function factory( lambda ) {
	if ( isnan( lambda ) || lambda < 0.0 || lambda === PINF ) {
		return constantFunction( NaN );
	}
	return logpdf;

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

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

@example
var logpdf = factory( 0.5 );
var y = logpdf( 3.0 );
// returns ~-2.193

y = logpdf( 1.0 );
// returns ~-1.193