bug#40671: [DOC] modify literal objects

[Paul Eggert]

On 4/22/20 5:49 PM, Michael Heerdegen wrote:

> +  A mutable object can become constant if it is passed to the
> +@code{eval} function, because you should not modify an object that is
> +being or might be executed.  The reverse does not occur: constant
> +objects should stay constant.
> 
> `eval' is used quite rarely.  Can what you describe happen under other
> circumstances, or does it only happen to `eval'?  E.g. what about this
> case for example:
> 
>   (let ((l (list 1 2 3)))
>     (funcall (lambda () l)))
> 
> Has the list become a constant?

No, because the list is not part of the expression that is being evaluated.
However, something like this could cause trouble:

(let ((l (list 'lambda '(x) '(setcdr l x))))
  (eval (list l l)))

because it modifies the list l while it is evaluating it. (As it happens, this
code behaves differently in Emacs 26 than it does in Emacs 27 - that's what you
can get with undefined behavior....)

> Maybe I misread "might be executed" as
> "might be executed in the future" and you actually meant something like
> "might (currently) be executed (as part of the expression the
> interpreter currently executes).
> 
> BTW, speaking about Lisp the term "evaluate" is probably preferable to
> "execute" I think.

Both good points. The word "executed" is already gone from the manual, and I
installed the attached patch to try to address the other point.

0001-Tweak-mutability-doc-a-bit-more.patch
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