function factory( mu, beta ) {
	var lbeta;
	if (
		isnan( mu ) ||
		isnan( beta ) ||
		beta <= 0.0
	) {
		return constantFunction( NaN );
	}
	lbeta = ln( beta );
	return logpdf;

	/**
	* Evaluates the logarithm of the probability density function (PDF) for a Gumbel distribution.
	*
	* @private
	* @param {number} x - input value
	* @returns {number} evaluated logPDF
	*
	* @example
	* var y = logpdf( 2.3 );
	* // returns <number>
	*/
	function logpdf( x ) {
		var z;
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x === NINF ) {
			return 0.0;
		}
		z = ( x - mu ) / beta;
		return -z - exp( -z ) - lbeta;
	}
}
Returns a function for evaluating the logarithm of the probability density function (PDF) for a Gumbel distribution with location parameter `mu` and scale parameter `beta`.

@param {number} mu - location parameter
@param {PositiveNumber} beta - scale parameter
@returns {Function} logPDF

@example
var logpdf = factory( 0.0, 2.0 );
var y = logpdf( 0.0 );
// returns ~-1.693