Re: Change of Lisp syntax for "fancy" quotes in Emacs 27?

[Paul Eggert]

I'm afraid this patch is heading in the wrong direction, as we should be 
more vigilant about confusables, not less.

Consider this example, abstracted from the auth-source-secrets-create 
source code:

    (if (eq r 'secret)
        (let ((data data))
          (lambda () data))
      data)

The intent of the (let ((data data)) ...) code is to create a thunk 
which, when evaluated, yields the current value of 'data' (not the value 
of 'data' when the thunk is called), and that is what any human reading 
the code will see. However, that is not what the code actually does. In 
the (let ((data data)) ...), the space between the two instances of 
'data' is really an EN SPACE (U+2002) so the 'let' is declaring an 
identifier 'data data' whose name contains an EN SPACE and whose value 
is nil, an identifier that is never used; so the thunk yields the later 
value of 'data', not the earlier one.

Because humans cannot reliably review source code containing characters 
that are easily confusable with the ASCII symbols that are a basic part 
of Elisp syntax, we should not be relaxing the reader to encourage 
developers to use these characters in their identifiers. On the 
contrary, we should be discouraging their use even more than we do now.