February 2010 - Haskell-Cafe

STArray newListArray
by Vojtěch Knyttl 21 Feb '10

21 Feb '10

Hello, I am trying to create STArray with newListArray like this: la x y = newListArray ((1,1),(x,y)) [(x'+y') | x' <- [1..x], y' <- [1..y]] – but it does not work: No instance for (MArray a Field m) I tried to define the type like this, but it would not work either: la :: Int -> Int -> STArray (Int,Int) Field It is obvious that I don't get the syntax of using it, so I will appreciate any suggestions. V. K.

3 4

darcs 2.4 release candidate 2
by Reinier Lamers 21 Feb '10

21 Feb '10

Hi all, The darcs team would like to announce the immediate availability of darcs 2.4 release candidate 2. darcs 2.4 will contain many improvements and bugfixes compared to darcs 2.3.1. Highlights are the faster operation of record, revert and related commands, and the experimental interactive hunk editing. This beta is your chance to test-drive these improvements and make darcs even better. Compared to darcs 2.4 beta 3, the performance of "darcs check" and "darcs repair" has been brought up to the level of darcs 2.3.1.` The easiest way to install darcs is using the Haskell Platform [1]. If you have installed the Haskell Platform or cabal-install, you can install this beta release by doing: $ cabal update $ cabal install --reinstall darcs-beta Alternatively, you can download the tarball from http://darcs.net/releases/darcs-beta-2.3.99.2.tar.gz , and build it by hand as explained in the README file. Interactive hunk editing ------------------------ To try out interactive hunk editing, press 'e' when you are prompted with a hunk patch by 'darcs record'. You will then be shown an editor screen in which you can edit the state you want to record between the last two ruler lines. You can find more information about the hunk editing feature on http://wiki.darcs.net/HunkEditor . Reporting bugs -------------- If you have an issue with darcs 2.4 release candidate 2, you can report it via the web on http://bugs.darcs.net/ . You can also report bugs by email to bugs(a)darcs.net. What's New ---------- A list of important changes since 2.3.1 is as follows (please let me know if there's something you miss!): * Use fast index-based diffing everywhere (Petr) * Interactive patch splitting (Ganesh) * An 'optimize --upgrade' option to convert to hashed format in-place (Eric) * Hunk matching (Kamil Dworakowski, tat.wright) * Progress reporting is no longer deceptive (Roman) * A 'remove --recursive' option to remove a directory tree from revision control (Roman) * 'show files' accepts arguments to show a subset of tracked files (Luca) * A '--remote-darcs' flag for pushing to a host where darcs isn't called darcs * Many miscellaneous Windows improvements (Salvatore, Petr and others) * 'darcs send' now mentions the repository name in the email body (Joachim) * Handle files with boring names in the repository correctly (Petr) * Fix parsing of .authorspellings file (Tomáš) * Various sane new command-line option names (Florent) * Remove the '--checkpoint' option (Petr) * Use external libraries for all UTF-8 handling (Eric, Reinier) * Use the Haskell zlib package exclusively for compression (Petr) A list of issues resolved since 2.3.1: * 183: do not sort changes --summary output * 223: add --remote-darcs flag to specify name of remote darcs executable * 291: provide (basic) interactive patch splitting * 540: darcs remove --recursive * 835: 'show files' with arguments * 1122: get --complete should not offer to create a lazy repository * 1216: list Match section in ToC * 1224: refuse to convert a repo that's already in darcs-2 format * 1300: logfile deleted on unsucessful record * 1308: push should warn about unpulled patches before patch-selection * 1336: sane error message on --last "" (empty string to numbers parser) * 1362: mention repo name in mail send body * 1377: getProgname for local darcs instances * 1392: use parsec to parse .authorspelling * 1424: darcs get wrongly reports "using lazy repository" if you ctrl-c old-fashioned get * 1447: different online help for send/apply --cc * 1488: fix crash in whatsnew when invoked in non-tracked directory * 1548: show contents requires at least one argument * 1554: allow opt-out of -threaded (fix ARM builds) * 1563: official thank-you page * 1578: don't put newlines in the Haskeline prompts * 1583: on darcs get, suggest upgrading source repo to hashed * 1584: provide optimize --upgrade command * 1588: add --skip-conflicts option * 1594: define PREPROCHTML in makefile * 1620: make amend leave a log file when it should * 1636: hunk matching * 1643: optimize --upgrade should do optimize * 1652: suggest cabal update before cabal install * 1659: make restrictBoring take recorded state into account * 1677: create correct hashes for empty directories in index * 1681: preserve log on amend failure * 1709: fix short version of progress reporting * 1712: correctly report number of patches to pull * 1720: fix cabal haddock problem * 1731: fix performance regression in check and repair * 1741: fix --list-options when option has multiple names Kind Regards, the darcs release manager, Reinier Lamers [1]: You can download the Haskell platform from http://hackage.haskell.org/platform/

