tips on using monads
I've got one of those algorithms which "threatens to march off the right edge" (in the words of Goerzen et al). I need something like a State or Maybe monad, but this is inside the IO monad. So I presume I need StateT or MaybeT. However, I'm still (slowly) learning about monads from first principles. I thought I might present my code and get some pointers... maybe someone could actually show me how to rewrite it, which would be a neat way to see MaybeT and StateT in action. I'm hoping to get anything from a one-line response to a rewrite of my code. Anything will help. Here's the program: {- This is a program which starts with a document containing "notes" about software requirements (in a particular format) and puts them into a database. Notes include details such as the "source" of the requirement (who gave it), the topic(s) to which it pertains, the date, etc. I have written a parser to take a text document typed up by me during a meeting and parse it into a NoteRecord structure. Here is the structure: -} data NoteRecord = NoteRecord { recordSource :: String, -- Name of person who gave req. recordDate :: [Int], -- Date in [<year>,<month>,<date>] recordSourceType :: String, -- "meeting", "phone", "email", etc. recordBugNum :: Maybe Int, -- Bugzilla # (if relevant) recordTopics :: [String], -- list of official topics pertaining recordText :: String } -- the text of the note itself deriving (Show) {- One other wrinkle. The source (person name) and topic must be one of a set of pre-determined strings. A person has an official full name which is stored in the database. Topics also have official descriptive strings. If I wasn't clever, then the note, as I type it up, must have the exact name and topic. But I hate trying to remember things like that. So I have implemented a "fuzzy string match" system so that I can type part of someone's name (or even misspell it) or part of a topic string, and the system will find the best match to an official string. In pseudocode, the function to insert a note in the database must do this: This function starts with a NoteRecord. - If text already exists in the database, give an error and skip to end. - Fuzzy-match strings to topics and source. - If no potential match can be found to some of topics or source, give error and skip to end. - Ask user to confirm if the matched topics and source look okay. - if user says no, skip to end. - Actually insert the record. -} insertNote :: NoteRecord -> Connection -> IO () insertNote nr conn = do -- Check if it exists in the database already. status <- checkPreExistingText nr conn if status then putStrLn "Skipping... text exists already." else do -- Find best fit for all topics and source. -- See type signatures below. bestFitTopics <- fitTopics nr conn bestFitSource <- fitSource nr conn case any isNothing bestFitTopics of True -> putStrLn "Error... some topic couldn't be matched." False -> case bestFitSource of Nothing -> putStrLn "Error.. source couldn't be matched." _ -> do b <- isUserOkay nr bestFitTopics bestFitSource if b then do -- Create a new NoteRecord with matched -- and validated topics/source. nrValidated = nr { recordTopics = bestFitTopics , recordSource = bestFitSource } insertRow nrValidated conn else putStrLn "Abort due to user request." checkPreExistingText :: NoteRecord -> Connection -> Bool -- There are multiple topics in the NoteRecord. For each one, -- find the best fuzzy match, or indicate if there is no plausible -- match at all. fitTopics :: NoteRecord -> Connection -> [Maybe String] -- There is one source. Try to find fuzzy match. fitSource :: NoteRecord -> Connection -> Maybe String -- Present user with all fuzzy matches and get a yes/no response if it's -- okay to proceed. isUserOkay :: NoteRecord -> [Maybe String] -> Maybe String -> Bool -- Do actual insert into database. insertRow :: NoteRecord -> Connection -> IO ()
Michael P Mossey wrote:
I've got one of those algorithms which "threatens to march off the right edge" (in the words of Goerzen et al). I need something like a State or Maybe monad, but this is inside the IO monad. So I presume I need StateT or MaybeT. However, I'm still (slowly) learning about monads from first principles. I thought I might present my code and get some pointers... maybe someone could actually show me how to rewrite it, which would be a neat way to see MaybeT and StateT in action. I'm hoping to get anything from a one-line response to a rewrite of my code. Anything will help.
Here's a version using ErrorT from mtl. I added some missing IO bits on your types; the type error that remains is for you to fix :-) With ErrorT you can use throwError when you want to break out of the block and give back an error, which seems to fit what you were doing. The downside is you have to add all these liftIO bits wherever you do a plain IO computation. insertNote :: NoteRecord -> Connection -> IO () insertNote nr conn = either putStrLn return =<< runErrorT (do -- Check if it exists in the database already. status <- liftIO $ checkPreExistingText nr conn when status $ throwError "Skipping... text exists already." -- Find best fit for all topics and source. -- See type signatures below. bestFitTopics <- liftIO $ fitTopics nr conn bestFitSource <- liftIO $ fitSource nr conn case any isNothing bestFitTopics of True -> throwError "Error... some topic couldn't be matched." False -> case bestFitSource of Nothing -> throwError "Error.. source couldn't be matched." _ -> do b <- liftIO $ isUserOkay nr bestFitTopics bestFitSource when (not b) $ throwError "Abort due to user request." -- Create a new NoteRecord with matched -- and validated topics/source. let nrValidated = nr { recordTopics = bestFitTopics , recordSource = bestFitSource } liftIO $ insertRow nrValidated conn ) checkPreExistingText :: NoteRecord -> Connection -> IO Bool fitTopics :: NoteRecord -> Connection -> IO [Maybe String] fitSource :: NoteRecord -> Connection -> IO (Maybe String) isUserOkay :: NoteRecord -> [Maybe String] -> Maybe String -> IO Bool insertRow :: NoteRecord -> Connection -> IO () Thanks, Neil.
On Mon, May 18, 2009 at 3:08 AM, Neil Brown
With ErrorT you can use throwError when you want to break out of the block and give back an error, which seems to fit what you were doing.
Of course, now that you are using throwError, you can remove a lot of the extra indentation:
insertNote :: NoteRecord -> Connection -> IO () insertNote nr conn = either putStrLn return =<< runErrorT (do -- Check if it exists in the database already. status <- liftIO $ checkPreExistingText nr conn when status $ throwError "Skipping... text exists already." -- Find best fit for all topics and source. -- See type signatures below. bestFitTopics <- liftIO $ fitTopics nr conn bestFitSource <- liftIO $ fitSource nr conn when (any isNothing bestFitTopics) $ throwError "Error... some topic couldn't be matched." when (isNothing bestFitSource) $ throwError "Error.. source couldn't be matched." b <- liftIO $ isUserOkay nr bestFitTopics bestFitSource when (not b) $ throwError "Abort due to user request." -- Create a new NoteRecord with matched -- and validated topics/source. let nrValidated = nr { recordTopics = bestFitTopics, recordSource = bestFitSource } liftIO $ insertRow nrValidated conn )
-- ryan
I've got one of those algorithms which "threatens to march off the right edge" (in the words of Goerzen et al). I need something like a State or Maybe monad, but this is inside the IO monad. So I presume I need StateT or MaybeT. However, I'm still (slowly) learning about monads from first principles. I thought I might present my code and get some pointers... maybe someone could actually show me how to rewrite it, which would be a neat way to see MaybeT and StateT in action. I'm hoping to get anything from a one-line response to a rewrite of my code. Anything will help.
Perhaps this is useful: http://www.haskell.org/haskellwiki/Equational_reasoning_examples#Coding_styl... Claus
participants (4)
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Claus Reinke -
Michael P Mossey -
Neil Brown -
Ryan Ingram