is-length.scss

//
// Check whether `$value` is a valid length.
/// @group Utilities
/// @access private
/// @param {*} $value - Value to validate.
/// @return {Boolean}
/// @require {function} contains
/// @link http://www.sitepoint.com/bulletproof-function-validate-length-values-sass/
//

@function is-length($value) {
  @return contains(0 'auto' 'initial' 'inherit', $value) or type-of($value) == 'number' and not unitless($value) or str-slice($value + '', 1, 4) == 'calc';
}