SassMeister-input.scss

// ----
// Sass (v3.3.0.rc.2)
// Compass (v1.0.0.alpha.17)
// ----

// Extend function, JS STYYYYLE !
// ------------------------------------------------------------
// @param (map) $obj: default object
// @param (map) $extObj: conf object
// ------------------------------------------------------------
// @return (map) extended object

@function extend($obj, $extObj) {
  @each $key, $value in $extObj {
    $obj: map-merge($obj, ($key: $value));
  }
  @return $obj;
}

// A mixin of yours 
@mixin app($conf: ()) {
  // What could be the defaults parameter for your mixin
  $conf: extend((
    speed    : 1000,
    theme    : dark,
    name     : 'default',
    duration : 250ms,
    width    : 100%,
    widget   : modular
  ), if(type-of($conf) != map, (), $conf));
  
  // Do something with the configuration objecy
  test: inspect($conf);
}

test {
  // Including the mixin with a custom conf object
  @include app((
    speed: 500,
    theme: light,
    name: 'test'
  ));
  
  // Or full default if you don't feel like specifying any parameter
  @include app();
}

SassMeister-output.css

test {
  test: (speed: 500, theme: light, name: "test", duration: 250ms, width: 100%, widget: modular);
  test: (speed: 1000, theme: dark, name: "default", duration: 250ms, width: 100%, widget: modular);
}