Haskell programm

Hey! Can anybody help me with this simple Haskell program? I need it to calculate derivate in a random point. I have no idea how to program in Haskell, I just need this one program! I get an error with this code I found online(the function main' is not defined in module main)

derivate function point step = result function point (1.0/step) 0
result function point estimate result | otherwise = (function(point + (estimate/2)) - function(point - (estimate/2)))/estimate

f1 x = 1
f2 x = x*x*x
f3 x = x * exp (x**2)

Thank you!!!