Golf! Yeah!
Here the low-level version:
(defn sift
[pred coll]
(letfn [(step
[sep [f :as s] xs]
(if (or (not s) (pred f))
(cons [sep xs] (sift pred s))
(recur sep (next s) (conj xs f))))]
(lazy-seq
(when-let [s (seq coll)]
(step (first s) (next s) [])))))
As lazy as you can get, not holding unto the head (I hope), traversing
each element of the sequence only once.
Sincerely
Meikel
PS: Since the problem is ill-specified I assume, that the first item
is always a separator (for the implementors convenience). Which makes
sense, since nil does not necessarily fulfill pred and I don't another
"default" value.
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