example.sass

=border(!width = 10px, !style = "solid", !color = blue)
  border= !width !style !color

/* Using inline arguments */
.foo
  +border(10px, "solid", green)
  &:hover
    +border(10px, "dashed", red)

/* Using Keyword arguments */
.bar
  +border
    !color: green
  &:hover
    +border
      !style: dashed
      !color: red

example.scss

/* Note that the syntax here is pretty tentative */

@mixin border($width, $style, $color) {
  @defaults {
    $width: 10px;
    $style: solid;
    $color: blue;
  }
  border: $width $style $color;
}

/* using inline arguments */
.foo {
  @include border(10px, solid, green);
  &:hover {
    @include border(10px, dashed, red);
  }
}

/* using keyword arguments */
.bar {
  @include border {
    $color: green;
  }
  &:hover {
    @include border {
      $style: dashed;
      $color: red;
    }
  }
}

generated.css

/* Both of these files produce the same CSS. They're just different surface syntaxes for the same underlying document. */

.foo {
  border: 10px solid green;
}

.foo:hover {
  border: 10px dashed red;
}

.bar {
  border: 10px solid green;
}

.bar:hover {
  border: 10px dashed red;
}