[Haskell-beginners] How does this function append the log to the beginning of the list?

[jugree at lavabit.com]

Hello.

Could you explain this example(0)? Could you show its step by step
execution?

gcd' :: Int -> Int -> Writer (DiffList String) Int
gcd' a b
    | b == 0 = do
        tell (toDiffList ["Finished with " ++ show a])
        return a
    | otherwise = do
        result <- gcd' b (a `mod` b)
        tell (toDiffList [show a ++ " mod " ++ show b ++ " = " ++ show (a
`mod` b)])
        return result

Why does the above append the log to the beginning of the list?

What value will result have in the following?

result <- gcd' 2 (3 `mod` 2)

(0) http://learnyouahaskell.com/for-a-few-monads-more#writer