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

	/**
	* Evaluates the cumulative distribution function (CDF) for an exponential distribution.
	*
	* @private
	* @param {number} x - input value
	* @returns {Probability} evaluated CDF
	*
	* @example
	* var y = cdf( 2.0 );
	* // returns <number>
	*/
	function cdf( x ) {
		if ( x < 0.0 ) {
			return 0.0;
		}
		return 1.0 - exp( -lambda * x );
	}
}
Returns a function for evaluating the cumulative distribution function (CDF) for an exponential distribution with rate parameter `lambda`.

@param {PositiveNumber} lambda - rate parameter
@returns {Function} CDF

@example
var cdf = factory( 0.5 );
var y = cdf( 3.0 );
// returns ~0.777

y = cdf( 1.0 );
// returns ~0.393