3 6

Restricted categories
by Sjoerd Visscher 21 Feb '10

21 Feb '10

Hi all, I want restricted categories, just like the rmonad package provides restricted monads. The ultimate goal is to have a product category: http://en.wikipedia.org/wiki/Product_category > {-# LANGUAGE TypeOperators, TypeFamilies, MultiParamTypeClasses, FlexibleInstances, FlexibleContexts, ScopedTypeVariables, UndecidableInstances #-} > import Prelude hiding ((.), id, fst, snd) > import qualified Prelude First lets create a clone of Data.Suitable from the rmonad package, but this time for types with two arguments: > class Suitable2 m a b where > data Constraints m a b > constraints :: m a b -> Constraints m a b > withResConstraints :: forall m a b. Suitable2 m a b => (Constraints m a b -> m a b) -> m a b > withResConstraints f = f (constraints undefined :: Constraints m a b) > withConstraintsOf :: Suitable2 m a b => m a b -> (Constraints m a b -> k) -> k > withConstraintsOf v f = f (constraints v) Now we can define a restricted category: > class RCategory (~>) where > id :: Suitable2 (~>) a a => a ~> a > (.) :: (Suitable2 (~>) b c, Suitable2 (~>) a b, Suitable2 (~>) a c) => b ~> c -> a ~> b -> a ~> c (->) (or "Hask") is an instance of this class: > instance Suitable2 (->) a b where > data Constraints (->) a b = HaskConstraints > constraints _ = HaskConstraints > instance RCategory (->) where > id = Prelude.id > (.) = (Prelude..) Now on to the product category. Objects of the product category are types that are an instance of IsProduct: > class IsProduct p where > type Fst p :: * > type Snd p :: * > fst :: p -> Fst p > snd :: p -> Snd p For example: > instance IsProduct (a, b) where > type Fst (a, b) = a > type Snd (a, b) = b > fst = Prelude.fst > snd = Prelude.snd Arrows from the product of category c1 and category c2 are a pair of arrows, one from c1 and one from c2: > data (c1 :***: c2) a b = c1 (Fst a) (Fst b) :***: c2 (Snd a) (Snd b) The instance for Suitable2 restricts objects to IsProduct, and requires the Fst and Snd parts of the objects to be suitable in c1 resp. c2: > instance (IsProduct a, IsProduct b, Suitable2 c1 (Fst a) (Fst b), Suitable2 c2 (Snd a) (Snd b)) => Suitable2 (c1 :***: c2) a b where > data Constraints (c1 :***: c2) a b = (IsProduct a, IsProduct b, Suitable2 c1 (Fst a) (Fst b), Suitable2 c2 (Snd a) (Snd b)) => ProdConstraints > constraints _ = ProdConstraints Finally the RCategory instance: > instance (RCategory c1, RCategory c2) => RCategory (c1 :***: c2) where > id = withResConstraints $ \ProdConstraints -> id :***: id > -- f@(f1 :***: f2) . g@(g1 :***: g2) = > -- withResConstraints $ \ProdConstraints -> > -- withConstraintsOf f $ \ProdConstraints -> > -- withConstraintsOf g $ \ProdConstraints -> > -- (f1 . g1) :***: (f2 . g2) Here I am running into problems. I get this error: Could not deduce (Suitable2 c2 (Snd a) (Snd b), Suitable2 c1 (Fst a) (Fst b)) from the context (IsProduct b, IsProduct c, Suitable2 c1 (Fst b) (Fst c), Suitable2 c2 (Snd b) (Snd c)) arising from a use of `withConstraintsOf' In the first argument of `($)', namely `withConstraintsOf g' I don't understand this, as I thought the constraints the error is complaining about is just what withConstraintsOf g should provide. I guess there's something about the Suitable trick that I don't understand, or possibly the type families Fst and Snd are biting me. Who can help me out? Thanks. greetings, Sjoerd

