Your First Program & Program Structure

3.1 Hello, World (Free Pascal)

Save as hello.pas:

Compile and run:

program Hello;

{$mode objfpc}{$H+}

begin
  Writeln('Hello, World!');
end.

fpc hello.pas
./hello          # Linux/macOS
hello.exe        # Windows

3.2 Hello, World (Delphi console)

program Hello;

{$APPTYPE CONSOLE}

uses
  System.SysUtils;

begin
  try
    Writeln('Hello, World!');
    Readln;  // keep the console window open when run from the IDE
  except
    on E: Exception do
      Writeln(E.ClassName, ': ', E.Message);
  end;
end.

3.3 Anatomy of a program

program Name; Name must be a valid identifier; ISO Pascal allows program Name(input, output); lists units (modules); System is always used implicitly label, const, type, var declaration sections; ISO Pascal requires this order, FPC/Delphi allow any order and repetition procedure and function declarations begin... end. main program block — the only end followed by a period

program Anatomy;              // 1. program heading (optional in FPC/Delphi, but conventional)

{$mode objfpc}{$H+}           // 2. compiler directives

uses                          // 3. uses clause: units this program needs
  SysUtils, Math;

const                         // 4. declaration sections, in any order and repeatable
  MaxItems = 10;

type
  TScore = 0..100;

var
  Total: Integer;
  Score: TScore;

procedure ShowTotal;          // 5. routines
begin
  Writeln('Total = ', Total);
end;

begin                         // 6. main block
  Total := 0;
  for Score := 1 to MaxItems do
    Total := Total + Score;
  ShowTotal;
  Writeln('Max of 3 and 7: ', Max(3, 7));
end.                          // 7. final "end" followed by a period

3.4 Statements and semicolons

In Pascal, the semicolon separates statements; it does not terminate them. That is why the last statement before end does not need one (an extra one creates a harmless empty statement):

The one place where a semicolon is a real error is before else in an if statement:

begin
  A := 1;
  B := 2;      // the ';' here is optional
end;

if X > 0 then
  Writeln('positive')    // no semicolon here!
else
  Writeln('not positive');

3.5 Compound statements

begin ... end groups several statements into one, wherever a single statement is expected:

if X > 0 then
begin
  Writeln('positive');
  Inc(Count);
end;