方案中奇怪的递归定义

Question

Consider the following Scheme definition:

(define f (lambda () (procedure? f)))

Strangely, when I evaluate (f) I get #t . The question is: why does this evaluation terminate? I was expecting it to loop indefinitely. What does the inner lambda evaluate to, given that f hasn't yet been defined?

Answer 1

There is no recursion involved here.

When you execute this code, a procedure f is defined (not executed):

> (define f (lambda () (procedure? f)))
> f
#<procedure:f>

When you then execute it, it checks if there is a procedure associated with the symbol f, which is true at that point in time, so it returns #t :

> (f)
#t

To be recursive, the procedure would have to call itself using (f) .