function factory( lambda ) {
	var scale;
	if ( isnan( lambda ) || lambda < 0.0 || lambda === PINF ) {
		return constantFunction( NaN );
	}
	scale = 1.0 / lambda;
	return pdf;

	/**
	* Evaluates the probability density function (PDF) for an exponential distribution.
	*
	* @private
	* @param {number} x - input value
	* @returns {number} evaluated PDF
	*
	* @example
	* var y = pdf( 2.3 );
	* // returns <number>
	*/
	function pdf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x < 0.0 ) {
			return 0.0;
		}
		return exp( -x / scale ) / scale;
	}
}
Returns a function for evaluating the probability density function (PDF) for an exponential distribution with parameter `lambda`.

@param {PositiveNumber} lambda - rate parameter
@returns {Function} probability density function (PDF)

@example
var pdf = factory( 0.5 );
var y = pdf( 3.0 );
// returns ~0.112

y = pdf( 1.0 );
// returns ~0.303