input: from typing import List
  
  def predict_temperature(temperature_list: List[float]) -> float:
      """ Predicts the next day's forecasted temperature using the exponential smoothing algorithm
      given the past three day's forecasted temperatures.
      >>> predict_temperature([21, 22, 20])
      21.0
      """
output:     alpha = 0.5
      return alpha * temperature_list[-1] + (1 - alpha) * (alpha * temperature_list[-2] + (1 - alpha) * temperature_list[-3])