方案中的清单程序

Question

I am studying for an exam right now but I am not sure if I understood the list procedure in Scheme. I know that it can be used for creating list variables like (define x (list 'a 'b 'c)) . However I saw another usage of it in procedure creation:

1 ]=> (define foo3
           (lambda (b lst)
                (if b
                    (car lst)
                    (cadr lst)
                )
           )
      )
;Value: foo3

1 ]=> (foo3 #f ’(a b))
;Value: b

1 ]=> ((foo3 #t (list cdr car)) ’(a b c))
;Value: (b c)

What does the (list cdr car) mean? (I know what cdr and car means in terms of referencing first and rest of the list)

Answer 1

In the code, (list cdr car) is just a list of procedures. foo3 will select one procedure from that list, according to the passed parameter b . In the second example, this snippet:

(foo3 #t (list cdr car))

... Will return cdr because the first parameter was #t , so in the end we're just evaluating this:

(cdr '(a b c))
=> '(b c)