November 2009 - Haskell-Cafe

ANN: wumpus-core-0.13.0
by Stephen Tetley 27 Nov '09

27 Nov '09

Hello All Wumpus-core is a library for drawing 2D vector pictures, supporting output to PostScript and SVG. It has no FFI dependencies, so should be portable across operating systems (assuming GHC with type families). Wumpus-core is functional, though quite low-level - its similar to PostScript but with a more 'coordinate free' style. I'm working on a complementary package for higher-level stuff, but it will be some time before it is release ready. Many thanks to Conal Elliott and Iavor S. Diatchki for their invaluable libraries (VectorSpace and MonadLib/xml respectively). Best wishes Stephen http://hackage.haskell.org/package/wumpus-core

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I really donot know how to use `newtype` ?
by zaxis 27 Nov '09

27 Nov '09

import Control.Monad.State import Control.Monad.Reader data XConf = XConf {} data XState = XState {} newtype X a = X (ReaderT XConf (StateT XState IO) a) > :t StateT StateT :: (s -> m (a, s)) -> StateT s m a > :t ReaderT ReaderT :: (r -> m a) -> ReaderT r m a then how to use `X` ? Would you mind explaining the newtype X in detail ? Sinerely! ----- fac n = foldr (*) 1 [1..n] -- View this message in context: http://old.nabble.com/I-really-donot-know-how-to-use-%60newtype%60---tp2654… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Re: Iteratee question
by Valery V. Vorotyntsev 27 Nov '09

27 Nov '09

On Fri, Nov 27, 2009 at 4:04 AM, John Lato <jwlato(a)gmail.com> wrote: > My apologies for not replying; I have been traveling and am only now > working through my email. > > Oleg's response is much better than anything I would have written. > I'd like to add one point. > > stream2list is very inefficient as he mentioned, however only for > large values of 'n'. For small n it should be fine. Assuming you're > using Word8 elements, "small" means "< 4096". This is because the > default chunk size reading from a file is 2048 elements, so for any n > < 4096 you have at most two concatenations in producing the > stream2list. > > Sincerely, > John Lato > > PS for one example of a binary data parser, please see > http://inmachina.net/~jwlato/haskell/iter-audio/ > > This is similar to the audio codec included with iteratee, but much > more efficient. In particular, the functions "convFunc" and > "unroller" in Sound.Iteratee.Codecs.Common are pretty highly > optimized. Wonderful! Sample code is very helpful to get familiar with iteratees. Thank you, John. Thanks to both of you. -- vvv

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Types
by Tsunkiet Man 27 Nov '09

27 Nov '09

Hello, I´ve a small question about Haskell. It´s really vague explained in "Haskell in 10 minutes" and sometimes it's really detailed explained which is very hard for people who just got into Haskell to read. Can someone explain me something about the type Integral? What exactly is it? I know that 2 is something of the type Integral, however what does this differ from Int? The same question goes for Fractional and Floating, what exactly is their commonality and what exactly is their difference? Thank you for explaining these basic things to me.

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Possible FGL bug
by Ivan Lazar Miljenovic 26 Nov '09

26 Nov '09

When developing my QuickCheck-2 test-suite for graphviz, I wrote the following Arbitrary instance for FGL graphs (which needs FlexibleInstances): ,---- | instance (Graph g, Arbitrary n, Arbitrary e, Show n, Show e) => Arbitrary (g n e) where | arbitrary = do ns <- liftM nub arbitrary | let nGen = elements ns | lns <- mapM makeLNode ns | trace ("Nodes: " ++ show lns) (return ()) | les <- listOf $ makeLEdge nGen | trace ("Edges: " ++ show les) (return ()) | return $ mkGraph lns les | where | makeLNode n = liftM ((,) n) arbitrary | makeLEdge nGen = liftM3 (,,) nGen nGen arbitrary | | shrink gr = map (flip delNode gr) (nodes gr) `---- However, when I try to run this, I occasionally get irrefutable pattern match failures as follows: ,---- | *Data.GraphViz.Testing.Instances.FGL Data.Graph.Inductive.Tree> sample (arbitrary :: Gen (Gr Int Char)) | | | 0:0->[] | | 0:-2->[] | 1:0->[('\a',0)] | 2:0->[] | | -4:-3->[('U',-3),('#',1)] | -3:3->[] | 1:-1->[('}',-3)] | | -8:8->[] | -3:2->[] | -1:-5->[('\US',-3),('&',0)] | 0:5->[('F',-1),('p',4)] | 4:-1->[] | | -2:8->[('\177',-2),('(',-2),('d',-2),('4',-2),('D',-2),('\US',-2),('d',-2),('u',-2)] | | -16:11->[] | -2:-2->[] | 0:11->[('@',1)] | 1:13->[('u',11)] | 9:-11->[('\231',11)] | 11:12->[('\226',1)] | 16:15->[] | | -10:2->[] | -4:8->[] | 1:30->[] | 26:26->[('<',1),('K',-4)] | 31:-21->[] | | -35:51->[('@',-29)] | -29:21->[('\132',-11)] | -11:-31->[('j',61)] | -4:40->[('a',-29)] | 0:6->[('z',-35),('9',28),('\170',-11),('\SUB',28)] | 23:8->[('P',-29),('(',61),('\\',28)] | 28:60->[] | 61:44->[('q',61)] | *** Exception: Data/Graph/Inductive/Graph.hs:250:26-59: Irrefutable pattern failed for pattern (Data.Maybe.Just (pr, _, la, su), g') `---- The actual error comes from the definition of insEdge: ,---- | -- | Insert a 'LEdge' into the 'Graph'. | insEdge :: DynGraph gr => LEdge b -> gr a b -> gr a b | insEdge (v,w,l) g = (pr,v,la,(l,w):su) & g' | where (Just (pr,_,la,su),g') = match v g `---- with the Graph instance for Tree-based graphs using this for its mkGraph method: ,---- | mkGraph vs es = (insEdges' . insNodes vs) empty | where | insEdges' g = foldl' (flip insEdge) g es `---- So, is this really a bug in FGL, or am I using mkGraph wrong? On another note, why doesn't the PatriciaTree graph type have a Show instance? :( -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

