Making a Sass mixin with optional arguments

A DRY'r Way of Doing It

And, generally, a neat trick to remove the quotes.

@mixin box-shadow($top, $left, $blur, $color, $inset:"") {
  -webkit-box-shadow: $top $left $blur $color #{$inset};
  -moz-box-shadow:    $top $left $blur $color #{$inset};
  box-shadow:         $top $left $blur $color #{$inset};
}

SASS Version 3+, you can use unquote():

@mixin box-shadow($top, $left, $blur, $color, $inset:"") {
  -webkit-box-shadow: $top $left $blur $color unquote($inset);
  -moz-box-shadow:    $top $left $blur $color unquote($inset);
  box-shadow:         $top $left $blur $color unquote($inset);
} 

Picked this up over here: pass a list to a mixin as a single argument with SASS

A so much better DRY way

is to pass $args... to the @mixin. That way, no matter how many $args you will pass. In the @input call, you can pass all args needed. Like so:

@mixin box-shadow($args...) {
  -webkit-box-shadow: $args;
  -moz-box-shadow: $args;
  box-shadow: $args;
}

And now you can reuse your box-shadow in every class you want by passing all needed args:

.my-box-shadow {
  @include box-shadow(2px 2px 5px #555, inset 0 0 0);
}