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Objective-CL

CL syntax extension to make working with objects easier. It introduces the [] syntax, which does two things:

  1. In method calls, object and method are swapped, so instead of passing the object as the first argument, you instead pass method to the object. Remaining method arguments follow the method.

  2. Inside [] object slots can be accessed by prefixing them with .. E.g., my-object.some-slot expands to (slot-value my-object 'some-slot).

A demonstration:

(defclass vec ()
  ((x :initarg :x)
   (y :initarg :y)
   (z :initarg :z)))

(defun make-vec (x y z)
  (make-instance 'vec :x x :y y :z z))

Then, in the REPL:

CL-USER> (objective-cl:enable)
T
CL-USER> (defgeneric len (o))
#<STANDARD-GENERIC-FUNCTION COMMON-LISP-USER::LEN (0)>
CL-USER> (defmethod len ((v vec))
           (sqrt (+ [v.x expt 2]
                    [v.y expt 2]
                    [v.z expt 2])))
#<STANDARD-METHOD COMMON-LISP-USER::LEN (VEC) {1004820213}>
CL-USER> (len (make-vec 3 4 0))
5.0
CL-USER> [(make-vec 1 2 3) len] ;;; Same as above, with different syntax
3.7416575
CL-USER> ;;; You can also use [obj.slot] just to extract slot-value from an object:
(defmethod print-object ((v vec) stream)
  (format stream "~A ~A ~A" [v.x] [v.y] [v.z]))
#<STANDARD-METHOD COMMON-LISP:PRINT-OBJECT (VEC T) {1001A97F83}>
CL-USER> (print (make-vec 2 3 4))

2 3 4
2 3 4

Of course, this syntax can be used for anything:

CL-USER> [2 + 3]
5 (3 bits, #x5, #o5, #b101)
CL-USER> ['(1 2 3 4) length]
4 (3 bits, #x4, #o4, #b100)
CL-USER> [add-nums defun (a b c) (+ a b c)]
ADD-NUMS
CL-USER> (add-nums 1 2 3)
6 (3 bits, #x6, #o6, #b110)

Packages may be specified as usual for both slots and objects:

CL-USER> (defpackage :bar (:use :cl)
                     (:export :some-slot))
#<PACKAGE "BAR">
CL-USER> (in-package :bar)
#<PACKAGE "BAR">
BAR> (defclass some-class () (some-slot unexported-slot))
#<STANDARD-CLASS BAR:SOME-CLASS>
BAR> (defvar *an-instance* (make-instance 'some-class))
*AN-INSTANCE*
BAR> (in-package :cl-user)
#<PACKAGE "COMMON-LISP-USER">
CL-USER> [bar::*an-instance*.bar:some-slot setf 34]
34 (6 bits, #x22, #o42, #b100010)
CL-USER> (slot-value bar::*an-instance* 'bar:some-slot)
34 (6 bits, #x22, #o42, #b100010)
CL-USER> (read-from-string "[bar::*an-instance*.bar:some-slot]")
(SLOT-VALUE BAR::*AN-INSTANCE* 'BAR:SOME-SLOT)
CL-USER> [bar::*an-instance*.bar::unexported-slot setf 12]
12 (4 bits, #xC, #o14, #b1100)
CL-USER> (slot-value bar::*an-instance* 'bar::unexported-slot)
12 (4 bits, #xC, #o14, #b1100)

Limitations

Since there's no way to tell "." in the package or variable name from the "." separating object and slot, variables and packages with "." in their names are not supported. For packages, this can be mitigated by using :local-nicknames. For slot names the usual slot-value must be used.

Installation

Copy (or symlink) to ~/quicklisp/local-projects, then type (ql:register-local-projects) in the REPL. Or just copy the file objective-cl.lisp straight into your project.

Usage

To enable the extension for all project, use :around-compile attribute:

(asdf:defsystem :your-awesome-system
  :license "Your favourite license"
  :depends-on (:<...>
               :objective-cl)
  :around-compile (lambda (next)
                    ;; uiop:symbol-call is needed because objective-cl system
                    ;; is not yet loaded by the time the defsystem form is read,
                    ;; so the package also doesn't exists, which causes an error
                    (uiop:symbol-call '#:objective-cl '#:enable)
                    (unwind-protect (funcall next)
                      (uiop:symbol-call '#:objective-cl '#:disable)))
  :components
  ((:file "packages")
    <...>

Please use responsibly.