Sort of sprintf for Sass

demo.scss

@for $i from 1 through 12 {
  @debug format(
    ('.on-', 2, '-columns-i-am-spanning-', 1),
    (12, $i)
  );
}

/*
DEBUG: .on-1-columns-i-am-spanning-12
DEBUG: .on-2-columns-i-am-spanning-12
[…]
DEBUG: .on-12-columns-i-am-spanning-12
*/

format.scss

@function format($format, $args

  // scope private var
  ,$_string:  null
  ,$_token:   null
) {
  @each $_token in $format {
    @if length($_token) > 1 {
      $_token: format($_token, $args);
    }
    @else if type-of($_token) == number {
      $_token: nth($args, $_token);
    }
    $_string: '#{$string}#{$_token}';
  }
  @return quote($_string);
}