b c (possibly class Arrow a => ArrowST a where liftAST :: Kleisli ST b c -> a b c ) 3. Why switch is needed? How to interpret switch with current continuation? I think switch is equivalent to ArrowChoice but do I miss something? Regards

3 3

Data Structures GSoC
by Nathan Hunter 06 Apr '10

06 Apr '10

Hello. I am hoping to take on the Data Structures project proposed two years ago by Don Stewart here<http://hackage.haskell.org/trac/summer-of-code/ticket/1549>, this summer. Before I write up my proposal to Google, I wanted to gauge the reaction of the Haskell community to this project. Particularly: -What Data Structures in the current libraries are in most dire need of improvement? -How necessary do you think a Containers Library revision is? -Should I attempt to build on the work Jamie Brandon did with Map as generalised tries, or is that beyond the scope of this project I am very excited to work with functional data structures, and I would greatly appreciate any input. -Nathan Hunter

5 5

Haskell JSON requests from your blog, roll your own Try Haskell!
by Christopher Done 05 Apr '10

05 Apr '10

For those interested, http://chrisdone.com/posts/2010-04-05-haskell-json-service-tryhaskell.html I've updated the Haskell JSON service I whipped up a month ago and made it much simpler, written some sample code for how to include this in your blog or tutorials. It supports the JSONP way of making requests, so you can use it from any domain. Thanks to that, Try Haskell can now be run by anyone who has the HTML/JavaScript code and an internet connection, which is nice. Lowers the barrier a bit for people who'd like to write interactive tutorials but can't be bothered messing around with FastCGI and a web server. Don't worry about using it too much, it's on a Linode account with 200GB of bandwidth a month. AJAX requests really won't use that up. ;) I hope someone uses it on a tutorial. *Hint hint* BONUS. Hope you guys find it useful!

1 0

Re: Hackage accounts and real names
by Hein Hundal 05 Apr '10

05 Apr '10

I don't like using my real name on line unless I am 100% sure that I want my statements recorded for all time and available to anyone. Using a pseudonym allows me to be more honest in my opinions and it allows me to join groups without wondering how my membership in that group will be viewed in 20 years. Simply put, if you don't want much input from me or people that think like I do about anonymity, then, go ahead and require a real name.

1 0

Where are the haskell elders?
by Günther Schmidt 05 Apr '10

05 Apr '10

Hi all, I notice that posts from the Haskell elders are pretty rare now. Only every now and then we hear from them. How come? Günther

10 12

Haskell on Debian
by Alex Rozenshteyn 05 Apr '10

05 Apr '10

I tend to install haskell packages from apt whenever possible. One such package is "unix", which appears to come provided by the ghc6 debian package. I'm trying to cabal install lambdabot and getting the following: $ cabal install lambdabot Resolving dependencies... Configuring lambdabot-4.2.2.1... Preprocessing executables for lambdabot-4.2.2.1... Building lambdabot-4.2.2.1... Main.hs:11:7: Could not find module `System.Posix.Signals': It is a member of the hidden package `unix-2.4.0.0'. Perhaps you need to add `unix' to the build-depends in your .cabal file. Use -v to see a list of the files searched for. cabal: Error: some packages failed to install: lambdabot-4.2.2.1 failed during the building phase. The exception was: ExitFailure 1 I have a feeling this has something to do with the interaction between cabal and apt... Does anyone have any advice for fixing this issue other than just adding "unix" to the lambdabot.cabal file? -- Alex R

3 6

Integers v ints
by Jens Blanck 05 Apr '10

05 Apr '10

I was wondering if someone could give me some references to when and why the choice was made to default integral numerical literals to Integer rather than to Int in Haskell. Also, if you are aware of similar discussions in other languages. I'd like to use this information to make an analogous case for defaulting real numerical literals (well, the literals are likely to be in scientific notation, i.e., floating point) to some data type of computable reals rather than to floating point Double. Jens

7 8

Hackage and popular packages: 2010 Q1 report
by Don Stewart 05 Apr '10

05 Apr '10

Hackage 2010 Q1 report http://donsbot.wordpress.com/2010/04/03/the-haskell-platform-q1-2010-report/ After the big move of Hackage from monk to the new abbot server, here's the first report on which packages are popular, and how Hackage is doing in general. -- Don

7 7

ANN: spec2code
by Greg Fitzgerald 05 Apr '10

05 Apr '10

After 5 years of R&D, I’m proud to announce the spec2code compiler. With spec2code, developers no longer need to acknowledge the mundane details of programming, such as memory allocation, bounds-checking, byte ordering, inheritance models or performance tuning. spec2code uses the latest techniques in compiler optimization to derive a deterministic implementation from only a high-level specification. For example, an optimal quicksort is derived from the following specification. sort :: Ord a => [a] -> (b | ordered b) sort = undefined ordered :: Ord a => [a] -> Bool ordered [] = True ordered [_] = True ordered (x1:x2:xs) = x2 > x1 && ordered (x2:xs) Read as, “the function 'sort' takes a list of orderable list items, and returns a list for which each list item is greater to the one preceding it.” How does it work? By telling the compiler about the logical properties of how its output relates to its input, the compiler is sufficiently constrained to apply its genetic algorithm. It iterates over the specification with all known optimization techniques, algebraically reducing the program to an underlying machine model. The compiler uses static analysis and complexity theory in conjunction with the machine model to order implementations by their performance characteristics and choose the optimal one. Conceptually, the first phase orders algorithms by complexity, using "average case", "worst case" and "memory consumption": [MergeSort (n*log n) (n*log n) n, QuickSort (n*log n) (n*n) (log n), InsertionSort (n*n) (n*n) 1, BubbleSort (n*n) (n*n) 1 ] After choosing a high-level algorithm, spec2code does the usual low-level optimizations via LLVM to map the algorithm to optimal native code. Does it scale? Absolutely, spec2code is not confined to specifications for which optimized algorithms are already known. spec2code can be used to implement operation systems, device drivers, build systems, package management tools, and even do your shirt laundry. spec2code will change your job from programmer to specification author, giving you more time for meetings, managing email, and browsing the web. Say goodbye to those dirty Perl scripts, broken C code, and ultimately, your job. spec2code is the future of programming and the beginning of the end of mankind. Looking forward to obsoleting you soon, Greg

2 1