Why doesn't repeatedly generate reproducible random numbers when using a seed in Clojure?

Question

I have some functions that require a series of random numbers so I\'ve taken some simple primitives such as #(inc (g/uniform 0 n)) and I can\'t seem to generate

Answer 1

Because of laziness. You return from binding before the sequence is realized. Therefore, the lazy sequence never sees the bindings you set. One solution would be to force realization with a doall on the sequence inside the binding.

Answer 2

If, as your comment on the other answer indicates, you want to preserve the laziness, then you would need to apply the binding in the function passed to repeatedly, capturing a seed that you created outside of the lazy seq. For instance:

(defn rand-seq [seed]
  (let [r (java.util.Random. seed)]
    (repeatedly #(binding [g/*rnd* r]
                   (inc (g/uniform 0 10))))))

(take 10 (rand-seq 42))
#=> (8 7 4 3 7 10 4 3 5 8)

(take 10 (rand-seq 42))
#=> (8 7 4 3 7 10 4 3 5 8)