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Christopher Diggins

Dr. Dobb's Bloggers

Proposed Design of the Heron Module System

December 23, 2009

I show here the design of the module system under consideration for Heron.

I am considering a design for the Heron module system that allows modules to be instantiated just like classes. While this system isn;t as powerful as that used inNewspeak andScala , it does seem to address some of the problems that I am aware of with other languages with weak or non-existent module systems:

1. No global mutable state.

2. Explicit programmer controlled initialization of modules

3. Parameters can be passed to module during construction

The Heron module bears a resemblance to the Scala approach of treating objects as modules, but is not as expressive (or complex). In fact that Heron module would be a class, that allows nesting of other classes in it, which ordinary classes don;t allow.

Here is an example of how the syntax might look in action:

    module M {

      fields {

        // State associated with particular module instances.

        x : int 


      methods {

        // A module initializer

        Constructor(n : Int) {

          x = n;



      classes {

        class C {

          methods {

            Get() : Int { return M.x; }

            Set(n : Int) { M.x = n; }





    module N {

      imports {

        M as module1;

        M as module2;

        Console; // Console as Console is implied here


      methods {

        Constructor() {

           // Construct the module instances

           // Order is specified. 

           // We could even do conditional module instantiation.

           m1 = new M();

           m2 = new M();

           Console = new Console();

           // Construct two different instances of class C, 

           // Both have the same type "M.C", but are associated 

           // with different instances of the module M 

           M.C c1 = new M1.C(0);

           M.C c2 = new M2.C(1);

           Console.WriteLine(c1.Get()); // 0

           Console.WriteLine(c2.Get()); // 1


           Console.WriteLine(c1.Get()); // 0

           Console.WriteLine(c2.Get()); // 42





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