Hi Chas,
Have you considered wrapping the computation in a lazy sequence?
(lazy-seq [(expensive-calculation)])
The, instead of (force ...), you just use (first ...). I think
equality and hashing will automatically work like you want them to.
Cheers,
Jason
On Apr 9, 11:55 am, Chas Emerick <cemer...@snowtide.com> wrote:
> I recently came across a situation where I very much wanted to delay
> the calculation of some values, but where I also wanted those delays
> to use their calculated values for equality determinations. This is
> particularly important when comparing vectors or maps that contain
> delayed values, where doing something like (= (force some-delay) some-
> value) isn't possible.
>
> So, I whipped up a tdelay (transparent delay) macro, an almost
> complete ripoff of clojure.core/delay, except that it creates
> instances of a subclass of clojure.lang.Delay that defers equality and
> hashcode calculations to the delay's value. The results:
>
> user=> (= {:a 5 :b (delay 12)} {:a 5 :b 12})
> false
> user=> (= {:a 5 :b (tdelay 12)} {:a 5 :b 12})
> true
>
> I get nervous when I screw around with equality in relatively sneaky
> ways like this, so I thought I'd toss this out on the list and see if
> anyone has any comments one way or the other.
>
> - Chas
>
> ------
>
> (import '(clojure.lang Delay Util))
>
> (defn make-tdelay
> [fn]
> (proxy [Delay] [fn]
> (equals [other]
> (Util/equals (.deref this) (Delay/force other)))
> (hashCode []
> (Util/hash (.deref this)))))
>
> (defmacro tdelay
> [& body]
> (list 'make-tdelay (list* `#^{:once true} fn* [] body)))
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