User defined exceptions

You can declare your own exception types

    TAN_ERROR : exception;

Example:

    procedure main is

        X, res    : FLOAT;
        TAN_ERROR : exception;


        function tan (X : FLOAT) return FLOAT is
        begin
            return sin(x)/cos(x);
        exception
            when NUMERIC_ERROR =>
                raise TAN_ERROR;
        end;


    begin

        PUT ("Enter a real number X : ");
        GET (X);
        res := tan(X);
        PUT ("Tan(X) is "); PUT(res); NEW_LINE;

    exception
        when TAN_ERROR =>
            PUT_LINE ("The tangent is too big");

    end;

Declaring exceptions in packages

NUMERIC_ERROR may arise in tan(x) function

Perhaps too unilateral to handle it locally, so prefer to pass an exception back to the caller.

What to pass back?

• NUMERIC_ERROR is too general
• more specific name desirable (eg TAN_ERROR)

Where to declare TAN_ERROR?

• not in function TAN (invisible outside)
• not in calling code (belongs with TAN)
• best is in a package, along with TAN

An example shows how a package specification can define an exception; the package body can raise that exception when appropriate; and a user program can recognise and handle the exception.

Another example shows a package to do with vector arithmetic. Some vector operations only make sense if they are performed with vectors of the same length. The package can define an exception LENGTH_ERROR, to be raised if an operation is invoked with vectors of different lengths.