- Haskell-Cafe - Haskell.org

"translating" recursively defined sequence
by Christopher Howard 05 Mar '13

05 Mar '13

Hi. My Haskell is (sadly) getting a bit rusty. I was wondering what would be the most straightforward and easily followed "procedure" for translating a recursively defined sequence into a Haskell function. For example, this one from a homework assignment. quote: -------- a_1 = 10 a_(k+1) = (1/5) * (a_k)**2 -------- (The underscore is meant to represent subscripting what follows it.) -- frigidcode.com

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Concurrency performance problem
by Łukasz Dąbek 05 Mar '13

05 Mar '13

Hello Cafe! I have a problem with following code: http://hpaste.org/83460. It is a simple Monte Carlo integration. The problem is that when I run my program with +RTS -N1 I get: Multi 693204.039020917 8.620632s Single 693204.039020917 8.574839s End And with +RTS -N4 (I have four CPU cores): Multi 693204.0390209169 11.877143s Single 693204.039020917 11.399888s End I have two questions: 1) Why performance decreases when I add more cores for my program? 2) Why performance of single threaded integration also changes with number of cores? Thanks for all answers, Łukasz Dąbek.

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Haskell + RankNTypes + (forall p. p Char -> p Bool) sound?
by Shachaf Ben-Kiki 05 Mar '13

05 Mar '13

I was trying to figure out a way to write absurd :: (forall p. p Char -> p Bool) -> Void using only rank-n types. Someone suggested that Haskell with RankNTypes and a magic primitive of type (forall p. p Char -> p Bool) might be sound (disregarding the normal ways to get ⊥, of course). Is that true? Given either TypeFamilies or GADTs, you can write absurd. But it doesn't seem like you can write it with just RankNTypes. (This is related to GeneralizedNewtypeDeriving, which is more or less a version of that magic primitive.) This seems like something that GADTs (/TypeFamilies) give you over Leibniz equality: You can write data Foo a where FooA :: Foo Char FooB :: Void -> Foo Bool foo :: Foo Bool -> Void foo (FooB x) = x Without any warnings. On the other hand data Bar a = BarA (Is a Char) | BarB (Is a Bool) Void bar :: Bar Bool -> Void bar (BarB _ x) = x bar (BarA w) = -- ??? Doesn't seem possible. If it's indeed impossible, what's the minimal extension you would need to add on top of RankNTypes to make it work? GADTs seems way too big. Shachaf

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What pattern is this (Something.T -> IO a) in Sound.ALSA.Sequencer
by Martin Drautzburg 05 Mar '13

05 Mar '13

Hello all, this was previously posted on Haskell Beginners, but only partially answered. In Sound.ALSA.Sequencer, there are a number of functions which together set up a midi environement (client, port, queue). They all have a type, where the last argument has a type like: (something.T -> IO a) i.e. *Main> :t SndSeq.withDefault SndSeq.withDefault :: SndSeq.OpenMode mode => SndSeq.BlockMode -> (SndSeq.T mode -> IO a) -> IO a *Main> :t Port.withSimple Port.withSimple :: SndSeq.T mode -> String -> Port.Cap -> Port.Type -> (Port.T -> IO a) -> IO a *Main> :t Queue.with Queue.with :: SndSeq.T mode -> (Queue.T -> IO a) -> IO a There is example code, where a full setup is created by a number of nested "do" blocks. The repeating pattern there is: something1 $ \x -> do something2 $ \y -> do something3 $ \z -> do What is this all about? I particularly would like to understand, when this parttern is needed and what determines the the number of nested "do" blocks. -- Martin

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simple parsec question
by Immanuel Normann 05 Mar '13

05 Mar '13

Hi, I am trying to parse a semi structured text with parsec that basically should identify sections. Each section starts with a headline and has an unstructured content - that's all. For instance, consider the following example text (inside the dashed lines): --------------------------- top 1: some text ... bla top 2: more text ... bla bla --------------------------- This should be parsed into a structure like this: [Section (Top 1) (Content "some text ... bla"), Section (Top 1) (Content "more text ... bla")] Say, I have a parser "headline", but the content after a headline could be anything that is different from what "headline" parses. How could the "section" parser making use of "headline" look like? My idea would be to use the "manyTill" combinator, but I don"t find an easy solution. Many thanks for any hint Immanuel

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Haskell syntax/indentation for vim
by Tristan Ravitch 05 Mar '13

05 Mar '13

Cafe, I've recently been playing with vim and wasn't quite satisfied with the existing syntax highlighting and indentation, so I thought I'd try my hand at a new Haskell mode: https://github.com/travitch/hasksyn It is minimal in that it doesn't provide support for running external commands over code or anything fancy. It just does syntax highlighting and reasonably-smart indentation. There is no support for literate Haskell since supporting both with one mode is very tricky. It might be useful to some people. Comments, bug reports, and suggestions welcome.

