Re: Evaluating arithmetic expressions with macros (eval)

On Wed, Oct 04, 2023 at 04:29:11PM -0400, Nick Bowler wrote:
> Hi,
> 
> On 2023-10-04, Sébastien Hinderer <[email protected]> wrote:
> > I find myself stuck with something which I assume is trivial. I define:
> >
> > m4_define([X], [9])
> > m4_define([Y], [3])
> >
> > And I would like to define Z as being the arithmetic sum of X and Y and
> > can seem to get it.
> >
> > I tried several variations of eval but had no success. I understand
> > that all the macros need to be expanded before eval is called but I
> > don't understand how to do it.
> 
> The short answer for your specific example is to simply not quote the
> arguments:
> 
>     m4_define([Z], m4_eval(X + Y))

Not being an m4 wizard, I can only ask whether it would be better to put
parentheses around the X and Y arguments, i.e.

	m4_define([Z], m4_eval((X) + (Y)))

... to guard against the possibility of X and Y being defined as more
complex expressions?  It might not be necessary in the case of addition,
but it might be in other cases.

I'm just drawing from what I know about similar macros in other
languages.

Regrads,
  Andreas

> 
> This approach is probably sufficient for most typical uses of m4_eval,
> as there would seem to be little chance of unwanted macro expansion.
> 
> Some details, when m4 sees the following macro expansion:
> 
> m4_define([Z], m4_eval(X + Y))
> 
> - The first argument contains no unquoted text, so no macro expansion is
>   performed.  The quotes are removed, and the first argument is "Z".
> 
> - The second argument contains unquoted text and a macro:
> 
>    m4_eval(X + Y).
> 
>    - The first argument of this contains unquoted text with two macros,
>      X and Y.  These are replaced with 9 and 3, respectively.  There
>      are no further macros to expand and no quotes to remove, so the
>      actual argument to m4_eval is 9 + 3.
> 
>    Now m4_eval(9 + 3) is expanded, giving 12.  There are no further
>    macros to expand and no quotes to remove, so the second argument
>    to m4_define is 12.
> 
> Now m4_define(Z, 12) is expanded.
> 
> Hope that helps,
>   Nick

-- 
Andreas (Kusalananda) Kähäri
Uppsala, Sweden

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