Data Structures and Algorithms

6.2 ADTs in Ada

Two Ada constructs are needed for defining an ADT: the data type and its methods are placed in an Ada package. For safety and information hiding, the data type is made private. Although a package's "client" can see the structure of the data type, the compiler prevents access to individual attributes of the type: thus effectively implementing the information hiding principle. The client can see the information, but can't do anything with it! (I believe that the reason for exposing the structure of the private type is purely pragmatic: compilers and linkers know how much space an ADT in a separately compiled package - for which only the specification might be available - requires.)

An Ada package for complex numbers would be implemented:

PACKAGE complex_numbers IS
    TYPE complex IS PRIVATE;
    I : CONSTANT complex;    -- 'i'

    FUNCTION "-"( complex a ) RETURNS complex; -- Unary minus

    FUNCTION "+"( complex a; complex b ) RETURNS complex;
    FUNCTION "-"( complex a; complex b ) RETURNS complex;
    FUNCTION "*"( complex a; complex b ) RETURNS complex;
    FUNCTION "="( complex a; complex b ) RETURNS boolean;

PRIVATE
    TYPE complex IS RECORD
        real, imag : FLOAT;
        END RECORD;
    I : CONSTANT complex := (0.0, 1.0);
END complex_numbers;

PACKAGE BODY complex_numbers IS

    FUNCTION "-"( complex a ) RETURNS complex IS -- Unary minus
	RETURN complex'(-a.real,-a.imag);
	END "-";

    FUNCTION "+"( complex a; complex b ) RETURNS complex IS
        RETURN complex'(a.real+b.real,a.imag+c.imag);
        END "+";

    FUNCTION "-"( complex a; complex b ) RETURNS complex IS
        RETURN complex'(a.real-b.real,a.imag-c.imag);
        END "-";

    FUNCTION "*"( complex a; complex b ) RETURNS complex IS
	RETURN complex'(a.real*b.real - a.imag*b.imag,
                        a.real*b.imag + a.imag*b.real );
	END "*";

    FUNCTION "="( complex a; complex b ) RETURNS boolean IS
	RETURN (a.real = b.real) AND (a.imag = b.imag);
	END "=";

END complex_numbers;

Note that Ada provides excellent operator overloading capabilities, which enable us to write mathematically "natural" code: e.g.

    complex a, b, c, z;

    IF a = b THEN
	c := z - a;
        z := -z;
    END IF;

You can also observe that Ada is an extreme case of the principle that programs are read many times more often than they are read (for modern languages at least - nothing is ever likely to match the verbosity of COBOL!). Keywords appear everywhere: IS, RETURNS, etc..