val it: α → α = fun

(define (tak x y z)
(if (<= (force x) (force y))
(force y)
(tak (delay (tak (- (force x) 1) y z))
(delay (tak (- (force y) 1) z x))
(delay (tak (- (force z) 1) x y)))))

(let-optionals*
(map string->number *argv*) ((x 10) (y 5) (z 0))
(format #t "tak(~A, ~A, ~A) = ~A\n" x y z (tak x y z))) > >% time ./tak.pl 100 50 0
tak(100, 50, 0) = 100
./tak.pl 100 50 0 0.38s user 0.01s system 96% cpu 0.406 total
% time ./tak.rb 100 50 0
tak(100, 50, 0) = 100
./tak.rb 100 50 0 0.44s user 0.02s system 95% cpu 0.485 total
% time ./tak.scm 100 50 0
tak(100, 50, 0) = 100
./tak.scm 100 50 0 0.06s user 0.01s system 86% cpu 0.080 total > >かように Scheme は滅茶苦茶速い。ほかにもたとえば OCaml にも遅延評価機構はある。こんな感じ。 > >open Lazy
open Array
open Sys
let rec tak x y z =
if force x <= force y
then force y
else tak (lazy (tak (lazy (force x - 1)) y z))
(lazy (tak (lazy (force y - 1)) z x))
(lazy (tak (lazy (force z - 1)) x y))