Backquote

Backquote

[Herbert Euler]

Hello,

To understand how ` works, I read lisp/emacs-lisp/backquote.el.
And I found:

    ;; When the Lisp reader sees `(...), it generates (\` (...)).
    ;; When it sees ,... inside such a backquote form, it generates (\, 
...).
    ;; For ,@... it generates (\,@ ...).

Do all Lisp readers have this feature?  And is behavior of backquote
in GNU Emacs simillar to other Lisp implementations, especially for
nested backquotes?

Thanks in advance.

Regards,
Guanpeng Xu

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Re: Backquote

[Stefan Monnier]

> To understand how ` works, I read lisp/emacs-lisp/backquote.el.
> And I found:

>    ;; When the Lisp reader sees `(...), it generates (\` (...)).
>    ;; When it sees ,... inside such a backquote form, it generates (\, ...).
>    ;; For ,@... it generates (\,@ ...).

> Do all Lisp readers have this feature?

More or less.  It's just a generalization of the original mapping from '<e>
to (quote <e>).  What the , ` and ,@ expand to depends on the specific
flavor of Lisp.  I don't know what it is in CommonLisp, but in Scheme `<e> is
(quasiquote <e>), ,<e> is (unquote <e>), and ,@<e> is (unquote-splicing <e>).

There may also be differences in the details: in Emacs Lisp, ,<e> only maps
to (\, <e>) if it occurs within a `<e>, whereas I think this mapping is done
unconditionally in most other Lisps.

> And is behavior of backquote in GNU Emacs similar to other Lisp
> implementations, especially for nested backquotes?

Yes.  Tho it depends which version of Emacs Lisp: the behavior of nested
backquotes was changed in Emacs-CVS (the change makes it behave more like
other Lisps and is clearly just a bugfix).


        Stefan