On Jan 18, 2011, at 6:16 AM, MiltondSilva wrote:
> Testing for laziness seems simple:
> (defn lazy? [coll]
> (= (type coll) clojure.lang.LazySeq))
It's fairly easy to get other types that are (effectively) lazy. For example,
(cons 1 (map identity [1 2 3]))
is a clojure.Lang.Cons but I bet the original author would consider that just
as good as a LazySeq.
> "For instance if you know a particular sequence could be particularly
> large or expensive in certain situations, you may want your tests to
> assert that it is not getting evaluated prematurely.."
>
> That's what lazy seqs exists for. You get something that doesn't
> evaluated prematurely... Or am I missing your point?
I think the original author wants to have some computation that returns a
sequential but be extra-special confident that elements have not yet been been
evaluated. My hunch is that type-checking won't do it. For example, I expect
he'd dislike it if his computation (or the functions it used) called #'doall
somewhere. However, #'doall doesn't change the type:
user> (type (doall (map identity [1 2 3])))
clojure.lang.LazySeq
If this were my problem, I'd wonder if I could make the computation accept
functions. Then you could do something like this:
(fact "After the first, sprouts are not created until needed"
(let [explosive-seed (fn [& rest] (throw (Error. "Boom!")))]
(first (sprout-maker explosive-seed)) => identity-sprout?
(second (sprout-maker explosive-seed)) => (throws Error #"Boom")))
-----
Brian Marick, Artisanal Labrador
Contract programming in Ruby and Clojure
Author of /Ring/ (forthcoming; sample: http://exampler.com/tmp/ring.pdf)
www.exampler.com, www.exampler.com/blog, www.twitter.com/marick
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