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#lang racket (define (accumulate op initial seq) (if (null? seq) initial (op (car seq) (accumulate op initial (cdr seq)))) );accumulate (define (append lst1 lst2) (if (null? lst1) lst2 (cons (car lst1) (append (cdr lst1) lst2))) );append (define (enumerate-interval i j) (if (> i j) ‘() (cons i (enumerate-interval (+ 1 i) j)) ));enumerate-interval (define (unique-pairs n) (accumulate append ‘() (map (lambda(i) (map (lambda (j) (list i j)) (enumerate-interval (+ i 1) n))) (enumerate-interval 1 n));map );accumulate );unique-pairs (unique-pairs 3) (define (flatmap proc seq) (accumulate append ‘() (map proc seq)) );flatmap (define (filter predicate seq) (cond ((null? seq) ‘()) ((predicate (car seq)) (cons (car seq) (filter predicate (cdr seq)))) (else (filter predicate (cdr seq))));cond );filter (define (unique-triple n) (flatmap (lambda (x) (map (lambda (j) (cons x j)) (unique-pairs (- x 1)));map );lambda (enumerate-interval 1 n));flatmap );unique-triple (unique-triple 5)
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原文地址:http://www.cnblogs.com/wizzhangquan/p/4457440.html