Traits Prototype

Introduction

Consider Shinmera’s example:

Here’s my use-case right now: typically an outside system using Alloy will have its own event types. I’d like it to have “zero cost” event mapping to Alloy’s event types by adding them as traits to the instances and implementing the necessary methods to fetch the data. In Alloy itself the event dispatch would then work as if you had created actual instances of the event classes.

So let’s start by defining a protocol and an associated trait [fn:1]:

(defgeneric pointer-x (event))
(defgeneric pointer-y (event))

(traits:deftrait (event) pointer-event ()
  (:method pointer-x ((event event)))
  (:method pointer-y ((event event))))

This defines a specializer pointer-event-event for the event role of the pointer-event trait (sorry, didn’t work on naming conventions yet).

Traits can form a (multiple inheritance) hierarchy:

(defgeneric pointer-button (event))

(traits:deftrait (event) pointer-button-event ((pointer-event event))

  (:method pointer-button ((event event))))

Traits are used by defining the required methods for classes that should play the roles defined by the trait:

(defclass my-event () ; note: no superclass
  ((%x :initarg :x :reader x)
   (%y :initarg :y :reader y)))

(defmethod pointer-x ((event my-event))
  (x event))

(defmethod pointer-y ((event my-event))
  (y event))

Specializers generated for the roles of a trait can be used in methods defined on generic functions that are instances of traits:trait-generic-function:

(fmakunbound 'event-in-rectangle-p)

(defgeneric event-in-rectangle-p (event x1 y1 x2 y2)
  (:generic-function-class traits:trait-generic-function))

;; All the work above is just so we can write POINTER-EVENT-EVENT
;; instead of MY-EVENT here.
(defmethod event-in-rectangle-p ((event pointer-event-event) x1 y1 x2 y2)
  (and (<= x1 (pointer-x event) x2) (<= y1 (pointer-y event) y2)))

Does it work?

(list (event-in-rectangle-p (make-instance 'my-event :x 1 :y 2) 0 0 5 5)
      (event-in-rectangle-p (make-instance 'my-event :x 1 :y 7) 0 0 5 5))

Footnotes

[fn:1] “trait” may not be the best term since protocols in the context of generic functions can involve multiple roles instead of revolving around a single object.