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

	/**
	* Evaluates the probability density function (PDF) for a Laplace distribution.
	*
	* @private
	* @param {number} x - input value
	* @returns {number} evaluated PDF
	*
	* var y = pdf( -3.14 );
	* // returns <number>
	*/
	function pdf( x ) {
		var z;
		if ( isnan( x ) ) {
			return NaN;
		}
		z = ( x - mu ) / b;
		return 0.5 * exp( -abs( z ) ) / b;
	}
}
Returns a function for evaluating 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} PDF

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

var y = pdf( 10.0 );
// returns 0.25

y = pdf( 5.0 );
// returns ~0.021

y = pdf( 12.0 );
// returns ~0.092