sicp 1.33 带filter的accumulate.

filtered-accumulate.scm

(define (filtered-accumulate combiner null-value term a next b filter?)
    (if (> a b)
        null-value
         (combiner (if (filter? a) (term a) null-value)
                (filtered-accumulate combiner null-value term (next a) next b filter?)
            )
    )
)

(define (filtered-sum term a next b filter?)
    (filtered-accumulate + 0 term a next b filter?)
)

(define (prime-sum a b)
    (define (term x) x)
    (define (next a) (+ a 1))
    ; prime? 前面练习已实现.(1 不是素数.)
    (filtered-sum term a next b prime?)
)

; 6
(prime-sum 1 3)

; 在数论中，如果两个或两个以上的整数的最大公约数是 1，则称它们为互质[2].
(define (filtered-sum term a next b filter?)
    (filtered-accumulate + 0 term a next b filter?)
)

(define (prime-product n)
    (define (term x) x)
    (define (next a) (+ a 1))
    (define (filter? x) (= (gcd n x) 1))

    (filtered-accumulate * 1 term 2 next (- n 1) filter?)
)

; 24
(prime-product 5)

(combiner (if (filter? a) (term a) null-value)
                (filtered-accumulate combiner null-value term (next a) next b filter?)
            )

比较特殊的可能是这句. 基础语法糖熟悉以后, 越来越倾向于 单行书写了. 的确精简很多.