bug#61281: “`(a \, b)” equals to “`(a . , b)”

[Drew Adams]

> > Good catch!
> >
> > `(a \, b) returns (a . <value-of-b>)
> >
> > Looks like a bug to me.
> 
> Isn't `(a \, b) just another read syntax for
> `(a . (\, b)), which is another syntax for
> `(a . ,b) ?

Well, yes.  And that's no doubt why we get that.

But (\, b) shouldn't be handled as ,b.  I know
that it is - in Elisp.  I don't think it should
be.  To me, that spells (faulty) implementation
leaking out.

> With other words, I think `(a \, b) and `(a . ,b) are different read
> syntaxes for the same expression, equivalent to (cons 'a b).

Yes, but see above.  I think \, should be read
as the symbol whose print name is ",".  To me,
`(a \, b) should be treated like (a foo b): a
list of 3 symbols - no evaluation.  And `(a \,b)
should be treated as a list of two symbols,
whose print names are "a" and ",b".

The symbol \, should be read as just a symbol.
The same is not true of just an unescaped comma
- outside a backquoted sexp that raises an error,
and inside one it's handled specially as part of
the backquote syntax.

\, is not just ,

I don't have another Lisp interpreter, but I'm
guessing that Common Lisp does what I expect.
(For Common Lisp also, \ escapes a character.)