Question

Today I’ve seen some references to tying the knot and circular data structures. I’ve been out reading some answers, and the solutions seem to involve using a ref to point back to the head of the list. One particular SO question showed a Haskell example, but I don’t know Haskell well enough to know if the example was using a the Haskell equivalent of a ref.

Is there a way to make a Clojure data structure circular without using a ref or similar construct?

Thanks.

Answer 1

I straightforwardly translated the Haskell example into Clojure:

user> (def alternates
          (letfn [(x [] (lazy-seq (cons 0 (y))))
                  (y [] (lazy-seq (cons 1 (x))))]
            (x)))
#'user/alternates
user> (take 7 alternates)
(0 1 0 1 0 1 0)

It works as expected. However I prefer the cycle function to mutually recursive functions using letfn:

user> (take 7 (cycle [0 1]))
(0 1 0 1 0 1 0)