CODEWITHSUNDEEP
clojure
zcaudate
zcaudate

Reputation: 14258

how do you implement a delayed object constructor in clojure?

this is motivated by the 'art of the propagator' paper by Radul and Sussman at:

http://web.mit.edu/~axch/www/art.pdf

when they are building a compound progator, they say:

a compound propagator is implemented with a procedure that will construct the propagator’s body on demand. We take care that it is constructed only if some neighbor actually has a value, and that it is constructed only once

the code on page 10 is:

(define (compound-propagator neighbors to-build)
  (let ((done? #f) (neighbors (listify neighbors)))
    (define (test)
      (if done?
          ’ok
        (if (every nothing? (map content neighbors))
            ’ok
          (begin (set! done? #t)
                 (to-build)))))
    (propagator neighbors test)))

How do we do this using clojure's persistent data structures?

a simplified version of this maybe:

(def m {:a (delayed (some-object-constructor))})

where (:a m) constructs the object on the first call and gives and then subsequent calls to (:a m) will access the object.

its sort of like memoize but on values rather than functions..

Upvotes: 2

Views: 102

Answers (1)

Beyamor
Beyamor

Reputation: 3378

As far as delayed execution, delay might be a place to start. It could be used to evaluate the constructor only on the first dereference. It might look something like this:

(defn some-object-constructor
  []
  (println "Making something!")
  :something)

(def m {:a (delay (some-object-constructor))})

(println "Doing some intermediate work.")

(println (deref (:a m)))

Upvotes: 1

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