Re: Suppress user-prompting when calling commands in programs

[Stefan Monnier]

> But what about a case like this, a command with several optional args,
> that are not always necessary. How to give the user the option to decide
> for which args he wants to give input (and use the defaults for the
> others)?

That's a UI issue more than a programming issue.  The general design we
try to follow in Emacs in this respect is to only have 2 options:
either minimal prompting (the default) or full prompting (which the user
can request with C-u).
But that's completely up to the package's author.

> Where to put the (prefix) 'arg argument in this case? How does Emacs
> know which of the optional arguments should be the prefix-argument?

These questions can't be answered directly because they only indicate
a misunderstanding of what is the prefix argument.

> Do I need a "P" or "p" then somewhere in the spec when
> I add 'arg' to the arguments list? 

"P" only means that the argument at that position will receive the value
of `current-prefix-arg'.  And "p" does the same for
(prefix-numeric-value current-prefix-arg).  You can call that argument
`arg', or `best-friends-for-ever', or any other name you like.

>  (defun iorg-scrape-repl (&optional port host how local)
>    "Run inferior Picolisp and setup process for GUI-scripting."
>    (interactive
>     (cond
>      ((equal current-prefix-arg nil) nil)
>      ((equal current-prefix-arg '(4))
>       (list
>        (read-number "Port: ")))
>      ((equal current-prefix-arg '(16))
>       (list
>        (read-number "Port: ")
>        (read-string "Host: ")))
>      ((equal current-prefix-arg '(32))
>       (list
>        (read-number "Port: ")
>        (read-string "Host: ")
>        (read-string "How: ")))
>      (t
>       (list
>        (read-number "Port: ")
>        (read-string "Host: ")
>        (read-string "How: ")
>        (read-string "Local: ")))))

That would work.

As mentioned above, we usually prefer not to distinguish (4) from (16)
or any other value, other than nil, so it would be just:

   (defun iorg-scrape-repl (&optional port host how local)
     "Run inferior Picolisp and setup process for GUI-scripting."
     (interactive
      (when current-prefix-arg
        (list
         (read-number "Port: ")
         (read-string "Host: ")
         (read-string "How: ")
         (read-string "Local: "))))

And then we'd try to reduce the worst-case prompting either by avoiding
some of the prompts depending on previous prompt's return value, or by
merging prompts.  E.g.:

   (defun iorg-scrape-repl (&optional port host how local)
     "Run inferior Picolisp and setup process for GUI-scripting."
     (interactive
      (when current-prefix-arg
        (let ((host+port (split-string (read-string "Host (and port): ")
                                       ":")))
          `(,(nth 1 host+port)
            ,(nth 0 host+port)
            ,@(unless (equal (nth 0 host+port) "localhost")
                (list (read-string "How: ")
                      (read-string "Local: "))))))))

-- Stefan