LESS-variable with value == null, undefinded? - css

Question

Just to get this clear, im talking about "LESS" as described on http://lesscss.org/
i read the documentation and searched a little around but i didn't find a possibilty to call an mixin with some "null" or undefined parameters.
For example this mixin from retina.js's less file:
.at2x(#path, #w: auto, #h: auto) has 2 optional parameters.
Now i only want to call it with a defined path and height. How do i do this?
things that don't work:
.at2x(EXAMPLEURL, '', 500px)
.at2x(EXAMPLEURL, null, 500px)
.at2x(EXAMPLEURL, , 500px)
.at2x(EXAMPLEURL,, 500px)
.at2x(EXAMPLEURL, undefined, 500px)
call that works of course:
.at2x(EXAMPLEURL, auto, 500px)

Answer 1

A Null Value
Is an escaped empty string, like so:
.at2x(EXAMPLEURL, ~'', 500px);
This would output "nothing" for the #w variable except a white space character. This will not necessarily eliminate the property it is used for, but it will make that property of no effect in the css. So you might get this if it is used for setting width: #w:
width: ;
Which would be ignored in css. Or if it is part of background-size: #w #h, you would get something like this:
background-size: 500px;
Of course, this is not truly NULL in the normal programming sense of the term, but I think it is what you are seeking for.

Answer 2

You can use "Named Parameters":
.at2x(EXAMPLEURL, #h: 500px);

Answer 3

I've the same problem with Less and i find a fix for this.
FIX :
#MyEmptyVar: e("");
Problem example :
Before :
/* Input (Style.less)*/
#IconPrefix: "";
/*
* With my less version this
* #IconPrefix: ; cause a compilation error
*/
.#{IconPrefix}glass:before {
content: "\f000";
}
/* Output (Style.css)*/
.""glass:before {
content: "\f000";
}
After:
/* Input (Style.less)*/
#IconPrefix: e("");
.#{IconPrefix}glass:before {
content: "\f000";
}
/* Output (Style.css)*/
.glass:before {
content: "\f000";
}