Re: About `macroexpand'

[Filipp Gunbin <fgunbin@fastmail.fm>]

On 30/12/2012 06:39, Xue Fuqiao wrote:

> I wrote the following code:
> (defmacro t-becomes-nil (variable)
>   (if (eq variable t)
>       (setq variable nil)))
> (setq foo t)
> (macroexpand '(t-becomes-nil foo))
>
> The return value of `macroexpand' is nil.  I don't know why.  I think it should be:
> (if (eq foo t)
>     (setq foo nil)))
>
> Can anybody help?

Inside your macro, the formal argument `variable' evaluates to `foo'
(that is, the actual argument, which _not yet evaluated_ itself). 
Then it's value (`foo') is compared by `eq' with 't' and possibly
overwritten with `nil'. The comparison doesn't make any sense and what
is actually overwritten is the local value of `variable'.

A correct version is like that:

(defmacro t-becomes-nil (variable)
  `(if (eq ,variable t)
       (setq ,variable nil)))

This returns the list after the backquote as written except the
`,variable' is substituted for the local value of `variable', which is
`foo'. Then the calling program can evaluate the resulting form to
obtain final result.

More on this: (info "(elisp) Wrong Time").

-- 
Filipp Gunbin