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From: sdl@MITRE-BEDFORD.ARPA ("Steven D. Litvintchouk")
Newsgroups: net.emacs
Subject: (error-occurred) and (interactive)
Message-ID: <8605241659.AA07159@mitre-bedford.ARPA>
Date: Sat, 24-May-86 12:59:36 EDT
Article-I.D.: mitre-be.8605241659.AA07159
Posted: Sat May 24 12:59:36 1986
Date-Received: Mon, 26-May-86 01:31:27 EDT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The MITRE Corp., Bedford, MA
Lines: 58



>   Can anybody tell me if there is a function in GNU Emacs similar to the
>   Gosling predicate (error-occurred)?

I use "condition-case".  "condition-case" is actually more general
than Gosling's "error-occurred", because "condition-case" can handle
different kinds of errors.  Here is the definition:

condition-case:
Regain control when an error is signaled.
 (condition-case VAR BODYFORM HANDLERS...)
executes BODYFORM and returns its value if no error happens.
Each element of HANDLERS looks like (CONDITION-NAME BODY...)
where the BODY is made of Lisp expressions.
The handler is applicable to an error
if CONDITION-NAME is one of the error's condition names.
When a handler handles an error,
control returns to the condition-case and the handler BODY... is executed
with VAR bound to (SIGNALED-CONDITIONS . SIGNAL-DATA).
The value of the last BODY form is returned from the condition-case.
See SIGNAL for more info.


And, (courtesy of GNU emacs Mocklisp support package mlsupport.el),
here is the way "error-occurred" can be implemented using
"condition-case":

(defmacro error-occurred (&rest body)
  (list 'condition-case nil (cons 'progn (append body '(nil))) '(error t)))


>   I would also appreciate any explanation about the difference in GNU Emacs
>   between a "function" and a "command".  As far as I can tell, it has to do
>   with the use of the (interactive) function.  What is the real meaning of
>   this function when used with no arguments?


Placing an (interactive) declaration just after the documentation
string of a function, declares the surrounding function to be
interactively callable as a command.  That is, it can be invoked via
M-x (Meta-x) followed by its name.  Otherwise, the only way you can
call the function is as a Lisp function; e.g.  via M-ESC,
eval-expression, or in Lisp interaction mode.  I guess the reason for
this is to avoid cluttering up the name space of interactive commands
with Emacs Lisp functions which are never expected to be invoked
interactively by an end user; this cuts down on the number of
displayed command completions, etc.

If the function expects arguments, then the arguments of (interactive)
enable you to specify how arguments are to be passed interactively to
the function.


Steven Litvintchouk
MITRE Corporation
ARPA:  sdl@mitre-bedford
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