.PAS File Format

Introduction

Pascal is the language Niklaus Wirth published in 1970, designed for teaching clear, structured programming. It spread on home and office computers through Borland’s Turbo Pascal, released in 1983. Object Pascal added classes to it, and Borland’s Delphi, released in 1995 and now sold by Embarcadero, made Object Pascal a language for Windows applications. Free Pascal and the Lazarus environment are free compilers and tools for the same language.

.pas is the general extension, used for programs and units. Free Pascal also accepts .pp. The main file of a Delphi project is a .dpr and of a Lazarus project a .lpr; both are ordinary Pascal programs.

.pp is also the extension of Puppet manifests, which have no relation to Pascal.

Structure

A Pascal file is plain text and the language ignores case. A program starts with program Name;, then a uses clause naming its units, then const, type and var sections and procedures and functions. The main block is begin … end., with a full stop after the last end.

A unit, the language’s module, starts with unit Name;, and has an interface section of what it exports and an implementation section with the code. Assignment is :=, equality is =, <> means not equal and strings use single quotes. Comments are { } and (* *), plus // in Delphi and Free Pascal. Compiler directives are comments starting with a dollar sign, such as {$mode objfpc}.

program Hello;

{$mode objfpc}

uses
  SysUtils;

type
  TGreeter = class
    Name: string;
    function Greeting: string;
  end;

function TGreeter.Greeting: string;
begin
  Result := 'Hello, ' + Name;
end;

var
  G: TGreeter;
begin
  G := TGreeter.Create;
  G.Name := 'world';
  WriteLn(G.Greeting);
  G.Free;
end.

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