Generated by SassMeister.com.

SassMeister-input.scss

// ----
// Sass (v3.2.17)
// Compass (v0.12.4)
// ----

$default: 1;

@mixin test($p: null) {
    @if ($p) {
      /* Do something if $p is passed */
      value: $p;
    } @else {
      $p: $default;
      /* Do something else if $p is not passed */
      value: $p;
    }
}

test1 {
  @include test();
}
test2 {
  @include test(1);
}

SassMeister-output.css

test1 {
  /* Do something else if $p is not passed */
  value: 1;
}

test2 {
  /* Do something if $p is passed */
  value: 1;
}