The Synchronous Blog

Good news received last week:

Dear Mr. Francisco Sant’Anna,

I am pleased to confirm that your paper “LuaGravity, a Reactive Language
Based on Implicit Invocation” has been accepted for presentation and
publication at SBLP 2009.

All papers went through a rigorous reviewing process by the program
committee. Out of 30 research papers and 3 tutorials submitted, 12
papers and 1 tutorial were accepted.

Please make sure that in the preparation of the final paper you
carefully address the reviewers’ comments. Additionally, at least one
author is required to register in the conference for your paper to
appear in the proceedings.

Congratulations again on having your paper accepted. We look forward to
seeing you in Gramado!

Reviewer’s comments already addressed and final version submitted! One reviewer in particular pointed several constructive observations, which we took very seriously in the final version.

Follows the abstract for the paper:

The reactive programming paradigm covers a wide range of applications, such as
games and multimedia systems.
Mainstream languages do not offer proper support for reactive programming,
lacking language-level primitives that focus on synchronism and interactions
within application parts.
We propose an imperative reactive language, called LuaGravity, based on
unconventional implicit invocation mechanisms.
LuaGravity allows dataflow programming, sequential imperative execution, and
deterministic use of shared-memory.
With this work, we intend to unite the essential features of reactive languages
while keeping a convenient imperative style of programming.

SBLP [1] is the main Brazilian congress on programming languages. This year it will be held in Gramado on August 18-21.

[1] http://sblp2009.ucpel.tche.br/

Ok, here’s the classical synchronization problem.
I’ll jump the description of the problem [1], as you probably remember it from your operating system class.
A common solution involves mutual exclusion to lock access to shared forks.
Besides deadlock, the solution should avoid starvation and livelock as well.

The result can be seen in the video below:

The solution in LuaGravity uses its AWAIT primitive.

Here’s a possible solution for the problem:

Phil = org.class('phil', function ()
    function init (self)
        self.state  = ''
        self.total  = 0
        self.text   = self.state .. ' (' .. self.total .. ')'
        local p = POS[self._i]
        screen:add(
            Text {
                text   = self.text,
                face   = 'vera.ttf',
                _color = {r=0,g=0,b=0},
                x = p.x,
                y = p.y, dy=13,
        })
    end
    function run (self)
        local left, right = self._left, self._right
        while true
        do
            -- think
            self.state = 'thinking'
            AWAIT(math.random(5))

            -- wait
            self.state = 'hungry!'
            while not (left:_isFree() and right:_isFree()) do
                AWAIT(left.put, right.put)
            end

            -- eat
            left:_get(self)
            right:_get(self)
            self.state = 'eating'
            self.total = self.total() + 1
            AWAIT(math.random(5))

            -- back to think
            left:put(self)
            right:put(self)
        end
    end
end )

The reactor run is a infinity loop where the philosopher thinks, waits for the forks and eats.
The think and eat steps are just an AWAIT on a random time between 1 and 5 seconds.

After thinking, the philosopher enters the inner while, checking whether his forks are available for use, otherwise he waits for them until he succeeds.
After leaving the inner while, the philosopher acquires his forks in order to eat.
Note that, in synchronous languages, there’s no need for mutexes or other means of mutual exclusion.

After acquiring the forks, the philosopher eats for a random time.
Then, he goes back to think, releasing his forks.
The call to reactor put awakes philosopher’s neighbors if they are in their inner while.

Just by changing the variables self.state and self.total is enough to update the screen – it’s all about reactivity.

The full example code can be found here.

[1] http://en.wikipedia.org/wiki/Dining_philosophers_problem