Does Clojure's 'into' hold on to head?

Question

I have a very large lazy sequence, and I'd like to convert it into a set. I know that the number of distinct elements in the sequence is small, so I'll easily be able to fit the set into memory. However, I may not be able to fit the entire lazy seq into memory. I want to just do (into #{} my-lazy-seq), but it occurred to me that depending on how into is implemented, this might pull the whole seq into memory at once.

Will into hold on to the head of the sequence as it operates?

Answer 1

I don't see any increased usage running this (takes a minute or so)

user=> (into #{} (take 1e8 (cycle [:foo :bar])))
#{:bar :foo}

More precise proof would be to check the source for into, and we see it's just a fancy call to reduce:

(defn into
  ([to from]
     ;...
       (reduce conj to from)))

If reduce holds on to the head, then into does. But I don't think reduce does that.