assign variables with LESS css using mixins - less

Question

Is it possible to assign a variable based on another variable using less css?
pseudo code:
#userpick = red; /* this changes based on user pick */
#color1 = red;
#color2 = blue;
if( #userpick == #color1)
#systempick = #color2
else( #userpick == #color2)
#systempick = #color1
I want to be able to use #systempick as a color value not as a css style. For example:
border: 5px dashed #systempick;

Answer 1

Doesn't use a mixin, but resolves your desire
If the #userpick is stored as a string instead, then that could be used to access another variable name based off of it (a variable variable), like so:
LESS
#userpick: "blue"; /* this changes based on user pick, saved as a string */
#setopposite: "#{userpick}-opp";
//set opposing values
#red-opp: blue;
#blue-opp: red;
#systempick: ##setopposite;
.test {
color: #systempick;
}
CSS Output (note how blue outputs #ff0000, i.e. red)
/* this changes based on user pick */
.test {
color: #ff0000;
}
Since you are dealing with colors, the #userpick needs to be saved as a string because otherwise, LESS will automatically assign it a hex value rather than the color name.

Answer 2

You can use guards in mixins to change the color that is used, e.g.
.a when (#userpick = #color1) {
color: #color2;
}
.a when (#userpick = #color2) {
color: #color1;
}
You might get code duplication but its the only way to get an if statement.
Other ways are possible (e.g. inline javascript if using less.js or a function plugin if using dotless) but are probably a little hacky.

Answer 3

I was able to get what I needed, however this may not work on the PHP port of Less.
This was a bit tricky since you can assign variables only once. There are also some restrictions regarding scope of variables. I'm also comparing color values using HSV (hue, saturation, value)
.set_colors(#F00, #00F, #F00); /* pass red, blue and user pick */
.set_colors(#c1, #c2, #u) when (hue(#c1) = hue(#u)) and (saturation(#c1) = saturation(#u)) and (lightness(#c1) = lightness(#u)){
#color1 = #c1;
#color2 = #c2;
#userpick = #c1;
#systempick = #c2;
}
.set_colors(#c1, #c2, #u) when (hue(#c2) = hue(#u)) and (saturation(#c2) = saturation(#u)) and (lightness(#c2) = lightness(#u)){
#color1 = #c1;
#color2 = #c2;
#userpick = #c2;
#systempick = #c1;
}