function factory( mu, b ) {
	if (
		isnan( mu ) ||
		isnan( b ) ||
		b <= 0.0
	) {
		return constantFunction( NaN );
	}
	return logpdf;

	/**
	* Evaluates the logarithm of the probability density function (PDF) for a Laplace distribution.
	*
	* @private
	* @param {number} x - input value
	* @returns {number} evaluated logarithm of PDF
	*
	* @example
	* var y = logpdf( -3.14 );
	* // returns <number>
	*/
	function logpdf( x ) {
		var z;
		if ( isnan( x ) ) {
			return NaN;
		}
		z = ( x - mu ) / b;
		return -( abs( z ) + ln( 2.0 * b ) );
	}
}
Returns a function for evaluating the logarithm of the probability density function (PDF) for a Laplace distribution with location parameter `mu` and scale parameter `b`.

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

@example
var logpdf = factory( 10.0, 2.0 );

var y = logpdf( 10.0 );
// returns ~-1.386

y = logpdf( 5.0 );
// returns ~-3.886

y = logpdf( 12.0 );
// returns ~-2.386