This is an implementation of the "worlds" model for scoping side effects, as described in http://www.vpri.org/pdf/tr2011001_final_worlds.pdf. It only depends on closer-mop to allow us to mess with the meta-object protocol, and trivial-garbage to create weak hash-tables (for tracking object changes without memory leaks), and bordeaux-threads cause we can since we imagine this could be very useful in multi-threaded programs.

Worlds act similar to "transactions" in databases, where no changes to the outside world are made until the world is committed. Using the metaclass WORLDS:WORLD-CLASS, classes can have changes to their instances' slots managed by the world system. Worlds can be created using the WORLDS:SPROUT function (which "sprouts" a world from the current world), can be entered using the WORLDS:WITH-WORLD macro and have their changes committed to their parents using the WORLDS:COMMIT function.

(defclass f ()
  ((bar :initarg :bar)
   (baz :initarg :baz))
  (:metaclass worlds:world-class))

(let ((world (worlds:sprout))
      (f (make-instance 'foo :bar "BAR")))
  (worlds:with-world (world)
     ;; changes in this body will only be seen within WORLD
     (setf (slot-value f 'bar) "QUUX")
     (print (slot-value f 'bar)))
  ;; the change in the slot BAR will not be seen here...
  (print (slot-value f 'bar))
  ;; ...until it is committed
  (worlds:commit world)
  ;; then the change will be seen
  (print (slot-value f 'bar)))
#|
prints
  "QUUX" 
  "BAR" 
  "QUUX" 
|#

This is the "BSD 0-clause license", which cl-worlds is licensed under.

Copyright (C) 2020 by Hayley Patton

Permission to use, copy, modify, and/or distribute this software for any purpose with or without fee is hereby granted.

THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.