passing dimensions to metapost

["Thomas A. Schmitz" <thomas.schmitz@uni-bonn.de>]

Hi all,

cleaning up the code in my module, I run into a very interesting  
problem. I want to pass a user-defined value to Metapost. Until now,  
the value "scale" had to be numeric, and I used it like this:

draw fullcircle scaled \MPvar{scale}mm

and Metapost was happy, and I was too. Now Aditya made the reasonable  
suggestion that users should be allowed to input any dimension and not  
be restricted to "mm." Easy enough. However, I also want a fallback  
just in case they input a number, in that case, I want to fall back to  
mm as a default. So I tried (after Taco's excellent explanation on the  
wiki):

\expandafter\processaction\expandafter[\CVERscale]%
      [\v!unknown=>{\doifdimensionelse%
		    {\CVERscale}%
		    {\setupMPvariables[CircleSomething][scale=\CVERscale]}%
		    {\doifnumberelse%
			{\CVERscale}%
			{\newdimen\CIscale \global\CIscale=\CVERscale mm  
{\setupMPvariables[CircleSomething][scale=\the\CIscale]}}%
			{\setupMPvariables[CircleSomething][scale=10mm\relax]%
			   \message{Please give a proper value for the "scale" argument!}}}},
       \v!default=>{\setupMPvariables[CircleSomething][scale=10mm 
\relax]}]


So what I want: first check if \CVERscale is a dimension; if it is,  
assign it to the "scale" parameter. If it isn't, check if it is a  
number; if this is the case, append the unit "mm" to it and put it  
into a dimension register. If it isn't a number either, make the value  
sufficiently absurd for the user to realize and write a message to the  
terminal. This is all fine and dandy and works, sort of: it works  
perfectly with dimensions, but when I try with a number as input, it  
doesn't fall back to the unit "mm," but to "pt," AFAICS. That's nice  
too, but I have no clue why, and I would prefer mm. If it helps, I can  
try to give you a minimal test file tomorrow, but maybe somebody spots  
what I'm doing wrong? (I tried both with mkii and mkiv, and always get  
the same result.)

All best

Thomas
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