computed-class

This library provides various tools that help dealing with constraint based change propagation in Lisp programs. One such tool is computed-class which is a metaclass for CLOS classes that support per instance lazy computation of slot values while keeping track of dependencies and managing invalidation of such computed results transparently. The engine behind this is fully integrated with CLOS but it is also independent from it. It makes it possible to use computed local variables that can even be captured by lambda's (see clet) and also to explicitly handle the computed-state structs (the cells) as first-class citizens.

(define-computed-universe compute-as :name "Default computed-class-test universe")

(deftest compute3 ()
  (let* ((object-1 (make-instance 'computed-test
                                  :slot-b (compute-as (1+ (slot-a-of -self-)))))
         (object-2 (make-instance 'computed-test
                                  :slot-a (compute-as (+ (slot-a-of object-1) (slot-b-of object-1)))
                                  :slot-b (compute-as (1+ (slot-a-of -self-))))))
    (signals unbound-slot (slot-a-of object-1))
    (setf (slot-a-of object-1) 0)
    (is (= 0 (slot-a-of object-1)))
    (is (= 2 (slot-b-of object-2)))
    (setf (slot-a-of object-1) 2)
    (setf (slot-b-of object-2) (compute-as (* 2 (slot-a-of -self-))))
    (is (= 5 (slot-a-of object-2)))
    (is (= 10 (slot-b-of object-2)))))

;; please note that the fake clet variables can even be captured by lambda's (actually that's happening in (compute-as (1+ a)))
(deftest clet4 ()
  (clet ((a (compute-as 2))
         (object (make-instance 'computed-test
                                :slot-a (compute-as (1+ a))
                                :slot-b (compute-as (1+ (slot-a-of -self-))))))
    (is (= a 2))
    (clet ((b (compute-as (+ a (slot-b-of object)))))
      (is (= b 6))
      (is (eq (cs-variable b-state) 'b))
      (is (= (slot-a-of object) 3))
      (is (= (slot-b-of object) 4))
      (setf (slot-a-of object) 42)
      (is (= a 2))
      (is (= b 45))
      (is (= (slot-a-of object) 42))
      (is (= (slot-b-of object) 43)))))

See test.lisp for more examples and/or read the Cells docs.

• Computed CLOS slots
• Capturable computed lexical variables with clet
• Standalone cells with public reader and writer api (called computed-state-value) for accessing the computed state when using in advanced scenarios.
• Memoizer-like defcfun that keeps a memoize cache for a side-effect-free defun. Functions defined by defcfun automatically drop and recalculate memoize entries when any cell is invalidated that was read while the memoize entry in question was calculated.

• computed-class-devel for all discussions

Attila Lendvai

Tamás Borbély

Levente Mészáros

You can browse the computed-class darcs repository or get the tree with

darcs get http://common-lisp.net/project/computed-class/darcs/computed-class

BSD

• We had trouble with Cells that we couldn't easily fix
• We needed full CLOS integration