| Ada provides all the expected features of a general purpose programming
language.
Ada also provides a programmer with an extensive set of tools for producing
programs with concurrently executing modules. Ada calls concurrently
executing
modules "tasks".
The following program demonstrates some simple Ada tasking capablities by
abusing the traditional "Hello World" program.
Each line of output in the following program is written by a separate task.
The tasks
are designed to take random amounts of time to finish their work. This
causes the
order of output to be different each time the program is run.
Three task objects are declared as M1, M2, M3. If the tasks did not run
concurrently
the output would always be in the expected order. The fact that the order of
the output
changes each time the program is run proves that the three tasks must be
executing
at the same time.
The Ada.Strings.Unbounded package is used inside each task to hold a
variable
length string with a maximum size of 72 characters, allowing you to force
each task to
produce a line full of output without causing formatting problems on the
average
screen.
A random number generator is used to cause each task to delay between 0.0
and 1.0 seconds. This random delay causes each task to take a different
amount
of time to complete. The task finishing first will write its output to the
screen first.
-----------------------------------------------------------------------
-- Hello World program in Ada using tasking
-----------------------------------------------------------------------
with Ada.Text_Io;
with Ada.Strings.Bounded;
with Ada.Numerics.Float_Random;
procedure HelloTask is
package Inner_Message is new
Ada.Strings.Bounded.Generic_Bounded_Length(72);
use Inner_Message;
task type Messenger is
entry Start(Message : in String);
end Messenger;
task body Messenger is
use Ada.Numerics.Float_Random;
Seed : Generator;
Msg : Bounded_String;
begin
accept Start(Message : in String)
do
Msg := To_Bounded_String(Message);
end Start;
Reset(Seed);
delay Duration(Random(Seed));
Ada.Text_Io.Put_Line(To_String(Msg));
end Messenger;
M1 : Messenger;
M2 : Messenger;
M3 : Messenger;
begin
M1.Start("Hello");
M2.Start("From");
M3.Start("Ada !");
end HelloTask;
| 
