11. Modules

A module is a collection of declarations of constants, types, variables, and procedures, together with a sequence of statements for the purpose of assigning initial values to the variables. A module constitutes a text that is compilable as a unit.

Module     = MODULE ident ";" [ImportList] DeclarationSequence
             [BEGIN StatementSequence] END ident ".".
ImportList = IMPORT Import {"," Import} ";".
Import     = [ident ":="] ident.

The import list specifies the names of the imported modules. If a module A is imported by a module M and A exports an identifier x, then x is referred to as A.x within M. If A is imported as B := A, the object x must be referenced as B.x. This allows short alias names in qualified identifiers. A module must not import itself. Identifiers that are to be exported (i.e. that are to be visible in client modules) must be marked by an export mark in their declaration (see Chapter 4).

The statement sequence following the symbol BEGIN is executed when the module is added to a system (loaded), which is done after the imported modules have been loaded. It follows that cyclic import of modules is illegal. Individual (parameterless and exported) procedures can be activated from the system, and these procedures serve as commands (see Appendix D1).

MODULE Trees;  (* exports: Tree, Node, Insert, Search, Write, Init *)
  IMPORT Texts, Oberon;  (* exports read-only: Node.name *)

  TYPE
    Tree* = POINTER TO Node;
    Node* = RECORD
      name-: POINTER TO ARRAY OF CHAR;
      left, right: Tree
    END;

  VAR w: Texts.Writer;

  PROCEDURE (t: Tree) Insert* (name: ARRAY OF CHAR);
    VAR p, father: Tree;
  BEGIN p := t;
    REPEAT father := p;
      IF name = p.name^ THEN RETURN END;
      IF name < p.name^ THEN p := p.left ELSE p := p.right END
    UNTIL p = NIL;
    NEW(p); p.left := NIL; p.right := NIL;
    NEW(p.name, LEN(name)+1); COPY(name, p.name^);
    IF name < father.name^ THEN
      father.left := p
    ELSE
      father.right := p
    END
  END Insert;

  PROCEDURE (t: Tree) Search* (name: ARRAY OF CHAR): Tree;
    VAR p: Tree;
  BEGIN p := t;
    WHILE (p # NIL) & (name # p.name^) DO
      IF name < p.name^ THEN p := p.left ELSE p := p.right END
    END;
    RETURN p
  END Search;

  PROCEDURE (t: Tree) Write*;
  BEGIN
    IF t.left # NIL THEN t.left.Write END;
    Texts.WriteString(w, t.name^); Texts.WriteLn(w);
    Texts.Append(Oberon.Log, w.buf);
    IF t.right # NIL THEN t.right.Write END
  END Write;

  PROCEDURE Init* (t: Tree);
  BEGIN NEW(t.name, 1); t.name[0] := 0X; t.left := NIL; t.right := NIL
  END Init;

BEGIN Texts.OpenWriter(w)
END Trees.

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