Problem Statement

Null (or nil) is the computer science concept of an undefined or unbound object. Some languages have an explicit way to access the null object, and some don't. Some languages distinguish the null object from undefined values, and some don't.

Show how to access null in your language by checking to see if an object is equivalent to the null object.

This task is not about whether a variable is defined. The task is about "null"-like values in various languages, which may or may not be related to the defined-ness of variables in your language.

Let's start with the solution:

MODULE Null;
IMPORT
  Out;
TYPE
  Object = POINTER TO ObjectDesc;
  ObjectDesc = RECORD
  END;

VAR
  o: Object;  (* default initialization to NIL *)

BEGIN
  IF o = NIL THEN Out.String("o is NIL"); Out.Ln END
END Null.