​meanList :: (Fractional a) => [a] -> a

meanList xs = (sumList xs) / (lengthList xs)

sumList :: (Fractional a) => [a] -> a

sumList [] = 0

sumList (x:xs) = x + (sumList xs)

lengthList :: (Fractional a) => [t] -> a

lengthList [] = 0

lengthList (_:xs) = 1 + (lengthList xs)