Exercise 4.53 · 习题

Exercise 4.53: With permanent-set! as
described in Exercise 4.51 and if-fail as in Exercise 4.52,
what will be the result of evaluating

(let ((pairs '()))

(if-fail

(let ((p (prime-sum-pair

'(1 3 5 8)

'(20 35 110))))

(permanent-set! pairs

(cons p pairs))

(amb))

pairs))

练习 4.53：在练习 4.51 所描述的 permanent-set! 以及练习 4.52 所描述的 if-fail 的基础上，对下列表达式求值将得到什么结果？

(let ((pairs '()))
(if-fail
(let ((p (prime-sum-pair
'(1 3 5 8)
'(20 35 110))))
(permanent-set! pairs
(cons p pairs))
(amb))
pairs))