Control.Monad.Journal.Class

MonadWriter on steroids.

MonadJournal is a more controlable version of MonadWriter because it enables you to access the Monoid being computed up. You can then access logs inside the computation itself, whereas you cannot with MonadWriter – unless you use specific functions like listen, but that still stacks Monoid in the monad.

Typically, you can use MonadJournal when you come across the logging problem and you need logs as long as you proceed.

MonadJournal

class (Monoid w, Monad m) => MonadJournal w m | m -> w where

This typeclass provides the ability to accumulate Monoid in a monad via the journal function; to get them via the history function and finally, to purge them all with the clear function.

In most cases, you won’t need history neither clear. There’s a cool function that combines both and enables you to deal with the Monoid: sink.

Methods

journal :: w -> m ()

Log something.

history :: m w

Extract the logs history.

clear :: m ()

Clear the logs history.

Instances

(Monad m, Monoid w, MonadJournal w m) => MonadJournal w (MaybeT m)

(Monad m, Monoid w, MonadJournal w m) => MonadJournal w (ListT m)

(Monad m, Monoid w, MonadJournal w m) => MonadJournal w (IdentityT m)

(Monad m, Monoid w, MonadJournal w m) => MonadJournal w (EitherT e m)

(Monad m, Monoid w, Monoid q, MonadJournal w m) => MonadJournal w (WriterT q m)

(Monad m, Monoid w, MonadJournal w m) => MonadJournal w (StateT s m)

(Monad m, Monoid w, MonadJournal w m) => MonadJournal w (ReaderT r m)

(Monoid w, Monad m) => MonadJournal w (JournalT w m)

(Monad m, Monoid w, MonadJournal w m) => MonadJournal w (RWST r w s m)

sink :: (MonadJournal w m, MonadIO m) => (w -> IO ()) -> m ()

Sink all logs history through MonadIO then clean it.

absorb :: MonadJournal w m => (a, w) -> m a

Absorb a logs history and pass around the value.