Less mixin to set rules conditionally

set.less

 .set(@property: null; @then: null; ) when not (@property = null ) and not ( @then = null ) 
{
  @{property}: @then;
}
 
.set( @property: null; @then: null; @when: null; )
{
  #check.condition( @when ); .output();
  
  .output() when (@condition) {
    .set(@property, @then); 
  }
  
}
 
.set( @property: null; @when: null; @and: null; @or: null; @then: null; @else: null; )
{
  #check.condition( @when ); .output();
  
  .output() when (@condition) and (@and = null) {       
     .set(@property, @then);      
  }
  
  .output() when (@condition) and not (@and = null) {    
    .set(@property, @then, @and);            
  }
  
  .output() when not (@condition) and not (@or = null) {   
    
    #check.condition(@or); ._output();
    
    ._output() when (@condition) {
      .set(@property, @then);
    }
    
   ._output() when not (@condition) {
      .set(@property, @else);
    }
                   
  }  
  
  .output(...) when not (@else = null) and ( default() ) {    
    .set(@property, @else);             
  }
    
}
 
#check {
  
  .condition( @condition ) when ( 3 = length( @condition ) ) 
  {
    .condition(
        extract( @condition, 1 ),
        extract( @condition, 2 ),
        extract( @condition, 3 )
    );
  }
  
  .condition(@a, eq, @b) when (@a = @b)     { @condition: true; }
  .condition(@a, gt, @b) when (@a > @b)     { @condition: true; }  
  .condition(@a, ge, @b) when (@a >= @b)    { @condition: true; }
  .condition(@a, lt, @b) when (@a < @b)     { @condition: true; }
  .condition(@a, le, @b) when (@a <= @b)    { @condition: true; }
  .condition(@a, ne, @b) when not(@a = @b)  { @condition: true; }
  .condition(true)                          { @condition: true; }
  .condition(...)        when (default())   { @condition: false;}
         
} // end #check
 
 
// use
 
@my-color: lightblue;
 
@my-value: 21px;
 
@my-switch: true;
 
@my-other-color: darkblue;
 
@my-other-value: 42em;
 
@my-other-switch: true;
 
.my-thing {
  
   // set background to @my-color, as long as @my-color isn't null (applies to all)
  .set(background; @my-color);
 
  // if a certain condition evaluates to true, set border-color to @my-other-color
  .set(border-color; @my-other-color; iscolor(@my-other-color) );  
    
  // if these two conditions evaluate to true, set color to @then
  .set(color;
      @when: @my-switch;
      @and: lightness(@my-other-color) ge 25%;
      @then: darken(@my-other-color, 10%);
  );  
  
  // if a certain condition evaluates to true, then set padding to @then. Otherwise, set it to @else
  .set(padding;
      @when: @my-value lt 24px; 
      @then: 1em; 
      @else: 1.5em; 
  );
  
  // if one or another condition evaluates to true, then set margin-botton to @then. Otherwise, set it to @else
  .set(margin-bottom;
      @when: @my-other-value ge 40em;
      @or: @my-other-switch;
      @then: 1.5em; 
      @else: 1em;
  );
 
  // static styles
  
  display: block;
  float: left;
  
}
 
 
/* output
**************************
 
.my-thing {
  background: #add8e6;
  border-color: #00008b;
  color: #000058;
  padding: 1em;
  margin-bottom: 1.5em;
  display: block;
  float: left;
}
*/