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
Filename extensions	.u
Website	github.com/urbiforge/urbi
Influenced by
C++,Self,Io

urbiscriptis a programming language for robotics.It features syntactic support for concurrency and event-based programming. It is aprototype-basedobject-orientedscripting language.It is dynamic:name resolutionis 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 managementis performed byreference counting.

Tightly bound to theUrbi platformit supports seamless integration of C++/Java components.

Syntax and semantics

Inspiration

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

Its prototype-based object-oriented design was influenced by theSelfand theIoprogramming languages.

It is designed to program, but also interact with robots;as such, it is influenced byUnix shellsand other languages that provide aread-eval-print loopstyle 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):

• Theifstatement,which conditionally executes a block of code, along withelse.
• The traditionalforstatement,as in C which iterates over an iterable object, capturing each element to a local variable for use by the attached block.
• Anotherforstatement,which iterates over an iterable object, capturing each element to a local variable for use by the attached block.
• Thewhilestatement,which executes a block of code as long as its condition is true.
• Thetrystatement,which allows exceptions thrown in its attached code block to be caught and handled bycatchclauses. An optionalelseclause is run if no exception was thrown. Clean-up code can be guaranteed to be run in every case when given in afinally-clause.
• Theassertstatement,used during debugging to check for conditions that ought to apply. urbiscript also featureassertblocks, which can be used to factor severalassertstatements.

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:

varstatus={if(closed)"closed"else"open"};
varpass={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 usingcoroutines.

Statement composition

Like in C, the semicolon denotes sequential composition:a;bstands for "run statementathen run statementb.Other tasks may be run betweenaandb.Another statement separator, pipe, denotes "tight sequential composition": no other task can be run betweenaandbina|b.

Similarly urbiscript features two means to compose statements concurrently. Witha,b,firstais run, and at some pointbwill be --- possibly whileais still running. This is very similar to the&operator in Unix shells. Alternatively, witha&b,bothaandbare started together; in interactive sessions, this means thatawon't be run untilbis fully entered and properly followed by either a;or a,.

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

{
{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(vari:[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 thefor&keyword runs the loop bodies concurrently:

for&(vari:[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,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

vare=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,varx)ifx%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:

varx=0;

[00002165] 0

vary=0;

[00002166] 0

varz=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

• Urbi

External links

• urbionGitHub
• Docker repository
• urbiscript Tutorial