Error in Ex2.6 Solution - Markdown

[pritesh-shrivastava]

You gave the following solution :
(define one (lambda (f) (lambda (x) (f (f x)))))

But this is infact 2 in Church numerals.
You can check the correct solution from the cummunity-scheme-wiki : http://community.schemewiki.org/?sicp-ex-2.6

[sgignoux]

Hi @pritesh-shrivastava Thanks for pointing this out! I look into it soon.

[ZeliTheZealot]

I agree with @pritesh-shrivastava . I've tried twice and got the same answer (see below). Besides, this source agrees with us.

(define zero (lambda (f) (lambda (x) x))); a constant function, independent of f
(define (add-1 n) (lambda (f) (lambda (x) (f ((n f) x)))))

Define one directly:

(add-1 zero); proceed by substitution
(lambda (f) (lambda (x) (f ((zero f) x)))); zero will take f as argument
(lambda (f) (lambda (x) (f ((lambda (f) (lambda (x) x) f) x)))); (substituting)
(lambda (f) (lambda (x) (f ((lambda (x) x) x)))); zero returned (lambda (x) x)
(lambda (f) (lambda (x) (f x))); (lambda (x) x) is the identity function