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Before yesterdayNews from the Ada programming language world

Coding Ada: Using multiple packages

By: spqr

8 February 2022 at 15:25

So Ada uses packages to implement a form of encapsulation, which actually existed before the whole hoopla with C++ evolved. Packages have a specification, and an implementation. In the example below, we will create three packages to generate an image comprised of random values, i.e. a noise image. Here is an example (it can be saved as a text image).

The first package only has a specification, and is called datas. It includes the data structures needed by the other packages (i) image, and (ii) print (and also the main program). Here is what the specification looks like (datas.ads):

package datas is
   type randRange is new Integer range 0..255;
   type randImg is array(1..500,1..500) of randRange;
end datas;

It includes a type randRange, which specifies integers in the range 0..255, and a type randImg, which is a 2D array of type randRange. Next is the package image – its task is to generate an image of random numbers in the range 0..255. Here is the specification (image.ads):

with ada.numerics.discrete_random;
with datas; use datas;

package image is
   package Rand_Int is new ada.numerics.discrete_random(randRange);
   use Rand_Int;
   procedure makeImage(img : out randImg);
end image;

It creates a new instance of the package discrete_random, to deal with generating numbers in the range 0..255. It also contains a function makeImage(), which uses the types in package datas to generate an image. Here is the package implementation (image.adb):

package body image is

procedure makeImage(img : out randImg) is
   gen : Generator;
begin
   reset(gen);
   for i in 1..img'last(1) loop
      for j in 1..img'last(2) loop
         img(i,j) := random(gen);
      end loop;
   end loop;
end;

end image;

The last package, print just prints out the image as a series of rows (to standard output, but it can be redirected to a file, e.g. a.out >randim.txt). Here is the specification (print.ads):

with ada.Text_IO; use Ada.Text_IO;
with ada.Integer_Text_IO; use Ada.Integer_Text_IO;
with datas; use datas;

package print is
   procedure printImage(img : in randImg);
end print;

And the associated implementation (print.adb):

package body print is

procedure printImage(img : in randImg) is
begin
   for i in 1..img'last(1) loop
      for j in 1..img'last(2) loop
         put(randRange'Image(img(i,j)));
      end loop;
      new_line;
   end loop;
end;

end print;

Finally the main program that calls them all (test.adb)

with image; use image;
with print; use print;
with datas; use datas;

procedure test is
   im : randImg;

begin
   makeImage(im);
   printImage(im);
end test;

This program can be compiled simply using the main file:

% gnatmake -Wall test.adb

Obviously packages can be compiled,bu they can’t be linked or executed. Each of the packages can also be compiled separately:

% gnatmake -Wall image.ads image.adb
% gnatmake -Wall print.ads print.adb
% gnatmake -Wall datas.ads
% gnatmake -Wall test.adb