I've run into a situation where I need to 'weave' two monads together, but I can't figure out how it's done. The first monad is "Parser a" from Attoparsec, and the second is one that I've created called "CBC a", with the following relevant functions: -- Decrypts a word in cipher-block-chaining mode decryptCBC :: Word128 -> CBC Word128 -- Performs a cipher-block-chaining mode operation with the given -- key and initialization vector. cbc :: Key -> IV -> CBC a -> a I've also defined a special parser for Attoparsec, for extracting the next 128 bit word from the stream: word128 :: Parser Word128 My problem is that I would like to write a function which reads a word, decrypts it, and then uses the result to make a decision about whether to read further words or return a result. So far, I think my function would have a result type of: func :: Parser (CBC foo) but within the function, if I do something like: func = do x <- word128 let r = do y <- decryptCBC x if (predicate y) then return bar else -- somehow read more from the parser? return r But within this function I run into the situation where my types turn into something like Parser (CBC (Parser a)). I have a feeling that I'm just going about this in a wrong way. I realize that I probably don't need to use Parser here, since I'm just reading a stream of words, but I'd still like to learn how to do this in the general case. Is there anyone that could point me in a better direction? Thanks, Ron