- Haskell-Cafe - Haskell.org

ANN: Bookshelf
by Emil Axelsson 14 May '09

14 May '09

This is the first release of Bookshelf, a simple document organizer with some wiki functionality. Documents in a directory tree are displayed as a set of HTML pages. Documents in Markdown format are converted to HTML automatically using Pandoc. The manual http://www.cs.chalmers.se/~emax/bookshelf/Manual.shelf.html describes the full functionality. Bookshelf is available on Hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Bookshelf or through its Darcs repository: http://www.cs.chalmers.se/~emax/darcs/Bookshelf/ I'm not aware of any bugs. I hope it works on Windows, but I haven't tested. Cheers, / Emil

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forall vs "=>"
by Daryoush Mehrtash 14 May '09

14 May '09

What is the difference between forall as in: runST<http://www.haskell.org/ghc/docs/6.10-latest/html/libraries/base/Control-Mon…>:: ( forall s. ST<http://www.haskell.org/ghc/docs/6.10-latest/html/libraries/base/Control-Mon…>s a) -> a and the "=>" as in evalStateT<http://www.haskell.org/ghc/docs/6.6/html/libraries/mtl/Control-Monad-State.…>:: Monad<http://www.haskell.org/ghc/docs/6.6/html/libraries/base/Control-Monad.html#…>m => StateT<http://www.haskell.org/ghc/docs/6.6/html/libraries/mtl/Control-Monad-State.…>s m a -> s -> m a thanks daryoush

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ANN: priority-sync-0.1.0.1: Cooperative task prioritization.
by Christopher Lane Hinson 14 May '09

14 May '09

http://hackage.haskell.org/cgi-bin/hackage-scripts/package/priority-sync $ cabal install priority-sync git clone http://www.downstairspeople.org/git/priority-sync.git Feedback will be greatly appreciated. This package is a spin-off from my work on roguestar, where I need to do significant background processing while retaining enough resources to perform smooth animation. The following is the front-page documentation for the package. In a simple use case, we want to run some expensive tasks in prioritized order, so that only one task is running on each CPU (or hardware thread) at any time. For this simple case, four operations are needed: simpleTaskPool, schedule, claim, and startQueue. let expensiveTask = threadDelay 1000000 pool <- simpleTaskPool forkIO $ claim Acquire (schedule pool 1) $ putStrLn "Task 1 started . . ." >> expensiveTask >> putStrLn "Task 1 completed." forkIO $ claim Acquire (schedule pool 3) $ putStrLn "Task 3 started . . ." >> expensiveTask >> putStrLn "Task 3 completed." forkIO $ claim Acquire (schedule pool 2) $ putStrLn "Task 2 started . . ." >> expensiveTask >> putStrLn "Task 2 completed." threadDelay 100000 -- contrive to wait for all tasks to become enqueued putStrLn "Starting pool: " startQueue pool threadDelay 4000000 -- contrive to wait for all tasks to become dequeued A TaskPool combines Rooms and Queues in an efficient easy-to-use-interface. Rooms provide fully reentrant synchronization to any number of threads based on arbitrary resource constraints. For example, the Room from a simpleTaskPool is constrained by GHC.numCapabilities. Queues provide task prioritization. A Queue systematically examines (to a configurable depth) all waiting threads with their priorities and resource constraints and wakes the most eagerly prioritized thread whose constraints can be satisfied. TaskPools are not thread pools. The concept is similar to IO Completion Ports. There are no worker threads. If a number of threads are waiting, the thread that is most likely to be processed next is woken and temporarily serves as a working thread. Rooms, Queues, and TaskPools are backed by carefully written STM (software transactional memory) transactions. A salient feature is that, because any thread can participate, a TaskPool supports both bound threads and threads created with forkOnIO. Friendly, --Lane

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List of exports of a module - are there alternatives?
by Maurício 13 May '09

13 May '09

Hi, When we want to list which declarations are exported by a module we do: module Mod ( list of exports ) where ... Are there propositions to alternatives to that (I could not find one)? Like, say, add a "do export" or "do not export" tag to declarations we want to (not) export? (I think something like that could be nice when we have modules with 200 declarations and just a few are (not) going to be exported.) Thanks, Maurício

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Fundep Curiosity
by Christopher Lane Hinson 13 May '09

13 May '09

I've noticed that a large majority of fundeps I see in other people's libraries are written: class C a b | b -> a Where the dependent parameter appears first in the MPTC. Is there a reason for this? AFAIK, there isn't any semantic significance to the order of parameters in an MPTC. Why do many haskellers find this configuration more intuitive? Friendly, --Lane

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Data.Map.Map Pattern Matching
by Nico Rolle 13 May '09

13 May '09

Hi I tried this but it diddn't work in ghci: import qualified Data.Map as Map test :: Map.Map [Int] [[Int]] -> Bool test (fromList[((i:is), (j:js))]) = [i] == j i get the : "Parse error in pattern Failed." error. regards

