toggle-viper-mode strangeness

toggle-viper-mode strangeness

[Giorgos Keramidas]

It's been several days since I updated cvs-emacs here, so forgive me if
this is fixed already.

Isn't the following in `toggle-viper-mode' strange?

        ;;;###autoload
        (defun toggle-viper-mode ()
          "Toggle Viper on/off.
        If Viper is enabled, turn it off.  Otherwise, turn it on."
          (interactive)
          (if (eq viper-mode t)
              (viper-go-away)
==>         (setq viper-mode nil)
            (viper-mode)))

Looking at the indentation I'd expect the if expression to end where the
arrow points, but it doesn't.  Or is an implicit (progn ...) added
automatically by Emacs around the third ... N-th elements of the if
expression?

Re: toggle-viper-mode strangeness

[Kevin Rodgers]

Giorgos Keramidas wrote:
> It's been several days since I updated cvs-emacs here, so forgive me if
> this is fixed already.
> 
> Isn't the following in `toggle-viper-mode' strange?
> 
>         ;;;###autoload
>         (defun toggle-viper-mode ()
>           "Toggle Viper on/off.
>         If Viper is enabled, turn it off.  Otherwise, turn it on."
>           (interactive)
>           (if (eq viper-mode t)
>               (viper-go-away)
> ==>         (setq viper-mode nil)
>             (viper-mode)))
> 
> Looking at the indentation I'd expect the if expression to end where the
> arrow points, but it doesn't.  Or is an implicit (progn ...) added
> automatically by Emacs around the third ... N-th elements of the if
> expression?

Yes:

,----[ C-h f if RET ]
| if is a special form.
| (if COND THEN ELSE...)
|
| If COND yields non-nil, do THEN, else do ELSE...
| Returns the value of THEN or the value of the last of the ELSE's.
| THEN must be one expression, but ELSE... can be zero or more expressions.
| If COND yields nil, and there are no ELSE's, the value is nil.
`----

-- 
Kevin Rodgers

Re: toggle-viper-mode strangeness

[Richard M. Stallman]

Here's what that code does:

	      (if (eq viper-mode t)
	          ;; Turn the mode off.
		  (viper-go-away)
		;; Turn the mode on.
		(setq viper-mode nil)
		(viper-mode)))

It looks correct to me, except perhaps for the eq call,
which treats non-nil non-t values as "off" rather than "on".
That is peculiar, but I don't know whether it is wrong.

Re: toggle-viper-mode strangeness

[Giorgos Keramidas]

On 2006-01-19 20:14, "Richard M. Stallman" <rms@gnu.org> wrote:
> Here's what that code does:
>
> 	      (if (eq viper-mode t)
> 	          ;; Turn the mode off.
> 		  (viper-go-away)
> 		;; Turn the mode on.
> 		(setq viper-mode nil)
> 		(viper-mode)))
>
> It looks correct to me, except perhaps for the eq call,
> which treats non-nil non-t values as "off" rather than "on".
> That is peculiar, but I don't know whether it is wrong.

I was under the impression that an (if cond then else) expression can
only have 4 parts within the parentheses.  This means that the above
would require a (progn) around the final two parts, i.e.:

        (if (eq viper-mode t)
            ;; Turn the mode off.
            (viper-go-away)
          ;; Turn the mode on.
          (progn
            (setq viper-mode nil)
            (viper-mode)))

It looks like I was wrong, and the else part can contain more than one
s-exp, returning the value of the last one.  Thanks :)

Re: toggle-viper-mode strangeness

[Johan Bockgård]

Giorgos Keramidas <keramida@ceid.upatras.gr> writes:

> I was under the impression that an (if cond then else) expression
> can only have 4 parts within the parentheses.

Different Lisp dialects are different in this respect.

-- 
Johan Bockgård

Re: toggle-viper-mode strangeness

[Stefan Monnier]

> I was under the impression that an (if cond then else) expression can
> only have 4 parts within the parentheses.  This means that the above

You're confusing Scheme and Lisp.


        Stefan

Re: toggle-viper-mode strangeness

[Luc Teirlinck]

Stefan Monnier wrote:

   > I was under the impression that an (if cond then else) expression can
   > only have 4 parts within the parentheses.  This means that the above

   You're confusing Scheme and Lisp.

Or Common Lisp (in which the `else' is _not_ an implicit progn) and Elisp.

Sincerely,

Luc.

Re: toggle-viper-mode strangeness

[Giorgos Keramidas]

On 2006-01-19 23:03, Stefan Monnier <monnier@iro.umontreal.ca> wrote:
> > I was under the impression that an (if cond then else) expression can
> > only have 4 parts within the parentheses.  This means that the above
>
> You're confusing Scheme and Lisp.

On 2006-01-19 22:14, Luc Teirlinck <teirllm@dms.auburn.edu> wrote:
> Or Common Lisp (in which the `else' is _not_ an implicit progn) and Elisp.

Common Lisp, in fact.  Too much ``M-x slime'' these last few days.

Thanks for the clarification everyone.  I'll make sure to check with the
info docs before I create more noise :)