How to generate repeatable random sequences with rand-int

Question

I want to be able to generate repeatable numbers using rand in Clojure. (Specifically, I want results of calls to rand-nth or Incanter's sample to be repeatable, and these call rand-int which in turn calls rand).

I figured out from this question that if I use clojure.data.generators, I can reset the random state:

(require '[clojure.data.generators :as gen])
(alter-var-root #'gen/*rnd* (constantly (java.util.Random. 437)))
(gen/reservoir-sample 5 (range 1000)) ; => [940 591 636 12 755]
(gen/reservoir-sample 5 (range 1000)) ; => [376 540 827 307 463]
; reset random state:
(alter-var-root #'gen/*rnd* (constantly (java.util.Random. 437)))
; now the same results are generated again:
(gen/reservoir-sample 5 (range 1000)) ; => [940 591 636 12 755]
(gen/reservoir-sample 5 (range 1000)) ; => [376 540 827 307 463]

However, that method only seems to affect functions in clojure.data.generators, which isn't surprising:

(alter-var-root #'gen/*rnd* (constantly (java.util.Random. 437)))
(rand) ; => 0.9372552374760151
(rand) ; => 0.2712729314667742
; reset random state:
(alter-var-root #'gen/*rnd* (constantly (java.util.Random. 437)))
; not same results as before:
(rand) ; => 0.630238593767316
(rand) ; => 0.426744420572015

How can I restore the random state in such as way as to get repeatable results from rand? So far I haven't found any documentation about this.

(Another question sounds as if it might be the same issue, but it's asking about something completely different.)

Answer 1

A clean way:

(ns designed.ly.rand)

(def ^:dynamic *rand* clojure.core/rand)

(defn rand-1
 ([]
   (*rand* 1))
 ([n]
   (*rand* n)))

(defmacro with-rand-seed
 "Sets seed for calls to random in body. Beware of lazy seqs!"
 [seed & body]
  `(let [g# (java.util.Random. ~seed)]
     (binding [*rand* #(* % (.nextFloat g#))]
      (with-redefs [rand rand-1]
        ~@body))))

It redefines rand within the scope. Example:

(with-rand-seed 9
  (rand 4)       ; => 2.9206461906433105
  (rand-int 10)) ; => 2

BTW. Beware of lazy seqs: http://kotka.de/blog/2009/11/Taming_the_Bound_Seq.html (Apparently, this link redirects to https without a trusted certificate so here's a link to a version at Web Archive: https://web.archive.org/web/20120505012701/http://kotka.de/blog/2009/11/Taming_the_Bound_Seq.html).

Answer 2

just wanted to add, for anyone still looking at this, there is already a library on clojars since 2015 that does this:

https://github.com/trystan/random-seed