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Re: How does letf work?
From: |
Florian Beck |
Subject: |
Re: How does letf work? |
Date: |
Wed, 29 Jan 2014 21:19:39 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.2.0 |
Thank you, I think I got it now.
If I understand correctly, it works like this:
{1} means pointer to cons cell 1; <...> means a shadowed cons cell
Simplified example.
(letf* ; x undefined
((x '(A B)) ; x={1} 1=[A|{2}] 2=[B|()]
((cdr x) '(C))) ; <2=[C|()]>
x ; x={1} 1=[A|{2}] <2=[C|()]>
) ; returns {1}, i.e. pointer to cell {1}
; which still point to {2}, but {2}'s
; original value has been restored
Same with car:
(letf* ; x undefined
((x '(A B)) ; x={1} 1=[A|{2}] 2=[B|()]
((car x) 'D) ; <1=[D|{2}]>
((cdr x) '(C))) ; <2=[C|()]>
x ; x={1} <1=[A|{2}]> <2=[C|()]>
) ; returns {1}, but both cons cells have
; been restored
But with setf, setcar, etc., there is nothing to restore:
(letf* ; x undefined
((x '(A B))) ; x={1} 1=[A|{2}] 2=[B|()]
(setcar x 'C) ; 1=[C|{2}]
x) ; returns pointer to {1}, which wasn't
; shadowed, so nothing to restore.
(Obviously, we don't need letf in the last example.)
Right? Let returns the "value of the last form". Straightforward in
hindsight.
--
Florian Beck