If you have a module with placeholder selectors, this pattern will help you avoid outputting them twice if imported twice.

defining_placeholders_once.scss

$module-placeholders-defined: false !default;
@mixin module-placeholders {
  @if not $module-placeholders-defined {
    $module-placeholders-defined: true;
    %module-object { color: red; }
    %module-object:hover { color: blue; }
  }
}

@include module-placeholders;

I must have missed something: what is %module-object?

In 3.2 % indicates a placeholder selector. It's like a class, but any selector it is used in will be omitted from the output unless it is extended.

@chriseppstein

Thank you for letting me know a good way! It's very useful.

But, If I use this with extend, it's not so much beautiful...

$use_foo: false !default;
@mixin use_foo() {
  @if $use_foo {}
  @else {
    $use_foo: true;
    .foo {
      /* style definition here */
    }
  }
}

@include use_foo;

p .baa {
  @extend .foo;
}

Do you know a better way?(any sass version ok,)