urbiscript

urbiscript
Paradigm multi-paradigm: object-oriented, event-driven, imperative, functional, procedural, reflective
Designed by Jean-Christophe Baillie
Developer Gostai et al.
First appeared 2003 (2003)
Stable release
2.7.4 / November 17, 2011 (2011-11-17)
Typing discipline duck, dynamic
OS Cross-platform
License BSD licenses[1]
Filename extensions .u
Website github.com/urbiforge/urbi
Influenced by
C++, Self,[2] Io[2]

urbiscript is a programming language for robotics.[3] It features syntactic support for concurrency and event-based programming. It is a prototype-based object-oriented scripting language. It is dynamic: name resolution is performed during the program execution (late binding); slots (member variables) can be added/removed at runtime, and even prototypes (superclasses) of an object can be changed at runtime.

Memory management is performed by reference counting.

Tightly bound to the Urbi platform it supports seamless integration of C++/Java components.

Syntax and semantics

Inspiration

From the syntactical point of view, urbiscript belongs to the C-family of programming languages.

Its prototype-based object-oriented design was influenced by the Self and the Io programming languages.[2]

It is designed to program, but also interact with robots;[2] as such, it is influenced by Unix shells and other languages that provide a read-eval-print loop style interactive toplevel. However, contrary to others, there is no prompt for user input but answers from the system are prefixed by a timestamp (in milliseconds) between square brackets:

 1 + 1; sleep(1s); 1 + 2 * 3;
[00005420] 2
[00006420] 7

Sequential statements and control flow

urbiscript statements include (among others):[4]

Actually, contrary to most C-like languages and despite what the syntax suggests, statements "have a value", and therefore are expressions, provided they are embedded in braces:

  var status = { if (closed) "closed" else "open" };
  var pass = { try { foo } catch { false } else { true } };

Concurrent statements and control flow

In urbiscript, some control-flow constructs come in several "flavors": two types of sequential composition, and two types of concurrent composition. Under the hood, concurrency is implemented using coroutines.[5]

Statement composition

Like in C, the semicolon denotes sequential composition: a;b stands for "run statement a then run statement b. Other tasks may be run between a and b. Another statement separator, pipe, denotes "tight sequential composition": no other task can be run between a and b in a|b.

Similarly urbiscript features two means to compose statements concurrently. With a,b, first a is run, and at some point b will be --- possibly while a is still running. This is very similar to the & operator in Unix shells. Alternatively, with a&b, both a and b are started together; in interactive sessions, this means that a won't be run until b is fully entered and properly followed by either a ; or a ,.

Scopes are boundaries for backgrounded jobs, as demonstrated in the following example:[5]

  {
    { sleep(2s); echo(2) },
    { sleep(1s); echo(1) }, 
  };
  echo(3);
[00012451] *** 1
[00013447] *** 2
[00013447] *** 3

Concurrent flavors of sequential constructs

Most looping constructs in urbiscript come in several "flavors", which are based on the four statement separators: ;, |, ,, and &.

For instance

  // This is actually "for;".
  for (var i : [0, 1, 2])
  {
    echo(i);
    echo(i ** 2);
  };

displays

[00002919] *** 0
[00002921] *** 0
[00002921] *** 1
[00002922] *** 1
[00002922] *** 2
[00002922] *** 4

i.e., the loop bodies are not executed sequentially, while the for& keyword runs the loop bodies concurrently:

  for& (var i : [0, 1, 2])
  {
    echo(i);
    echo(i ** 2);
  };
[00021680] *** 0
[00021680] *** 1
[00021680] *** 2
[00021682] *** 0
[00021682] *** 1
[00021682] *** 4

Event-based programming

Aiming at the development of portable robotic applications,[6] urbiscript relies on specific syntactic constructs to specify reactive behaviors such as "go to the charging dock when the battery is low", "play a friendly sound when a known face is recognized", or "stop when an obstacle is detected".

Explicit event handling

Event handling goes into three steps. First, define an event

  var e = Event.new;

Second, specify event handlers

  at (e?)
    echo("received event e");

Third, "emit" this event

  e!;
[00014333] *** received event e

Events can have payloads, and event handlers enjoy pattern matching on the payload:

  at (e?(1, var x) if x % 2 == 0)
    echo("received event e(1, %s)" % x);
  e!(1, 1);
[00014336] *** received event e
  e!(1, 2);
[00014336] *** received event e
[00014336] *** received event e(1, 2)

Implicit events

The urbiscript language also allows to monitor expressions:

  at (batteryLevel <= 0.2)
    robot.goToChargingDock;

The following example demonstrates the feature:

  var x = 0;
[00002165] 0
  var y = 0;
[00002166] 0
  var z = 0;
[00002167] 0
  at (x + y == z)
    echo("%s + %s == %s" % [x, y, z]);
[00002168] *** 0 + 0 == 0
  x = 1;
[00002169] 1
  z = 1;
[00002170] 1
[00002170] *** 1 + 0 == 1

See also

References

  1. "Urbi, the open source operating system for robots". Retrieved 2012-10-27.
  2. 1 2 3 4 Baillie, Jean-Christophe; Demaille, Akim; Nottale, Matthieu; Hocquet, Quentin; Tardieu, Samuel (2008). "The Urbi Universal Platform for Robotics" (PDF). Retrieved 6 October 2011.
  3. Baillie, Jean-Christophe. "Urbi: a new parallel & event-driven script language for robotics, games and more". YouTube. Retrieved 6 Oct 2011.
  4. "urbiscript Language Reference Manual". Retrieved 2011-09-20.
  5. 1 2 Baillie, Jean-Christophe; Demaille, Akim; Nottale, Matthieu; Hocquet, Quentin (2010). "Tag: Job Control in urbiscript" (PDF). Retrieved 6 October 2011.
  6. Baillie, Jean-Christophe; Demaille, Akim; Nottale, Matthieu; Hocquet, Quentin (2010). "Events! (Reactivity in urbiscript)". arXiv:1010.5694Freely accessible.
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