Patchage 0.4.1

patchage 0.4.1 has been released. Patchage is a modular patch bay for Jack and ALSA based audio/MIDI systems. For more information, see -- David Robillard

Iteration in Scheme

Thoughts brought on by using SWIG to make Scheme bindings for C/C++ code:

In Scheme, being a LISP, pretty much everything is usually a list. This is great and makes for elegant code, but as a data structures guy I cringe to see the ridiculous amount of O(n) searching done in LISP code. Why is the underlying structure (a list) a dependency of the code using it? Lack of abstraction: car and cdr only work on cons cells, and lists are made of cons cells - period. This sucks for plenty of reasons (the one that encouraged this rant is making Scheme bindings for C++ code).

Say you want to expose a C or C++ list in Scheme. To be really useful in Scheme, it will have to work with car and cdr. Luckily any list has the concept of 'get the value at this position' (car) and 'get the next position' (cdr). For C++ the position is an iterator, car is *, and cdr is ++. Unfortunately making 'nice' (ie typical and un-weird) Scheme bindings for these means you literally have to duplicate the entire C++ list and build a Scheme list out of it, because there's no functional abstraction going on here, and the type system is nonexistant.

I guess this boils down to a rant about needing iterators in Scheme. I cringe at sounding so much like an OO "patterns" kind of guy, but there it is :)

Lets say our Scheme dialect has some a type system with polymorphism. Scheme code is generally something like:

(define (do-something list)
  (something (car list))
  (if (not (null? (cdr list)))
    (do-something (cdr list))))

(define a-list '(1 2 3 4 5))

(do-something a-list)

Rename car "get" and cdr "next" to avoid confusion, and use an iterator:

(define (do-something iter)
  (something (get iter))
  (if (valid? (next iter))
    (do-something (next iter))))

(define a-collection (whatever))

(do-something (start a-collection))

The idiom for do-something (the fundamental LISP idiom if there ever was one) is the same. The only difference is the call to do-something must pass an iterator, here with the start function. Slightly unfortunate maybe, but the benefits are huge:

  • Common list-like iteration for all data structures (e.g. trees)
  • Different styles of iteration (use reverse-end instead of start and have do-something work in reverse without modification)

I suppose since I'm assuming a stronger/better/faster Scheme dialect with a good type system, a collection could just automatically convert to the 'start' iterator, and the calling code could even be the same if you just want classic LISP style code. I'm a huge fan of LISP style languages (mostly because the code is a simple list structure itself), but there really needs to be an abstraction between the underlying data structure and the code that uses it.


There's been a bit of talk in the GNOME camp lately about using DOAP instead of the unstructured text files that are the current norm for source packages. On the one hand, people want the benefits of having machine readable data in projects, OTOH, RDF/XML is a nightmare ("I'll never maintain such bloat!" - "That is one hell of an ugly file.").

This is how RDF/XML hurts RDF. The original loathed file:

<?xml version="1.0"?>
<?xml-stylesheet type="text/xsl"?>
<rdf:RDF xml:lang="en"
  <Project rdf:about="">
    <license rdf:resource="" />
    <name>Apache Ant</name>
    <homepage rdf:resource="" />
    <asfext:pmc rdf:resource="" />
    <shortdesc>Java-based build tool</shortdesc>
    <description>Apache Ant is a Java-based build tool. In theory, it is kind of like Make, but without Make\'s wrinkles.</description>
    <bug-database rdf:resource="" />
    <mailing-list rdf:resource="" />
    <download-page rdf:resource="" />
    <category rdf:resource="" />
        <name>Apache Ant 1.7.0</name>
        <location rdf:resource=""/>
        <browse rdf:resource=""/>

and the equivalent in Turtle (a subset of N3) (automatically generated with rapper -o turtle doap_Ant.rdf):

@prefix rdf: <> .
@prefix : <> .
@prefix asfext: <> .

    a :Project;
    :created "2006-02-17"@en;
    :license ;
    :name "Apache Ant"@en;
    :homepage <>;
    asfext:pmc <>;
    :shortdesc "Java-based build tool"@en;
    :description "Apache Ant is a Java-based build tool. In theory, it is kind of like Make, but without Make's wrinkles."@en;
    :bug-database <>;
    :mailing-list <>;
    :download-page <>;
    :programming-language "Java"@en;
    :category <>;
    :release [
        a :Version;
        :name "Apache Ant 1.7.0"@en;
        :created "2006-12-13"@en;
        :revision "1.7.0"@en
    :repository [
        a :SVNRepository;
        :location <>
        :browse <>
    ] .

I wouldn't want to hand maintain for RDF/XML version either, but the Turtle version? Sure. It's the exact same information, far more human readable, and about as terse as it could be while representing the same information.

The best thing about a syntax independent model like RDF is.. well, it's syntax independent. Choose one that doesn't suck :)

Ardour: back on track. Maybe.

Well, I've been working on Ardour but not directly on my SoC project.. finally got around to fixing that whole mixer-strip-element thing that's been on the table since last year. Things are more open to extensibility now (and a unified bus implementation should appear soon, so we'll have MIDI busses or even MIDI/audio (instrument) busses). Maybe we'll see things like this actually happen some time soonish...

It would be nice to figure out a really good MIDI meter (something more clever than just an audio peak meter abused), and a MIDI fader that can work in various ways, or even instrument plugin support, but it's probably time to get on the actual piano roll editing part of the project and leave that stuff until afterwards.

Kinda ruins my flow though, you know?

Back to the Hack

Well.. finally fully moved; I guess it's time to stop screwing around and get back into hacking. Specifically Ardour, since I'm being payed to do so and all.

Looks like we don't have a fancy new cairo canvas to play with. Maybe it will be worth my time to do that first, but I'm going to do some fiddling with displaying MIDI data with gnomecanvas first to get a grasp on things. Wouldn't hurt to have some visually obvious clue that I am, in fact, actually doing something. This summer's project should be more rewarding than last in that sense; most of what I need to do is visual stuff which tends to be more fun since you have something nice and tangible at the end of the day.

Of course, displaying data is one thing. One relatively easy thing. Actually editing it on the other hand.......... that's where the "fun" (i.e. hard) part comes in.

I think a top-down (GUI->implementation) strategy is best here. The Grand Battle Plan(TM) goes something like this:

  • Get MIDI data displayed in regions, notes visible as one canvas item (rect) per note
  • Attach event signals from note canvas items (move, click, etc) to a set of stub methods that encompass all the editing operations
  • Now there's a nice centralized area where all the editing operations need to be implemented
  • Figure out how the hell to implement them (ie ???????????)
  • Implement them (ie Profit!)

Unix Modular Synth

I've decided to use my blog to archive random ideas until I figure something better out (on that note, anyone know of any good RDF based mind-map software, or something generic that can easily be used as one?)

Until then:

<drobilla> crazy thought of the day: write an LV2 extension for text
stream input/output ports, and you can write a trivial unix cmd line
utility wrapper plugin
<drobilla> anyone want to convert midi to text then run it through
sed and grep and convert back to midi in a modular? :)
<gordonjcp> ahahaha
<gordonjcp> you sick sick man
<drobilla> unix modular, best thing ever

Ingix? Really wouldn't be that hard. Could even make it parallelize forkey pipelines, if the LV2 contexts extension is powerful enough. More on that later...

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