Re: passing dimensions to metapost

[Hans Hagen <pragma@wxs.nl>]

Thomas A. Schmitz wrote:
> 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.)

\startluacode
document.mymodule = document.mymodule or { }

function document.mymodule.checkdimen(str)
     return (str:find("%a%a$") and str) or (str .. "mm")
end
\stopluacode

\ctxlua{document.mymodule.checkdimen("#1")}

or more fancy:

function document.mymodule.checkdimen(str)
     if not str:find("%a%a$") then
         str = str .. "mm"
         ctx.writestatus("mymodule","you forgot a dimension unit, so I 
made it up for you: %s",str)
     end
     return str
end

for mkii you can use \doifdimensionelse (it reminds me to implement that 
in mkiv)

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                                           Hans Hagen | PRAGMA ADE
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