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From: mesard@bbn.com (Wayne Mesard)
Newsgroups: comp.emacs
Subject: Re: GNU Emacs special form "interactive"
Message-ID: <33979@bbn.COM>
Date: 30 Dec 88 18:52:38 GMT
References: <8812301512.AA10827@decwrl.dec.com>
Sender: news@bbn.COM
Reply-To: mesard@labs-n.bbn.com (Wayne Mesard)
Organization: Bolt Beranek and Newman Inc., Cambridge MA
Lines: 37

In article <8812301512.AA10827@decwrl.dec.com> ellis@ultra.dec.com (David Ellis) writes:
>What I'd like to do is have a function taking an argument that gets
>inserted into a prompt string.

>I can do this if I can get the special form "interactive" to take as its
>argument a string-valued expression in place of a specific string, for
>example
>
>	(interactive (format "s(This is a %s prompt string) : " foo-arg))
>
>Unfortunately, GNU Emacs rewards this effort with the error message
>
>	Wrong type argument: listp, "s(This is a Type A prompt string) : "
>
>saying that "interactive" expects a list (huh?) but has been given a string.

As the docs for interactive say, if the argument is not a literal
string, it is assumed to be lisp code which, when evaluated, will
produce a LIST containing an element to be bound to each of the function
parameters.

Since interactive acts as a declaration, its args are examined at
compile time.  (Thus your attempt to use format would not produce the
desired result even if you had wrapped a (list ...) call around the
format.)  The following prompts for a single argument to a function
placing a timestamp in the prompt.

 (interactive
  (list (read-string (format "The time is %s: " (current-time-string)))))

In summary, if the arg to interactive is a literal string, it gets used
directly by the code which does the argument binding.  Otherwise, it is
evaluated and must produce a list, each element of which is bound to one
of the functions args.

-- 
void Wayne_Mesard();   Mesard@BBN.COM   Bolt Beranek and Newman, Cambridge, MA