Modify method for BEM

modify.scss

@mixin modify ($modifier) {
  $class: quote(#{&});
  $index: str-index($class, '__');
  $parent: str-slice($class, 1, $index - 1);
   @at-root #{selector-nest(#{$parent}--#{$modifier}, &)} {
     @content;
   }
}

.parent {
  &__title {
    color: purple;
    
    @include modify("error"){
      color: red;
    }
  }
  
  &__child {
    &__grandChild {
      @include modify("valid"){
        color: green;
      }  
      
      &__deep {
        @include modify("error") {
          color: red;
        }
      }
    }
  }
}

renders.css

.parent__title {
  color: purple;
}
.parent--error .parent__title {
  color: red;
}

.parent--valid .parent__child__grandChild {
  color: green;
}

.parent--error .parent__child__grandChild__deep {
  color: red;
}