Church计数 my code：
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;Church计数
(define zero (lambda (f) (lambda (x) x)))
(define (add-1 n) (lambda (f) (lambda (x) (f ((n f) x)))))
(define (show-num n)((n (lambda (x) (+ 1 x))) 0))
(define (add a b) (lambda (f) (lambda (x) ((a f) ((b f) x)))))
(define one (lambda (f) (lambda (x) (f x))))
(define two (lambda (f) (lambda (x) (f (f x)))))
(show-num zero)
(show-num one)
(show-num two)
(newline)
(show-num (add-1 zero))
(show-num (add-1 (add-1 zero)))
(show-num (add-1 (add-1 (add-1 zero))))
(show-num (add-1 (add-1 (add-1 (add-1 zero)))))
(newline)
(display "my add\n")
(show-num (add zero zero))
(show-num (add (add-1 zero) (add-1 zero)))
(show-num (add (add-1 zero) (add-1 (add-1 zero))))
(define (multi a b)
(lambda (f)
(lambda (x)
((a (b f) ) x))))
(define (expon a b)
(lambda (f)
(lambda (x)
(((a b) f) x))))
(show-num (multi two two))
(show-num (multi (add one two) two))
(show-num (expon two two))
(show-num (expon (expon two two) two))
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Church计数，通过(lambda (f))把f作为参数，这样f就不会被求值，而f的执行次数就代表了数字。
lambda中的形参其值是未知的，所以不会被求值。
形参好比糖果的包装纸，要把一段代码包起来，把它放到一个匿名函数里就行了
比如有一个函数
(lambda (a b) (+ a b))
要把它包装起来，可以写成
(lambda (x) (lambda (a b) (+ a b)) )
要剥开糖纸，传个参数就行了。
Church计数就是这个思想。
show-num用来把Church计数方式的数字转换成普通数字。