Haskell: exception handling in non-IO monads

exception, haskell, monads, yesod

Solution

You can try something like shown below, basically `Handler` is `HandlerT` which is monad transformer (I haven't type checked the code below :))

tryInsert :: Text -> Text -> Text -> Handler Bool
tryInsert email user pwd = HandlerT (\d -> do pwdbs <- hashedPwd pwd
                                              catch (unHandlerT (runDB $ insert $ User email user pwdbs) d >> return True)
                                                    (\e -> return False))

And check the returned bool value if there was exception or not.

Problem

I started using Yesod to develop a little project, this is the first time I use Haskell to do something real. This code that handles a registration form works fine: ``` postRegisterR :: Handler () postRegisterR = do email <- runInputPost $ ireq textField "email" user <- runInputPost $ ireq textField "user" pwd <- runInputPost $ ireq textField "pwd" cpwd <- runInputPost $ ireq textField "cpwd" if pwd == cpwd && isValidEmail email then do tryInsert email user pwd setSession "user" user redirectUltDest SessionR else do redirect HomeR tryInsert :: Text -> Text -> Text -> Handler () tryInsert email user pwd = do pwdbs <- liftIO $ hashedPwd pwd _ <- runDB $ insert $ User email user pwdbs return () ``` Now the problem is: if I sign in twice with the same credentials I get an `InternalServerError`. This is right, because in my model configuration there is `UniqueUser email username`. So I'd like to catch and handle this error in some way. How can I do that and, in general, how exception handling works in Haskell when you are dealing with non-IO monads defined in an external library or framework? PS: I read this tutorial, but that is useful if you are designing a new library. I tryed to use the catch function, but I got a lot of type errors. Edit Thank you Ankur, your code worked with a little modification, to remove this error: ``` Ambiguous type variable `e0' in the constraint: (Exception e0) arising from a use of `catch' Probable fix: add a type signature that fixes these type variable(s) ``` code: ``` tryInsert :: Text -> Text -> ByteString -> Handler Bool tryInsert email user pwd = HandlerT (\d -> catch (unHandlerT (runDB $ insert $ User email user pwd) d >> return True) (\(e :: SomeException) -> return False)) ``` With `ScopedTypeVariables` extension enabled Edit 2 Final version, after bennofs' hint: ``` {-# LANGUAGE ScopedTypeVariables #-} import Control.Exception.Lifted (catch) import Control.Monad (void) postRegisterR :: Handler () postRegisterR = do email <- runInputPost $ ireq textField "email" user <- runInputPost $ ireq textField "user" pwd <- runInputPost $ ireq textField "pwd" cpwd <- runInputPost $ ireq textField "cpwd" if pwd == cpwd && isValidEmail email then do pwdbs <- liftIO $ hashedPwd pwd success <- tryInsert email user pwdbs case success of True -> do setSession "user" user redirectUltDest SessionR False -> redirect HomeR else do redirect HomeR tryInsert :: Text -> Text -> ByteString -> Handler Bool tryInsert email user pwd = do void $ runDB $ insert $ User email user pwd return True `catch` (\(e :: SomeException) -> do return False) ```

Original source