[plt-scheme] macros that need to eval

At Tue, 28 Jan 2003 07:45:35 -0500, Eli Barzilay wrote:
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> 
> On Jan 28, Doug Orleans wrote:
> > Here's a riddle: is there a way to write a macro that needs to
> > evaluate one of its arguments at expansion time-- without using
> > eval?
> > 
> > For example, can you write a macro arity-thunk such that
> >     (arity-thunk 4 (newline))
> > expands into
> >     (lambda (x1 x2 x3 x4) (newline))
> > ?  (The parameters don't have to be named that way; they could be
> > generated temporaries.)
> 
> I think that requiring that
> 
>     (arity-thunk (* 2 2) (newline))
> 
> would be a better question.  If not, then you can just look at the
> contents of the syntax object -- except that's a limited of form
> cheating -- relying on the fact that there is a number there and not
> something like the original string or you'd have to parse "#b100".

I don't know whether it's cheating. If `arity-thunk' expects a literal
number after `arity-thunk', that seems fine to me. That doesn't seem
like evaluation any more than detecting a keyword.

> Would you want this:
> 
>   (let ((x 4))
>     (arity-thunk x (newline)))
> 
> to expand to the same thing?

This is definitely not the sort of thing I would encourage. :)

Matthew