Re: Backquotes and commas and ",@"

[Ted Zlatanov <tzz@lifelogs.com>]

On Sat, 10 Dec 2011 09:10:55 +0100 Steinar Bang <sb@dod.no> wrote: 

SB> So, what's still unclear, is:
SB>  1. Why backquote (or for that matter: "why quote?") the body of a
SB>     defun?  What does that mean?
#+begin_src emacs-lisp
  (defun gnus-sync-lesync-pre-save-group-entry (url nentry &rest passed-props)
    (let* ((loc "gnus-sync-lesync-save-group-entry")
	   (k (car nentry))
	   (revision (gnus-sync-lesync-get-prop 'rev k))
	   (sname gnus-sync-lesync-name)
	   (topic (gnus-group-topic k))
	   (topic-offset (gnus-sync-topic-group-position k topic))
	   (sources (gnus-sync-lesync-get-prop 'source k)))
      ;; set the revision so we don't have a conflict
      `(,@(when revision
      ...
#+end_src

The function returns a list.  It was more convenient to build the list
all at once, with interpolation, than imperatively with push/pop or
cons.

SB>  2. What does ",@" mean?  "Splice the result of the evaluation into
SB>     the list"...?  Ah... I think I actually understand now.  Looking
SB>     at the elisp manual examples for ",@", I see that it means that if
SB>     the result of the comma-evaluation is a list, the elements are
SB>     added as element of the list, not as a single element that is a
SB>     list.

,@ says "whatever list follows, splice it in."  If the list is empty,
nothing happens; it's convenient when you want some content to appear
only if it's not nil.  So for example

#+begin_src emacs-lisp
      ,@(if topic (list (cons 'topic topic)) nil)
#+end_src

will result in an alist member like '(topic "xyz") if the `topic'
variable was defined, and nothing will be inserted otherwise.

SB> This means that the gnus-sync-lesync-pre-save-group-entry function
SB> returns a list.  And that list, is:
SB>  - If there is a revision, start the list with the symbol '_rev
SB>    followed by the revision
SB>  - Then the list will have an element that is a two element list,
SB>    consisting of the symbol '_id and the first element of the nentry
SB>    argument to the function (I don't understand why you have to have
SB>    the "." there, though...?)
SB>  - Then passed props are spliced in (what happens if there is no
SB>    passed-props argument sent to the function?  It's optional...?)
SB>  - Then an element that is a list starting with "source" (whatever
SB>    that is), followed by the result of evaluating the "if"
SB>  - Then an element that is a two element list starting with 'level and
SB>    with a number(?) as the second element
SB>  - Then splice in the elements resulting from evaluating two "if"s
SB>    relating to topics
SB>  - Then an element that is a list containing the read marks
SB>  - Then an element that is a list containing other marks, and this is
SB>    the one that broke down for me, because it got a conflict marker
SB>    instead of the other marks

SB> And I'm guessing this list is a translation of JSON into lisp lists...

It's prep work for the actual save to JSON, converting arbitrary group
data (especially marks) into a more neutral format that json.el can
handle.

Ted