Ada Engineering

Module 3 – Turn repeated reasoning into small components

Page 1 of 3 · About 15 minutes per lesson, including practice.

A function makes a promise about a result

The function below answers one question: how many lessons remain? Its inputs and result are Natural, Ada's predefined subtype of nonnegative integers. Our explicit policy is that an equal or larger completed count leaves zero remaining. Read the early return before the subtraction: it prevents a negative result for the stated input domain.

with Ada.Text_IO;

procedure Main is
   function Remaining (Total, Done : Natural) return Natural is
   begin
      if Done >= Total then
         return 0;
      end if;
      return Total - Done;
   end Remaining;
begin
   Ada.Text_IO.Put_Line (Natural'Image (Remaining (4, 1)));
end Main;

The call produces three remaining lessons, printed with a leading space by Natural'Image. Trace calls with (4, 4), (4, 5), and (0, 0). For each, identify which return is reached. A useful function name and a written policy let a reviewer judge the behavior independently of the implementation.

Reference: AdaCore: functions, procedures, and parameters.

Example code on Agila Forge ↗