3 4

ANN: HaskellTorrent v0.0
by Jesper Louis Andersen 20 Feb '10

20 Feb '10

Hello, I am happy to announce HaskellTorrent v0.0, a concurrent bittorrent client. The current release is a rather early preview release: it can download torrents and seed them, but currently there are many optimizations and tweaks needed before it is ready for day-to-day use. In particular, we are not yet fully compliant with the bittorrent specification and need proper clients to connect to in order to operate. Basically, you install the cabal package and then run HaskellTorrent foo.torrent and it will then download the contents of the torrent to the current directory. We would like to hear about any kind of problems you might encounter. Send them to jesper.louis.andersen(a)gmail.com for now. We have only tested the software on GHC 6.10.4 and 6.12.1, mostly on Unix platforms. It is known to work on Debian/Unstable with GHC610 and GHC612. If you want to hack code, get in touch with me. There are something for any level of skill to do. *** HaskellTorrent is using the CML library to get concurrency. It easily runs some 200-300 processes when it operates and its process model is somewhat loosely inspired by the model of Erlang. Looking forward, the next goal is to optimize concurrency, memory usage and CPU usage in the client. It currently hogs resources due to naive data structure choices and some expensive correctness assertions. *** Links of interest: http://hackage.haskell.org/package/HaskellTorrent http://github.com/jlouis/haskell-torrent *** This release was brought to you by: Alex Mason: A lot of Wireprotocol and BCode serialization work through the use of the cereal package. Astro: GHC 6.12 support. Support for multi-file torrents. In addition, patches were contributed by: Jesper Louis Andersen John Gunderman Thomas Christensen -- J.

1 0

Computing sums
by Andrew Coppin 20 Feb '10

20 Feb '10

The other day, I found myself writing the following code: data Property x y = forall s. Property s (x -> s -> s) (s -> y) step :: x -> Property x y -> Property x y step x (Property s f g) = Property (f x s) f g read :: Property x y -> y read (Property s _ g) = g s pure :: (x -> y) -> Property x y pure f = Property undefined const f fold :: (x -> y -> y) -> y -> Property x y fold f y = Property y f id fold1 :: (x -> x -> x) -> Property x x fold1 f = Property Nothing (\x mx -> maybe (Just x) (Just . f) mx) fromJust (>==>) :: Property x y -> Property y z -> Property x z p0 >==> p1 = Property (p0,p1) (\x (q0,q1) -> let w0 = step x q0 in (w0, step (read w0) q1)) (\(_,q1) -> (read q1)) (>==<) :: Property x y -> Property x y' -> Property x (y,y') p0 >==< p1 = Property (p0,p1) (\x (q0,q1) -> (step x q0, step x q1)) (\(q0,q1) -> (read q0, read q1)) sum = fold (+) 0 count = pure (const 1) >==> sum minimum = fold1 min maximum = fold1 max Have I just invented arrows? (And if so, why is the above code nowhere to be seen in the libraries?)

4 6

What happened in Ohloh?
by Paul Johnson 20 Feb '10

20 Feb '10

If you go to http://www.ohloh.net/languages/compare?l0=haskell&measure=projects and look at the number (not percentage) of Haskell projects you see it rise exponentially until the start of 2008 and then suddenly drop away. Does anyone know what happened? Assuming this is just an artefact because they aren't scanning Haskell project hosts, can we get them to fix it? I'd like to use this kind of graph at work as evidence that Haskell is on a growth trajectory. Paul.

