function factory( alpha, beta ) {
	var betalnAB;
	if (
		isnan( alpha ) ||
		isnan( beta ) ||
		alpha <= 0.0 ||
		beta <= 0.0
	) {
		return constantFunction( NaN );
	}
	betalnAB = betaln( alpha, beta );
	return logpdf;

	/**
	* Evaluates the natural logarithm of the probability density function (PDF) for a beta distribution.
	*
	* @private
	* @param {number} x - input value
	* @returns {number} evaluated natural logarithm of the PDF
	*
	* @example
	* var y = logpdf( 0.3 );
	* // returns <number>
	*/
	function logpdf( x ) {
		var out;
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x < 0.0 || x > 1.0 ) {
			// Support of the Beta distribution: [0,1]
			return NINF;
		}
		if ( x === 0.0 ) {
			if ( alpha < 1.0 ) {
				return PINF;
			}
			if ( alpha > 1.0 ) {
				return NINF;
			}
			return ln( beta );
		}
		if ( x === 1.0 ) {
			if ( beta < 1.0 ) {
				return PINF;
			}
			if ( beta > 1.0 ) {
				return NINF;
			}
			return ln( alpha );
		}
		out = -betalnAB;
		out += ( ( alpha-1.0 )*ln(x) ) + ( ( beta-1.0 )*log1p(-x) );
		return out;
	}
}
Returns a function for evaluating the natural logarithm of the probability density function (logPDF) for a beta distribution with first shape parameter `alpha` and second shape parameter `beta`.

@param {PositiveNumber} alpha - first shape parameter
@param {PositiveNumber} beta - second shape parameter
@returns {Function} logPDF

@example
var logpdf = factory( 0.5, 0.5 );

var y = logpdf( 0.8 );
// returns ~-0.228

y = logpdf( 0.3 );
// returns ~-0.364