week    actual_ sales(a)     forecast(f)                        forecast(f)                              forecast(f)
 1            20                          f1 = a1                              f1 = 20                                    f1 = 20
 2            27                          f2 = f1 + 0.5*(a1 - f1)        f2 = 20 +0.5(20-20)                f2 = 20
 3            25                          f3 = f2 +  0.5*(a2-f2)         f3 = 20+0.5*(27-20)               f3=23.5
 4            22


When I execute a function to achieve this in Haskell, I get the forecast for the fourth period. Can I save the values for intermediate period(1..3) also?

Please find my Haskell code below:

a = 0.5
ipt = [20,27,25,22]
avg :: [Double] -> Double
avg (x:xs) = (a*x) + (1-a)*(avg xs)

avg [] = 0

Could you please help me with this, thanking you in anticipation