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separate cabal installations on separate machines
by Brian Denheyer 26 Nov '09

26 Nov '09

Hello, I have server where my home directory resides. I use cabal from (at least) two machines with different processors (i386, powerpc) and the common home directory. I noticed that there is a --global option which does the right thing and places the installs in /usr/local, but it looks like it still stores some info, particularly about configuration state, in /home/${USER}/.cabal. How do I invoke cabal to make sure that the entire installation is machine specific ? Thanks, Brian

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some help on do and preservingMatrix
by Zsolt Ero 26 Nov '09

26 Nov '09

Sorry to ask such a basic question here, but I am trying to write my first Haskell OpenGL animation and just encontered with these questions: Could someone point me to some help on how does "do" work and how does preservingMatrix? I am trying to write a program starting from the OpenGL tutorial on the wiki, but I never used do's before. My very basic question is that how do I generally repeat a command many times? I understand "do" can do it, but it's not clear to me how to use it and how it works. So if I would like to write the following in one line, how could I write it?: spiral 0.7 spiral 0.8 spiral 0.9 spiral 1.0 I don't understand in which cases I can use "map something [0.7,0.8..1]" and in which cases I need do and how could I map using do. >From the tutorial, I could write this line, which works, but I don't really know what does mapM_ and preservingMatrix do. mapM_ (\x -> preservingMatrix $ do scale x x (1::GLfloat) rotate (x*(200)) $ Vector3 0 0 (1::GLfloat) spiral ((sin(a/50))^2/2+0.5) ) $ ([0.5,0.6..1::GLfloat]) Zsolt

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hackage unix broken
by Brian Denheyer 26 Nov '09

26 Nov '09

System/Posix/Process/Internals.hs:10:17: Module `GHC.Conc' does not export `Signal' The breakage is reported on hackage. The GHC.Conc page is still showing Signal, so it seems like this shouldn't be a problem. Is this a ghc library bug, i.e. should I start a ticket, or has it simply moved somewhere else ? Thanks, Brian

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How does cabal determine version ranges in the dependencies?
by David Fries 26 Nov '09

26 Nov '09

Hi everyone I recently started porting cabal-install to Freebsd. When I looked at its dependencies on hackage, I noticed HTTP (>=4000.0.2 && <4001). However the latest HTTP version on hackage is 4000.0.8. That struck me as kinda odd. How can cabal tell that it won't be compatible with HTTP version 4001? regards david -- Preisknaller: GMX DSL Flatrate für nur 16,99 Euro/mtl.! http://portal.gmx.net/de/go/dsl02

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ANN: Elerea 1.1
by Patai Gergely 26 Nov '09

26 Nov '09

Hello all, Elerea is a simple pull-based FRP library, which allows you to do stream-oriented programming using a primarily applicative style. Unlike traditional stream-based languages, streams (which I call 'signals' in the library) are first-class entities. I uploaded a new version with an experimental branch that reimplements the original idea in a completely different way: - without unsafePerformIO (although with some restructuring it could be used to make the interface 'more applicative') or existential types - signals are Monad instances, so you can use do notation to sample them - for the same reason the 'sampler' combinator is gone; you can use join instead, which is also well-behaved in comparison (i.e. the output of signals doesn't depend any more on how you observe them) - since the library didn't build on the time step being a Double in any way, I relaxed its type (see the Param branch) - the semantics is cleaned up somewhat (I plan to elaborate on this point later in a blog post, but until then you can check the Simple branch for a general idea) Gergely -- http://www.fastmail.fm - Accessible with your email software or over the web

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