It's worth understanding how to go back and forth between accumulator-style
and a lazy construction. You can convert the above non-lazy accumulator
version into a similar version that is lazy but has no risk of stack
overflow in the realization of that lazy flattened list:
(defn my-flatten
[xs]
(if (empty? xs) ()
(let [x (first xs), ys (rest xs)]
(if (sequential? x)
(if (seq x)
(recur (cons (first x) (cons (rest x) ys)))
(recur ys))
(lazy-seq (cons x (my-flatten ys)))))))
Basically, you just get rid of the accumulator, and in the place where you
would have conj'd in the next atomic element, you just build the lazy
sequence.
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