Stylus `calc()` mixin

calc-mixin.styl

// Adds the vendor prefixes for calc.
//
// Usage:
// width: calc('30% - 10px');
// If variables are needed they need to be specified this way.
// width: calc('40% \- %s' % (10px));
calc() {
  // vendors to be used by this function
  calc-vendors = webkit moz;

  if current-property {
    // arguments get unquoted
    arguments = unquote(arguments);

    // iterates over the vendor prefixes
    for prefix in calc-vendors {
      add-property(current-property[0], s('-%s-calc(%s)', prefix, arguments));
    }
    s('calc(%s)', arguments);
  } else {
    error('calc() must be used within a property');
  }
}