How to define a macro correctly?

How to define a macro correctly?

[Johan Andersson]

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Hey,

I want to create a macro that sets a variable value and then executes body.
I know how to solve it, but I want to know which way is the best (more
correct). I came up with these solutions:

a)
(defmacro mac (&rest body)
  `(progn
     (setq var t)
     ,@body))

b)
(defmacro mac (&rest body)
  (setq var t)
  `(progn ,@body))

c)
(defmacro mac (&rest body)
  (cons 'progn (cons (list 'setq 'var t) body)))

I noticed that (using macroexpand) macro a and c expands to the same list. b
however sets the variable in the macro and then only return the list body.
What does that mean exactly, that I set the variable in the macro and do not
return it as a list?

What way is the best? Or is there some other way that is better?

Thanks!

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Re: How to define a macro correctly?

[Barry Margolin]

In article <mailman.8.1287429628.15066.help-gnu-emacs@gnu.org>,
 Johan Andersson <johan.rejeep@gmail.com> wrote:

> Hey,
> 
> I want to create a macro that sets a variable value and then executes body.
> I know how to solve it, but I want to know which way is the best (more
> correct). I came up with these solutions:
> 
> a)
> (defmacro mac (&rest body)
>   `(progn
>      (setq var t)
>      ,@body))
> 
> b)
> (defmacro mac (&rest body)
>   (setq var t)
>   `(progn ,@body))
> 
> c)
> (defmacro mac (&rest body)
>   (cons 'progn (cons (list 'setq 'var t) body)))
> 
> I noticed that (using macroexpand) macro a and c expands to the same list. b
> however sets the variable in the macro and then only return the list body.
> What does that mean exactly, that I set the variable in the macro and do not
> return it as a list?
> 
> What way is the best? Or is there some other way that is better?
> 
> Thanks!

a and c are equivalent.  The backquote expression in a is just shorthand 
for the cons stuff you wrote in c (or something equivalent to it).

b is wrong.  The setq happens once at compile time, not execution time.  
You'll notice the difference when you byte-compile a file that uses the 
macro.

-- 
Barry Margolin, barmar@alum.mit.edu
Arlington, MA
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