On 2010 Mar 23, at 8:26 AM, Meikel Brandmeyer wrote:
here some notes:
Thanks!
I would use vector instead of list in the example since vector is more
idiomatic in Clojure.
Ok. Just my old lisp roots showing. Still haven't gotten the fingers/
spine
rewired to use [] around defn parameters. :)
(if (seq seq1)
(lazy-seq
Use when-let (more idiomatic) and move lazy-seq outside to achieve
full laziness. Otherwise realisation of the seq is always one step
ahead.
Thanks. I wasn't sure how high to lift the lazy-seq. Still annoying that
(if seq1 ...) and (if (seq seq1) ...) aren't the same. Oh well,
that'll take
some repetition to get into the reflexes as well.
(let [[s1 & s1tail] seq1
Don't use this destructuring in this case, because it forces again the
realisation of the seq one step ahead.
If I need s1 anyways, what is the "one step ahead part" that you refer
to?
That is just my translation of the Haskell idiom. I'm sad to see that
there
isn't a parallel clojure idiom with the lazy semantics.
So here is my try:
(defn semi-map
"Lazy sequence of seq1, optionally combined with seq2.
When (pred (first seq1)) item is true, yield
(fun (first seq1) (first seq2)) otherwise yield (first seq1)
without advancing seq2.
Example:
(semi-map vowel? vector \"Hellow Word\" (iterate inc 1))
-> (\\H [\\e 1] \\l \\l [\\o 2] \\w \\space \\W [\\o 3] \\r \\d)"
[pred f seq1 seq2]
(lazy-seq
(when-let [seq1 (seq seq1)]
(let [fst1 (first seq1)]
(if (pred fst1)
(cons (f fst1 (first seq2))
(semi-map pred f (rest seq1) (rest seq2)))
(cons fst1 (semi-map pred f (rest seq1) seq2)))))))
Thanks!
-Doug
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