In Clojure, how can I write my own recursive function to work with lazy lists?

Question

I was just trying to demonstrate laziness to someone, and I wrote a simple recursive function to process a list.

I assumed that it would be fine on an infinite list. But suddenly I got a "too much recursion" internal error.

Huh? I always write code that does that kind of thing. What's the problem?

But of course, normally I use built-in functions like map as the basis of other list-processing functions. This time I was trying to write my own recursive traversal. And, of course, that can't possibly work.

Here's what I wrote.

(defn q [xs] 
  (if (empty? xs) () 
    (cons (* (first xs) (first xs)) (q (rest xs)) )))

(take 10 (q (cycle '(1 2 3 4))))

So, how, in fact DO I write my own traversal functions that can handle lazy data structures in Clojure? Is there some kind of equivalent of a "yield"?

See clojuredocs.org/clojure.core/lazy-seq

Charles Duffy
– Charles Duffy

2017-05-05 23:11:03 +00:00
Commented May 5, 2017 at 23:11 — Charles Duffy
– Charles Duffy, Commented May 5, 2017 at 23:11

Charles Duffy · Accepted Answer · 2017-05-05 23:14:00Z

6

(defn q [xs] 
  (if (empty? xs) () 
    (lazy-seq  ;; <- here's our magic!
       (cons (* (first xs) (first xs)) (q (rest xs)) ))))

(take 10 (q (cycle '(1 2 3 4))))

answered May 5, 2017 at 23:14

Charles Duffy

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1 Comment

amalloy Over a year ago

Good, but should be (defn q [xs] (lazy-seq (if ...))), so that an item from the input sequence is only realized when an item from the output sequence is demanded. As written, you always consume one more item than needed from the input seq. As a rule of thumb, when you produce a lazy sequence you should make sure that lazy-seq is wrapped around anything that forces an element of the input (like seq, first, rest, or empty?).

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