(defn fill [coll subs]
(lazy-seq
(when-let [[x & nx :as xs] (seq coll)]
(if-let [[y & ny :as ys] (seq subs)]
(if (nil? x)
(cons y (fill nx ny))
(cons x (fill nx ys)))
xs))))
On Aug 17, 6:30 am, Rising_Phorce <[email protected]> wrote:
> Hi All:
>
> First of all thanks for the replies to my last post the clojure
> community is great!
>
> I've been trying to find a succinct way to do the following. Given
> two sequences, product a new sequence which takes items from the first
> sequence unless null in which case it will take an item from the
> second, until the first sequence is exhausted. If nil is encountered
> in the first sequence and the second sequence is exhaused, nil will be
> returned:
>
> e.g. user> (nil-coalesce (nil 1 2 3 nil nil) (4 5))
> (4 1 2 3 5 nil)
>
> I ended up with a scheme like natural recursion. I see two
> drawbacks. The result (presumably empty) must be passed in and the
> recursion will stack overflow for large sequences. I guess trampoline
> would fix this, but that seems a bit of a dirty trick and workaround
> until Java 7 gives us proper tail calls. Anyway I looked at partition
> and interleave but without a predicate, I don't see how to make either
> of these work. Anyone have a more elegant solution?
>
> (defn nil-coalesce [result maybe-nil replacements]
> (if (empty? maybe-nil)
> (if (empty? replacements)
> result
> nil)
> (let [x (first maybe-nil) xs (rest maybe-nil)]
> (if (nil? x)
> (recur (conj result (first replacements)) xs (rest
> replacements))
> (recur (conj result x) xs replacements)))))
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