bug#40671: [DOC] modify literal objects

[Paul Eggert]

On 4/19/20 10:54 PM, Drew Adams wrote:

> A mutable object cannot be changed to a constant

Sure they can. This idea is common in other languages, e.g., see Object.freeze 
method in JavaScript. There's no reason Emacs Lisp can't use the idea.


> > +A mutable object can become constant if it is passed to the
> > +@code{eval} function,
>
> How so?  What's an example?

(let ((x (make-string 1 ?a)))
  (eval `(progn
           (defun foo ()
             (let ((a ,x))
               (aset x 0 ?b)
               (list a "a" (equal a "a"))))
           (byte-compile 'foo)
           (foo))))

This code is not well-formed because it modifies the string x after passing it 
to eval (such strings should be constant). As a result, the behavior of the 
program is unpredictable. On master it currently yields ("b" "b" t) but there's 
no guarantee of this.