Re: Question about let binding behavior

[<tomas@tuxteam.de>]

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On Tue, Oct 08, 2024 at 06:21:03AM +0000, Louis-Guillaume Gagnon wrote:
> Dear all,
> 
> I'm a long time user but I've only recently started to dive into elisp 
> programming. I'm a bit surprised by the following behavior.
> 
> Let's say I write the following silly function:
> 
> (defun foo (bar)
>    (let ((baz '((quux . 0) (quuz . 0))))
>      (if (> 10 bar)
>      (setcdr (assoc 'quux baz) bar)
>        (setcdr (assoc 'quuz baz) bar))
>      baz))
> 
> Then the following evals:
> 
> (foo 1)
> => ((quux . 1) (quuz . 0))
> 
> (foo 100)
> => ((quux . 1) (quuz . 100))
> 
> I'm surprised that the list is not reset since I thought the binding was 
> local to the function, but clearly I don't have the right mental model 
> for elisp evaluation.

The binding (i.e. the "thing" linking the symbol baz to the value
'(...) is local. But what you do with that (setcdr... ) is to change
the value itself. Your function returns this (possibly changed) value.

Lisps have a different "accent" than more pure functional languages,
where values are more immutable.

> For the actual application I ended up using 
> copy-tree but I'd like to understand this better -- could someone 
> explain or point out the relevant sections of the manual?

Did that help?

Cheers & enjoy Lisp :-)
-- 
tomás

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