On 2010 Mar 23, at 9:14 AM, Per Vognsen wrote:
Remember the one-liner I gave you last time?
Yup. It was the 'hard coded + 0' parts that had been ruminating in the
back of my mind as being something that could be abstracted out. :)
Since the one you just posted didn't have all the features of my
version, I've taken the liberty of
adjusting it:
; Variation on Per Vognsen's version.
(defn semi-map
[pred fun seq1 seq2]
"Lazy sequence of seq1 optionally combined with seq2.
when pred on seq1 item is true, yield fun of seq1 and seq2
otherwise yield seq1 item without advancing seq2.
Example: (semi-map vowel? vector \"Hello Word\" (iterate inc 1))
-> (\\H [\\e 1] \\l \\l [\\o 2] \\space \\W [\\o 3] \\r \\d)
"
(map #(if (pred %1) (fun %1 (first %2)) %1)
seq1
(reductions #(if (pred %2) (rest %1) %1)
seq2 seq1)))
;;;
What I don't understand (yet) is how to do a lazyness analysis on this
similar to what
Meikel Brandmeyer did with my original code.
So, for my own understanding:
the outermost map is lazy, hence nothing will happen it the first
result needs to be realized.
OK, so far so good.
when the first result of the map needs to be realized, I'm
getting a bit foggy...
the 2nd parameter to map 'seq1' will have to have its first
element (and tail?) realized
in order to call the mapping function #(if...)
the 3rd parameter to map '(reductions...)' will have to have its
first element realized
in order to call the mapping function #(if...) of the
outermost map.
then:
In order to realize the first value of the result of reductions,
both the first value of seq1
has to be realized to pass in as the second parameter to the
reductions' function #(if ...)
Ok, so now my head hurts. But in a good way.
I think I have convinced myself that the only "eager" realization/
evaluation might be of the tail of seq1, but I'm not sure.
Thanks Per!
-Doug
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