Re: Warn about comparing quoted lists (etc) using `eq`

["Mattias Engdegård" <mattias.engdegard@gmail.com>]

15 dec. 2022 kl. 05.17 skrev Juanma Barranquero <lekktu@gmail.com>:

> Note that your patch has a false positive with the empty string, which is optimized to be always the same object

That's only an implementation aspect that we give no guarantees about on the Lisp language level, and in fact as you correctly observe:

> there are two, a unibyte one and a multibyte one

Precisely, and often there's no telling which one we compare against in (eq x "") -- that expression may return t or nil when x is an empty string.
Thus it's definitely a well-motivated warning, toy examples like

> (let ((x "")) (eq x ""))

notwithstanding. We simply do not guarantee the identity of literal strings and the empty string is no exception.

15 dec. 2022 kl. 07.34 skrev Dr. Arne Babenhauserheide <arne_bab@web.de>:

> I miss a suggestion, though: the warning as it is is not actionable. It
> would be great if it could say something along the lines of
> 
> "... (arg 2). Consider using `equal'."

While this sounds like a useful suggestion on the surface, it's actually slightly dangerous: blindly replacing `eq` with `equal` (and `memq` with `member`, and so on) may very well break working code (that silently relied on the condition not being true). It's also a poor remedy against pratfalls like

  (memq x '(one 'two 'three))

which do occur. What we really want is to make a human look at the code, and think.