Interpolation

Its important to have the $side in the beginning of the arguments in the assemble mixin as this is the only one without a default value it has to be declared first.

@mixin assemble($side, $bg: #fff, $pad: 10px) {
  background: $bg;
  border-#{$side}: 1px solid #ccc;
  padding: $pad;
}

.factory {
  @include assemble($side: left);
}
.highrise {
  @include assemble($side: right,$bg: #797979, $pad: 20px);
}