Editing Modula-3 (section)

=== Revelation ===
Modula-3's <code>REVEAL</code> construct provides a conceptually simple and clean yet very powerful mechanism for hiding implementation details from clients, with arbitrarily many levels of ''friendliness''. A full revelation of the form <code>REVEAL T = V</code> can be used to show the full implementation of the <code>Person</code> interface from above. A partial revelation of the form <code>REVEAL T <: V</code> merely reveals that T is a supertype of V without revealing any additional information on T.<ref>{{Cite journal |last1=Cardelli |first1=Luca |last2=Donahue |first2=James |last3=Glassman |first3=Lucille |last4=Jordan |first4=Mick |last5=Kalsow |first5=Bill |last6=Nelson |first6=Greg |date=August 1992 |title=Modula-3 language definition |url=https://dl.acm.org/doi/10.1145/142137.142141 |journal=ACM SIGPLAN Notices |language=en |volume=27 |issue=8 |pages=15–42 |doi=10.1145/142137.142141 |issn=0362-1340}}</ref>

<syntaxhighlight lang="modula2">
MODULE Person;

REVEAL T = Public BRANDED 
OBJECT 
  name: TEXT;   (* These two variables *)
  age: INTEGER; (* are private. *)
OVERRIDES
  getAge := Age;
  init := Init;
END;

PROCEDURE Age(self: T): INTEGER =
  BEGIN
    RETURN self.age;
  END Age;

PROCEDURE Init(self: T; name: TEXT; age: INTEGER): T =
  BEGIN
    self.name := name;
    self.age := age;
  RETURN self;
  END Init;

BEGIN
END Person.
</syntaxhighlight>

Note the use of the <code>BRANDED</code> keyword, which "brands" objects to make them unique as to avoid structural equivalence. <code>BRANDED</code> can also take a string as an argument, but when omitted, a unique string is generated for you.

Modula-3 is one of a few programming languages which requires external references from a module to be strictly qualified. That is, a reference in module <code>A</code> to the object <code>x</code> exported from module <code>B</code> must take the form <code>B.x</code>. In Modula-3, it is impossible to import ''all exported names'' from a module.

Because of the language's requirements on name qualification and [[method overriding]], it is impossible to break a working program simply by adding new declarations to an interface (any interface). This makes it possible for large programs to be edited concurrently by many programmers with no worries about naming conflicts; and it also makes it possible to edit core language libraries with the firm knowledge that no extant program will be ''broken'' in the process.