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Re: [Haskell-cafe] caml build
by Vasili I. Galchin 13 May '09

13 May '09

oops ... yes. On Wed, May 13, 2009 at 11:29 AM, Nicolas Pouillard < nicolas.pouillard(a)gmail.com> wrote: > Excerpts from Vasili I. Galchin's message of Wed May 13 18:21:55 +0200 > 2009: > > Hello, > > > > I did a "cabal build -?" and discovered -v/--version. > > So **caml** build was a typo right ? > > > > > Vasili > > > > On Wed, May 13, 2009 at 2:42 AM, Nicolas Pouillard < > > nicolas.pouillard(a)gmail.com> wrote: > > > > > Excerpts from Vasili I. Galchin's message of Tue May 12 23:23:51 +0200 > > > 2009: > > > > Hi Nicolas, > > > > > > > > I am starting a "caml build". I want line by line which module > is > > > > being built so when I get an error I have a context to reason about > to > > > fix > > > > the problem. Got it? > > > > > > Maybe you mean "cabal build" ? > > > > > > > On Tue, May 12, 2009 at 7:23 AM, Nicolas Pouillard < > > > > nicolas.pouillard(a)gmail.com> wrote: > > > > > > > > > Excerpts from Vasili I. Galchin's message of Tue May 12 00:27:26 > +0200 > > > > > 2009: > > > > > > Hello, > > > > > > > > > > > > I have forgotten whether I sent this posting out. Sorry if > I > > > did (I > > > > > > didn't see on Haskell cafe archive). > > > > > > > > > > > > I am building Swish and getting an error. I want to follow > the > > > > > > progress of "swish build" ... I don't see an additional parameter > > > like > > > > > > verbose mode that will tell which swish component is being built. > ??? > > > > > > > > > > I don't get the relation with the subject of your post, can you > > > elaborate? > > > > > > > > > > Best regards, > > > > > > > > > > -- > > > > > Nicolas Pouillard > > > > > > > > > > > -- > > > Nicolas Pouillard > > > > > -- > Nicolas Pouillard >

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Qt declarative UI
by david48 13 May '09

13 May '09

I thought that it might interest some people : http://labs.trolltech.com/page/Projects/Graphics/Kinetic/DeclarativeUI

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caml build
by Vasili I. Galchin 13 May '09

13 May '09

Hello, I have forgotten whether I sent this posting out. Sorry if I did (I didn't see on Haskell cafe archive). I am building Swish and getting an error. I want to follow the progress of "swish build" ... I don't see an additional parameter like verbose mode that will tell which swish component is being built. ??? Regards, Vasili

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Stream-fusion without the lists
by Reiner Pope 13 May '09

13 May '09

Hi everyone, With stream-fusion, we can write functions which construct and destruct lists, such as (this is the main example from the Stream Fusion paper[1]) f :: Int -> Int f n = sum [k * m | k <- [1..n], m <- [1..k]] and the rewrite rules in stream-fusion replace these operations on lists with fusible operations on the Stream (non-recursive) datatype. In this example, the domain and codomain of the function don't mention the list datatype. There seem to be many functions like this: the lists are being used internally as composable loops rather than as data-structures. The Stream datatype seems to be much better suited to representing loops than the list datatype is. So, instead of programming with the lists, why don't we just use the Stream datatype directly? For example: In Data.Stream (from the stream-fusion package) we can find most of the Prelude list functions but with Stream in all the types instead of []. For example, Data.Stream.sum :: Num a => S.Stream a -> a Using this module, we can rewrite f without mentioning lists. We first need a Monad instance for Data.Stream.Stream: > import qualified Data.List.Stream as S > > instance Monad S.Stream where > return = S.return > (>>=) = S.concatMap Now we can write > f :: Int -> Int > f n = S.sum $ do > k <- S.enumFromToInt 1 n > m <- S.enumFromToInt 1 k > return (k*m) which is essentially the same as the original f, although lacking the syntactic sugar of the list comprehension. To me, it seems that Stream is more sensible data type to use than [] when the algorithm being expressed is just a loop (rather than something which needs a [] as a cache/buffer), for the following reasons: 1. If I am programming with lists and I need some function which I can't express with Prelude functions, I have to write it myself. I will probably lose fusion in this case, because I will write it recursively in terms of lists. On the other hand, if I am programming with Streams, I will write it myself in terms of Streams, and it should be easier to maintain fusion because it won't be recursive. 2. Holding on to a [] too long can cause a space leak. This is not the case for Stream, because a Stream only ever contains one "state". More generally, the memory use of Stream is more easily predictable than that of [], since "running" a Stream only holds on to one "state" at a time, and we often know how big the "state" is. 3. Fusion doesn't rely on rewrite rules firing. I consider this point less significant than the other two. So, thoughts? Do people program with Streams directly? What have I not considered? Cheers, Reiner [1] http://www.cse.unsw.edu.au/~dons/papers/stream-fusion.pdf

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