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LambdaJam 2013
by Tony Morris 05 Mar '13

05 Mar '13

Hey haskell guys, LambdaJam 2013 is a functional programming conference to be held in Brisbane in early May. Call for submissions ends this Friday 08 March. If you are planning to make a submission, please make sure you do soon! If you are stuck somehow, I'd love to be able to help you get to the goal. So feel free to send me an email or I am on IRC (dibblego), lurking around the #haskell channel or privmsg if you like. Personally, I would love see more haskell submissions :) LambdaJam2013 call for papers: http://www.yowconference.com.au/lambdajam/Call.html -- Tony Morris http://tmorris.net/

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Re: [Haskell-cafe] Trouble installing and using Chart/cairo on windows 7
by Brent Yorgey 04 Mar '13

04 Mar '13

On Mon, Mar 04, 2013 at 02:30:55PM -0800, briand(a)aracnet.com wrote: > On Mon, 4 Mar 2013 17:27:29 -0500 > Brent Yorgey <byorgey(a)seas.upenn.edu> wrote: > > > On Sun, Mar 03, 2013 at 05:38:02PM -0800, briand(a)aracnet.com wrote: > > > On Sun, 3 Mar 2013 19:58:37 -0500 > > > Brent Yorgey <byorgey(a)seas.upenn.edu> wrote: > > > > > > > > > > > Good access to fonts and font metrics is the kicker. Otherwise I'd > > > > say to switch to using diagrams as a backend, hence getting a whole > > > > bunch of actual backends for free. I would love to see development of > > > > some good Haskell font packages -- maybe it would even make a good > > > > GSoC project? Unfortunately I don't know enough about it to even know > > > > what would be involved, or how much work it would be. > > > > > > > > > > I assume that to use diagram the font package would have to be a vector font system, or could bit-mapped fonts be used ? > > > > A vector font system would be ideal, as then you could go crazy > > turning glyphs into paths and doing whatever the heck you want with > > them using the diagrams framework. However, bit-mapped fonts could be > > used too (as long as there is a backend to support them), it would > > just be less useful. > > > > yes - seems to me that vector fonts are definitely the way to go. > > it seems like this problem has to already have a solution. maybe all that's needed is a font importer which understands some, already established, vector font representation. > Well, there is the SVGFonts package: http://hackage.haskell.org/package/SVGFonts I don't have a good sense of how widely supported this representation is, or how easy it is to convert between it and other font representations. -Brent

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Trouble installing and using Chart/cairo on windows 7
by Arnaud Bailly 04 Mar '13

04 Mar '13

Hello, I am trying to install timeplot and splot on a windows 7 host, using Haskell platform 2012.2.0.0, and I have troubles installing HSChart. First difficulties were installing cairo, which requires Gtk+ libraries and headers. I installed those using a Gtk+-bundle, first in c:\Program Files\. I had to move to d:\soft (eg. a directory without spaces) to be able to install cairo. Then I tried to install Chart and at first failed with a cryptic 'Exit Failure 1' error from cabal. I succeeded when I redirected cabal output to a file, eg: cabal install > out 2<&1 whereas a plain 'cabal install' failed. Then I managed to install splot and timeplot. I then tried to use Chart to draw a simple chart, following http://hackage.haskell.org/packages/archive/Chart/0.16/doc/html/Graphics-Re… it hang forever with a CPU at 100%. Suggestions are more than welcomed. Regards, Arnaud

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ANN: pipes-network-0.1.0 - Stream your network sockets using the pipes and pipes-safe libraries
by Renzo Carbonara 04 Mar '13

04 Mar '13

I'm happy to announce the release of pipes-network 0.1.0. The pipes-network package allows using network sockets together with the pipes and pipes-safe libraries, for streaming purposes. Currently, just TCP sockets are supported. http://hackage.haskell.org/package/pipes-network-0.1.0 The package is split into two halves: one “Safe” half that can be used if proxies need to safely and timely acquire and release new sockets within a pipeline, using the facilities provided by the pipes-safe package; and another simpler and faster half that doesn't depend on pipes-safe, but it also doesn't provide any means for safely acquiring and releasing new sockets within a pipeline itself. There's plenty of documentation and some examples in the Haddocks. Many thanks to Gabriel Gonzalez, author of pipes, for discussing the design of the library with me; and to Paolo Capriotti, author of a previous pipes-network version, for his original work and for handing me the maintenance of this package. ¡Enjoy your TCP streams! Regards, Renzo Carbonara.

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