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

	/**
	* Evaluates the moment-generating function (MGF) for a Poisson distribution.
	*
	* @private
	* @param {number} t - input value
	* @returns {number} evaluated MGF
	*
	* @example
	* var y = mgf( 0.5 );
	* // returns <number>
	*/
	function mgf( t ) {
		return exp( lambda * ( exp(t) - 1.0 ) );
	}
}
Returns a function for evaluating the moment-generating function (MGF) of a Poisson distribution with mean parameter `lambda`.

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

@example
var mgf = factory( 2.0 );
var y = mgf( 0.1 );
// returns ~1.234