mixin option-map pattern

SassMeister-input.scss

// ----
// Sass (v4.0.0.alpha.1)
// ----

@mixin foo($opts: null, $bar: 1, $baz: 2, $bif: 3) {
  @if $opts { @include foo($opts...); } @else {
    bar: $bar;
    baz: $baz;
    bif: $bif;
  }
}

.test {
  @include foo((bar: 5, bif: 1));
}

SassMeister-output.css

.test {
  bar: 5;
  baz: 2;
  bif: 1;
}