You don't want to know how long the `mean`function took, just because I added the spec `myLength :: [a] -> Int`

CH03.hs

myLength :: (Num a) => [t] -> a
myLength []     = 0 
myLength (_:xs) = 1 + myLength xs

mySum :: (Num a) => [a] -> a

mySum [] = 0 
mySum (x:xs) = x + (mySum xs)

mean :: Fractional a => [a] -> Maybe a

mean :: Fractional a => [a] -> Maybe a

mean [] = Nothing
mean xs = Just (sum / len)
       where sum = mySum xs
             len = myLength xs