I ran into a problem with "take" today (thanks to wwmorgan on IRC for
helping steer me away from blaming "filter").
My actual code had to do with computing a subset of primes, but let's
take a simpler example.
(defn painful-seq []
(lazy-cat
[0 1 2]
(comment lots of hard work here)
(list 99)))
user=> (lazy-cat [0 1 2] (painful-list))
(0 1 2 99)
This is meant to represent 3 easy-to-compute values, followed by the
hard-to-compute number 99. So now if we take only those first three,
we avoid having to compute 99, right?
user=> (take 3 (painful-seq))
(0 1 2)
Well, it looks good. But let's "instrument" the code, as the cool
kids put it:
(defn painful-seq []
(lazy-cat
[0 1 2]
(println "hurt me plenty")
(list 99)))
user=> (take 3 (painful-seq))
(0 1 hurt me plenty
2)
Whoops. The return value is correct as we saw earlier, but before 2
gets returned, we apparently go on and compute that next expensive
value. This is despite the fact that nobody even wants it.
So I wrote up an new "take" function that seems to fix this:
user=> (my-take 3 (painful-seq))
(0 1 2)
Of course if you actually want the fourth value, there's nothing I can
do for you.
(0 1 hurt me plenty
2 99)
Here's "my-take" renamed to "take" for easy patching of boot.clj:
http://paste.lisp.org/display/69146
--Chouser
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