a random state.

implementation-dependent.

The current random state, which is used, for example, by the function random when a random state is not explicitly supplied.

 (random-state-p *random-state*) → true 
 (setq snap-shot (make-random-state)) 
 ;; The series from any given point is random, 
 ;; but if you backtrack to that point, you get the same series. 
 (list (loop for i from 1 to 10 collect (random)) 
       (let ((*random-state* snap-shot)) 
         (loop for i from 1 to 10 collect (random))) 
       (loop for i from 1 to 10 collect (random)) 
       (let ((*random-state* snap-shot)) 
         (loop for i from 1 to 10 collect (random)))) 
→ ((19 16 44 19 96 15 76 96 13 61) 
    (19 16 44 19 96 15 76 96 13 61) 
    (16 67 0 43 70 79 58 5 63 50) 
    (16 67 0 43 70 79 58 5 63 50))

The implementation.

random.

make-random-state, random, random-state

Binding *random-state* to a different random state object correctly saves and restores the old random state object.