Some people wonder where you can use a simple exception in Ada. Functions are an ideal place. Below is a program which contains a function isNumber() which returns true if the string passed to it is considered a number (integer). So 1984 us a number, but x24 is not. The program takes a word as input from the user (as an unbounded_string), and passes it to isNumber(). The function isNumber() converts the unbounded_string to a string, and then to an integer using integer’value(). If the conversion works, then the function returns true. If however, the conversion fails, then a Constraint_Error exception will be triggered, and false will be returned.

with ada.Text_IO; use Ada.Text_IO;
with ada.Integer_Text_IO; use Ada.Integer_Text_IO;
with ada.strings.unbounded; use ada.strings.unbounded;
with ada.strings.unbounded.Text_IO; use ada.strings.unbounded.Text_IO;

procedure number is

   aWord : unbounded_string;
   num: integer;
 
   function isNumber(s: unbounded_string) return boolean is 
      temp: integer;
   begin
      temp := integer'value(to_String(s));
      return true;
      exception 
         when Constraint_Error => return false;
   end isNumber;

begin
   put_line("Enter a word: ");
   get_line(aWord);
 
   if (isNumber(aWord)) then
      num := integer'value(to_String(aWord));
      put(num);
   else
      put_line("Not a number");
   end if;
end number;

This sort of code could be used to count numbers in a document or something similar. You can use float’value to do the same for floating-point numbers.