Hello. I'm trying to understand Monads. In order to do so, I decided to create my own Monad for a simple domain-specific language. The idea is to define a way to describe a multi-value replacement inside do-notation. Example of a function doing what I want (without Monads): replaceAll :: (a -> Maybe a) -> [a] -> [a] replaceAll f xs = go f xs [] where go :: (a -> Maybe a) -> [a] -> [a] -> [a] go _ [] acc = acc go f (x:xs) acc = let acc' = acc ++ [fromMaybe x (f x)] in acc' `seq` go f xs acc' Example of a replacement table: table :: Char -> Maybe Char table x = case x of 'a' -> Just 'b' 'A' -> Just 'B' _ -> Nothing Example of use: \> replaceAll table "All I want" "Bll I wbnt" Now, want I tried to do... As Monads are used for sequencing, first thing I did was to define the following data type: data TableDefinition a = Match a a (TableDefinition a) | Restart So, to create a replacement table: table' :: TableDefinition Char table' = Match 'a' 'b' (Match 'A' 'B' Restart) It look like a Monad (for me), as I can sequence any number of replacement values: table'' :: TableDefinition Char table'' = Match 'a' 'c' (Match 'c' 'a' (Match 'b' 'e' (Match 'e' 'b' Restart))) In order to run the replacement over a list, I've defined the following function: runTable :: Eq a => TableDefinition a -> [a] -> [a] runTable t = go t t [] where go _ _ acc [] = acc go s Restart acc (x:xs) = let acc' = (acc ++ [x]) in acc' `seq` go s s acc' xs go s (Match a b m) acc ci@(x:xs) | x == a = let acc' = (acc ++ [b]) in acc' `seq` go s m acc' xs | otherwise = go s m acc ci The result is still the same: \> runTable table' "All I want" "Bll I wbnt" I'd like to define the same data structure as: newTable :: TableDefinition Char newTable = do 'a' :> 'b' 'A' :> 'B' But I can't figure a way to define a Monad instance for that. :( Can you help me? Thanks, Thiago.