Offset mixing by Hugo Giraudel http://hugogiraudel.com/2013/08/05/offsets-sass-mixin/

offset.scss

@mixin _position($position, $args) {
  $offsets: top right bottom left;
  
  @each $o in $offsets {
    $i: index($args, $o);
    @if $i
    and $i + 1 <= length($args) 
    and type-of( nth($args, $i + 1) ) == number {
      #{$o}: nth($args, $i + 1);
    }
  }
  
  position: $position;
}

/**
 * Mixin triplet calling the private mixin
 * with the appropriate position value
 */
@mixin absolute($args) {
  @include _position(absolute, $args);
}

@mixin relative($args) {
  @include _position(relative, $args);
}

@mixin fixed($args) {
  @include _position(fixed,    $args);
}