input: from typing import List
  
  def predict_temperature(temperature_list: List[float]) -> float:
      """This function uses the exponential smoothing algorithm to predict the next day's forecasted temperature 
      given the past three day's forecasted temperatures.
      >>> predict_temperature([21, 22, 20])
      20.5
      """
output:     alpha = 0.5
      smoothed_value = alpha * temperature_list[-1] + (1 - alpha) * (alpha * temperature_list[-2] + (1 - alpha) * temperature_list[-3])
      return smoothed_value