function pdf( x, lambda ) {
	var scale;
	if (
		isnan( x ) ||
		isnan( lambda ) ||
		lambda < 0.0 ||
		lambda === PINF
	) {
		return NaN;
	}
	if ( x < 0.0 ) {
		return 0.0;
	}
	scale = 1.0 / lambda;
	return exp( -x / scale ) / scale;
}
Evaluates the probability density function (PDF) for an exponential distribution with rate parameter `lambda` at a value `x`.

@param {number} x - input value
@param {PositiveNumber} lambda - rate parameter
@returns {number} evaluated PDF

@example
var y = pdf( 0.3, 4.0 );
// returns ~1.205

@example
var y = pdf( 2.0, 0.7 );
// returns ~0.173

@example
var y = pdf( -1.0, 0.5 );
// returns 0.0

@example
var y = pdf( 0, NaN );
// returns NaN

@example
var y = pdf( NaN, 2.0 );
// returns NaN

@example
// Negative rate:
var y = pdf( 2.0, -1.0 );
// returns NaN