3 4

Haskell - Heterogeneous Data Structures - Nested Pairs and functional references
by Günther Schmidt 19 Feb '10

19 Feb '10

Hallo Heinrich, entschuldige bitte wenn ich Dich ausserhalb der Cafe-Liste kontaktiere, aber so langsam wird's mir peinlich.... Auf meinen Post "Heterogeneous Data Structures - Nested Pairs and functional references" vom 16.02.2010 in der Cafe-Liste habe ich eine interessante Antwort erhalten von Alexander Solla, die ich nicht verstanden haben. Als ich nachgehakt habe, hat er freundlicherweise sofort eine Antwort geliefert die ich noch weniger verstanden habe ... Jetzt trau' ich mich nicht mehr nochmals zu fragen, denn ich glaube ich würd's wieder nicht verstehen. Kannst Du mir vielleicht erklären was er mir da eigentlich geschickt hat? Grüße Günther

1 0

fast, strongly typed heterogeneous collections
by Alberto G. Corona 19 Feb '10

19 Feb '10

Hi What is the speed of hProjectByLabel/s in Data.HList.Record? . The documentation in hackage <http://hackage.haskell.org/packages/archive/HList/latest/doc/html/Data-HLis…>does not mention it. Perhaps this is because Hlist is designed for supposedly short records and this does not matter too much. looking at the code I guess that everything in HList is O(n) because it uses type-level list structure, but I think that nothing prevents from using a type level balanced tree, Data.Map style, instead. This would permit to implement large heterogeneous Maps with fast access and strongly typed. AFAIK, the alternative is to use a fast data container with data elements encapsulated within Data.Dynamic, that is weakly typed, produces type errors at runtime, which is unsatisfactory and dangerous. Any comments?

4 4

Cabal broken on Mac OS X?
by Oleg Lobachev 19 Feb '10

19 Feb '10

Hello all, I suspect that it's me and not the cabal, but everything looks fine and still fails. Would someone mind to tell me, what am I doing wrong? $ cabal -V cabal-install version 0.6.2 using version 1.6.0.3 of the Cabal library $ ghc -V The Glorious Glasgow Haskell Compilation System, version 6.12.1 $ which ghc /usr/bin/ghc $ which cabal /usr/local/bin/cabal $ cabal install cabal-install cabal: failed to parse output of 'ghc-pkg dump' That's the problem! As it is Mac OS X 10.6.2, here is some information on the architecture and the executables used: $ uname -v Darwin Kernel Version 10.2.0: Tue Nov 3 10:37:10 PST 2009; root:xnu-1486.2.11~1/RELEASE_I386 $ file /Library/Frameworks/GHC.framework/Versions/612/usr/lib/ghc-6.12.1/ghc /Library/Frameworks/GHC.framework/Versions/612/usr/lib/ghc-6.12.1/ghc: Mach-O executable i386 $ file `which cabal` /usr/local/bin/cabal: Mach-O executable i386 $ file `which ls` /bin/ls: Mach-O universal binary with 2 architectures /bin/ls (for architecture x86_64): Mach-O 64-bit executable x86_64 /bin/ls (for architecture i386): Mach-O executable i386 $ ghc --make hello.hs [1 of 1] Compiling Main ( hello.hs, hello.o ) Linking hello ... $ ./hello Hello world! $ file ./hello ./hello: Mach-O executable i386 The output of ghc-pkg dump is here: http://www.mathematik.uni-marburg.de/~lobachev/tmp/ghc-pkg.txt I have installed the Haskell Platform and then the new GHC via official pkg files. Greetings, Oleg

2 2

Why is Haskell used so little in the industry?
by Gabi 19 Feb '10

19 Feb '10

I posted a question about it in StackOverflow (http://stackoverflow.com/questions/2284875/why-is-haskell-used-so-little-in…) Haskell gets some beating there. Please Hasellers, join the discussion and give your opinion there. -- Regards, Gabi http://bugspy.net

1 0