Is it possible to lookup for variable that doesn't exist?

Hello! Is it possible to lookup for variables that doesn’t exist? I have a list of generated variables for every icon glyph. Some icons consist from two 2 glyphs (need for ::before + ::after for 2 colors icon). I want to pass base icon name to mixin and get correct output, but i get an error of looking @@icon2, and it doesn’t matter if i use when or if. Also tried to use mixin guards, but no changes. Some code:

// generated list of icons
@icon__glyph_about_20__1: '\E00C';
@icon__glyph_about_32__1: '\E00D';
@icon__glyph_about_32__2: '\E00E';

// common mixin for icons
.nbl-icon__use(@iconName) {
  @icon1: ~'icon__glyph_@{iconName}__1'; // always exists
  @icon2: ~'icon__glyph_@{iconName}__2'; // sometimes doesn't exist
  
  &::before {
    content: @@icon1;
  }
  
  & when not (@@icon2 = undefined) {
    &::after {
      content: @@icon2;
    }
  }
}

.test {
  .nbl-icon__use(about_20);
}

Example of error: Message: variable @icon__glyph_about_20__2 is undefined in file ...

P.S. Unfortunately i can’t generate list of vars with null’s. P.S.S. sry for my eng, hope u understand me

[matthew-dean]

@theexplay The short answer is that no, there’s no current way to do this in exactly the way you mention. One workaround, though, would be to specify all the var names in a file loaded before the optional definitions, and set those values to something you would check later. In other words:

// default values
@icon__glyph_about_20__1: false;
@icon__glyph_about_20__2: false;
@icon__glyph_about_32__1: false;
@icon__glyph_about_32__2: false;

// generated values
@icon__glyph_about_20__1: '\E00C';
@icon__glyph_about_32__1: '\E00D';
@icon__glyph_about_32__2: '\E00E';

// common mixin for icons
.nbl-icon__use(@iconName) {
  @icon1: ~'icon__glyph_@{iconName}__1'; // always exists
  @icon2: ~'icon__glyph_@{iconName}__2'; // sometimes doesn't exist
  
  &::before {
    content: @@icon1;
  }
  
  & when not (@@icon2 = false) {
    &::after {
      content: @@icon2;
    }
  }
}

.test {
  .nbl-icon__use(about_20);
}

[matthew-dean]

@seven-phases-max

Oh, shoot. That’s a good catch. This is one of those cases where doing a lot of Less refactoring has tricked my brain towards what Less will do vs. what Less currently does do.

It should be:

  & when not (@@icon2 = false) {
    &::after {
      content: @@icon2;
    }
  }

You’re right that Less currently expects something close to === equality, although not consistently.