Re: How does letf work?

[Michael Heerdegen <michael_heerdegen@web.de>]

Joost Kremers <joost.m.kremers@gmail.com> writes:

> (letf (((cdr test-x) '(a b c d)))
>   (cdr test-x))
>
> = > (KEY a b c d)
>
> Taking what you say at face value, one might expect that at the moment
> the value of (cdr test-x) is printed, it's already been changed back to
> the original value, so it should say (KEY 1 2 3 4), but it doesn't.

(cdr test-x) here references an object being a list equal to '(a b c d).
But after the letf has been left, that object is not anymore the cdr of
test-x, because its cdr has been restored.  But that doesn't make the
return value "switch" to a different object.

The analogy with boxes is very helpful here.

Michael.