[metapost] (newbie) macro with a sequence of pairs

[Nicola]

In article <4B2742EA.7060001 at elvenkind.com>,
 Taco Hoekwater <taco at elvenkind.com> wrote:

> luigi scarso wrote:
> > I'm pretty new about metapost programming
> > and I need to write a "polyline" macro
> > that take a with a "sequence of pairs"  like
> > (0,0) (10,10) (30,10)
> > and draw  a line
> > ie
> > polyline (0,0) (10,10) (30,10)
> > is
> > draw (0,0)--(10,10)--(30,30)
> > 
> >  Is it possible ?
> 
> Not without a lot of trickery, I think. Why do you
> not want to use
> 	
>    polyline (0,0)--(10,10)--(30,30);

Here is some trickery:

def next text p =
   -- begingroup [p]
enddef;

vardef polyline(expr x,y) text points =
   save (,),[,];
   delimiters [ ];
   let ( = next;
   let ) = endgroup;
   draw [x,y] points
enddef;

beginfig(0);
   polyline (0,0) (10,10) (30,10) ;
   polyline (5,7); % Boundary case
endfig;

Don't expect this to work if you deviate, even slightly, from the given 
syntax (e.g., if you want to use variables to denote pairs).

Regards,
Nicola