# [plt-scheme] should `map' reuse cons cells?

Jens Axel Søgaard writes:
> The function map have can't determine whether the given function
> behaves like the identity on all elements of the list without
> first building a list with the mapped elements and then compare
> this list to the original.
> [...]
> In other words, I interpret the "where possible" not to mean
> "theoretically possible" but to mean "possible without speed
> or memory sacrifices".
I'm not sure what the big sacrifice is:
(define (map f l)
(if (null? l)
null
(let ((h (car l)) (t (cdr l)))
(let ((fh (f h)) (ft (map f t)))
(if (and (eq? h fh) (eq? t ft))
l
(cons fh ft))))))
>* (define l '(0 1 2 3))
*>* (define fl (map (lambda (x) (if (zero? x) 10 x)) l))
*>* (eq? l fl)
*#f
>* (eq? (cdr l) (cdr fl))
*#t
OK, it's not tail-recursive, but I'll leave that as an exercise for
the reader...
--dougo at place.org