Objects are fast and space-efficient, especially when they don't contain any state forms. If you have a small compound data structure (say, a geometric primitive, or the return value from a function), it's usually best to implement it as a class, rather than a table or an array.

(defclass Rect
  (field x)
  (field y)
  (field w)
  (field h)

  (met init (@x @y @w @h))

  (met area ()
    (* @w @h))

  (met op-clone ()
    (Rect @x @y @w @h))

  (met op-eq? (other)
    (let [x y w h] other)
    (and (== @x x) (== @y y) (== @w w) (== @h h))))

That's a lot of code for something so simple! We provide the defstruct macro to get rid of the boilerplate:

(defstruct Rect 
  x y w h

  (met area ()
    (* @w @h)))

(prn Rect) ; prints #<class:Rect>
(let rect (Rect:new 10 10 20 20))
(prn rect) ; prints #<obj:Rect>
(prn [rect 'x]) ; prints 10
(prn (.area rect)) ; prints 400

As demonstrated above, the defstruct macro defines a class with the given named fields. After the list of field names, defstruct also accepts zero or more met, prop and const clauses. Other class clauses, like init, state, fsm, wrap and mixin, are forbidden. Classmacros are also forbidden.

Struct Initialization

When programming in Rust, you will have encountered tuple structs: structs which identify their fields by position, rather than by name.

fn main() {
struct Raster {
    pixels: Vec<u32>,
    width: u32,
    height: u32,
    format: ImageFormat

// vs.

struct Raster(Vec<u32>, u32, u32, ImageFormat);

Tuple structs with more than one field are generally discouraged. They're hard to understand, hard to refactor, and they require more documentation. This is doubly true in a dynamically-typed language. For example, if you wanted to change the order of a tuple's fields in a Python codebase, the language would give you no help at all; it would be a completely manual task.

To encourage the use of named (rather than positional) struct fields, defstruct registers a global initialization macro which shares the struct's name. This macro behaves like a Rust struct initializer, or like the tab macro:

(defstruct Hit

(let hitbox (Rect @x @y w h))

(let fire-punch (Hit
  (strength 35)
  (element 'fire)))

(let ice-punch (Hit
  (element 'ice)

; the Hit macro resembles a table constructor
(let fire-punch-table (tab
  ('hitbox hitbox)
  ('strength 35)
  ('element 'fire)))

The original class is bound to the global Name:new. This enables you to easily opt in to using positional arguments, when you think they would be the better choice.

(defstruct Rgb r g b)

(let named (Rgb (r 32) (g 139) (b 32)))
(let positional (Rgb:new 32 139 32))

(prn (eq? named positional)) ; prints #t

Finally, the global Name? is bound to a function which tests whether or not a value is a Name struct. This is more convenient and intuitive than writing (is? val Name) every time.

(let chartreuse (Rgb 0x7f 0xff 0x00))
(prn (Rgb? chartreuse)) ; prints #t

Operator Overloading

The default behaviour of the eq? function, when comparing two objects, is to test them for identity using the same? function. This means that two objects can belong to the same type, and store the same values, but still compare unequal to one another.

You can override this default behaviour by defining a method named op-eq?.

As noted above, op-eq? is automatically implemented by the defstruct macro. It will compare each struct field in turn using eq?.

(defclass Spawner
  (field level) ; a large, immutable table
  (field to-spawn) ; a class
  (field remaining) ; an integer counter

  (met op-eq? (other)
      (same? @level [other 'level])
      (same? @to-spawn [other 'to-spawn])
      (== @remaining [other 'remaining]))))

By default, the clone and deep-clone functions only duplicate a reference to an object; they don't copy the object's storage. You can provide op-clone and op-deep-clone methods to override this behaviour.