Lambda the Ultimate

I redesigned the constructors in Theme-D version 2.0.0, see [1]. The design is inspired by the OCaml object system. Constructors are special procedures used to create instances (objects) of classes. They are not defined like other procedures but Theme-D creates them using the construct statement in a class and field initializers in a class. The translator-generated default constructor is sufficient in many cases. For example, consider the class <complex> defined in the standard library:

  (define-class <complex>
    (attributes immutable equal-by-value)
    (inheritance-access hidden)
    (fields
     (re <real> public hidden)
     (im <real> public hidden))
    (zero (make <complex> 0.0 0.0)))

The translator-generated default constructor takes two real arguments and sets the first to the field re and the second to the field im.

The programs objects1.thp and objects2 in subdirectory theme-d-code/examples of the source code [1] demonstrate user-defined constructors. Here is the first example:

  (define-class <widget>
    (fields
     (str-id <string> public module)))

  (define-class <window>
    (superclass <widget>)
    (construct ((str-id1 <string>) (i-x11 <integer>) (i-y11 <integer>)
		(i-x21 <integer>) (i-y21 <integer>))
	       (str-id1))
    (fields
     (i-x1 <integer> public module i-x11)
     (i-y1 <integer> public module i-y11)
     (i-x2 <integer> public module i-x21)
     (i-y2 <integer> public module i-y21)
     (i-width <integer> public module (+ (- i-x21 i-x11) 1))
     (i-height <integer> public module (+ (- i-y21 i-y11) 1))))

The constructor of class <window> passes the first argument str-id1 to the constructor of its superclass <widget>. The constructors also initialize the fields using their arguments. Note that the field initializers may contain more complex expressions than just copying an argument variable.

Here is the second example:

  (define-class <widget>
    (construct ((str-id1 <string>)) () (nonpure))
    (fields
     (str-id <string> public module
	     (begin
	       (console-display "new widget: ")
	       (console-display-line str-id1)
	       str-id1))))

  (define-class <window>
    (superclass <widget>)
    (construct ((str-id1 <string>) (i-x11 <integer>) (i-y11 <integer>)
		(i-x21 <integer>) (i-y21 <integer>))
	       (str-id1) (nonpure))
    (fields
     (i-x1 <integer> public module i-x11)
     (i-y1 <integer> public module i-y11)
     (i-x2 <integer> public module i-x21)
     (i-y2 <integer> public module i-y21)
     (i-width <integer> public module (+ (- i-x21 i-x11) 1))
     (i-height <integer> public module (+ (- i-y21 i-y11) 1))))

Here we log the calls to the constructor of <widget> to the console. Note that we have to declare the constructors as nonpure if they have side effects.

[1] Theme-D Homepage