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

	/**
	* Evaluates the moment-generating function (MGF) for an exponential distribution.
	*
	* @private
	* @param {number} t - input value
	* @returns {number} evaluated MGF
	*
	* @example
	* var y = mgf( 0.5 );
	* // returns <number>
	*/
	function mgf( t ) {
		if ( isnan( t ) || t >= lambda ) {
			return NaN;
		}
		return lambda / ( lambda - t );
	}
}
Returns a function for evaluating the moment-generating function (MGF) of an exponential distribution with rate parameter `lambda`.

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

@example
var mgf = factory( 4.0 );
var y = mgf( 3.0 );
// returns 4.0

y = mgf( 0.5 );
// returns ~1.143