Basic Construction

Felix provides a dynamic object construction mechanim.

object ob (x:string) = {
    var age = 0;
    method fun name () => x;
    method fun underage () => age < 21;
    method proc setage (y:int) { age = y; }
 var joe = ob "joe";
 joe.setage 42;
 println$ "name " + joe.name() + " is " +
   if joe.underage()
   then "underage"
   else "of age"

Here, ob is an ordinary function with one special property: the return is done automatically, and the returned value is a record of closures of the functions and procedures specified as method. They are each closed over the local data frame of the constructor function ob.

Applying the object constructor ob to an argument that matches its parameter x creates the actual object, which is an ordinary record value.

Object type

The type of the object above is given by:

typedef ob_t =
  name: 1->string,
  underage: 1->bool,
  setage: int -> 0

An alternative syntax is:

interface ob_t  {
  name: 1->string;
  underage: 1->bool;
  setage: int -> 0;

You can use this to specify the return type of the constructor:

object ob (x:string) implements ob_t = {
    var age = 0;
    method fun name () => x;
    method fun underage () => age < 21;
    method proc setage (y:int) { age = y; }


Felix objects are more flexible than Java or C++ because they’re not defined by classes, they’re just values contructed by a function. You can, in fact, constuct an object with an ordinary function: the only magic with an object bound function is the automatic return of the record of method closures.

Because an instance object is just a record, you can apply any record operations to it!

However, the local variables of the constructor to which the method closures are bound are protected by functional abstraction, and so can be considered private. This privacy cannot be bypassed.

But you can do this:

var joe = ob "joe";
joe = (joe with name = (fun () => "Joe")); // change name method
println$  "name " + joe.name(); // prints Joe not joe

var xjoe = extend joe with (surname = "Blogs") end;
println$ xjoe.surname;

Note, the type of xjoe is no longer the same, it has an extra field called surname. This field is just a string, not a method.