Re: How does letf work?

[Nicolas Richard]

Florian Beck <fb@miszellen.de> writes:
>
> (letf* ((x (copy-list  test-x))
>       ((cdr x) '(a b c d)))
>   x)
>
> => (KEY 1 2 3 4)
>
> I'm still confused.

test-x, which I recall is (KEY 1 2 3 4) here, is simply a cons cell. Its
car is KEY, its CDR is (1 2 3 4).

After the first line of the letf*, test-x and x are different cons
cells. In fact their CAR are `eq', and their CDR are not. OTOH nothing
of this is relevant, test-x doesn't matter here.

After the second line, you changed the CDR of x to the list '(a b c d).
But that is a temporary binding that'll be reverted as soon as we exit
the letf* form.

At the third line, before the closing paren, you say: return the value
of x, so you indeed get that cons cell, which is '(KEY a b c d).

But now, the closing paren happens, i.e. letf* does its job and reverts
the variable value of x to what is was before (empty, I presume, making
x an unbound symbol) and the place represented by (cdr x), i.e. the cdr
of the cons cell that just got returned, to its initial value: '(1 2 3
4). So when letf returns, the cons cell that we received changes. And
that's what gets printed.

-- 
Nico.