src/Ganeti/Logging/WriterLog.hs

{-# LANGUAGE FlexibleInstances, FlexibleContexts, TypeFamilies,
             GeneralizedNewtypeDeriving,
             StandaloneDeriving, MultiParamTypeClasses, UndecidableInstances,
             CPP #-}

{-| A pure implementation of MonadLog using MonadWriter

-}

{-

Copyright (C) 2014 Google Inc.
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2. Redistributions in binary form must reproduce the above copyright
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-}

module Ganeti.Logging.WriterLog
  ( WriterLogT
  , WriterLog
  , runWriterLogT
  , runWriterLog
  , dumpLogSeq
  , execWriterLogT
  , execWriterLog
  ) where

-- The following macro is just a temporary solution for 2.12 and 2.13.
-- Since 2.14 cabal creates proper macros for all dependencies.
#define MIN_VERSION_monad_control(maj,min,rev) \
  (((maj)<MONAD_CONTROL_MAJOR)|| \
   (((maj)==MONAD_CONTROL_MAJOR)&&((min)<=MONAD_CONTROL_MINOR))|| \
   (((maj)==MONAD_CONTROL_MAJOR)&&((min)==MONAD_CONTROL_MINOR)&& \
    ((rev)<=MONAD_CONTROL_REV)))

import Control.Applicative
import Control.Monad
import Control.Monad.Base
import Control.Monad.IO.Class
import Control.Monad.Trans.Control
import Control.Monad.Writer
import qualified Data.Foldable as F
import Data.Functor.Identity
import Data.Sequence

import Ganeti.Logging

-- * The data type of the monad transformer

type LogSeq = Seq (Priority, String)

type WriterSeq = WriterT LogSeq

-- | A monad transformer that adds pure logging capability.
newtype WriterLogT m a =
  WriterLogT { unwrapWriterLogT :: WriterSeq m a }
  deriving (Functor, Applicative, Alternative, Monad, MonadPlus, MonadIO,
            MonadTrans)

deriving instance (MonadBase IO m) => MonadBase IO (WriterLogT m)

type WriterLog = WriterLogT Identity

-- Runs a 'WriterLogT', returning the result and accumulated messages.
runWriterLogT :: WriterLogT m a -> m (a, LogSeq)
runWriterLogT = runWriterT . unwrapWriterLogT

-- Runs a 'WriterLog', returning the result and accumulated messages.
runWriterLog :: WriterLog a -> (a, LogSeq)
runWriterLog = runIdentity . runWriterLogT

-- | Runs a 'WriterLogT', and when it finishes, resends all log messages
-- to the underlying monad that implements 'MonadLog'.
--
-- This can be used to delay logging messages, by accumulating them
-- in 'WriterLogT', and resending them at the end to the underlying monad.
execWriterLogT :: (MonadLog m) => WriterLogT m a -> m a
execWriterLogT k = do
  (r, msgs) <- runWriterLogT k
  F.mapM_ (uncurry logAt) msgs
  return r

-- | Sends all log messages to the a monad that implements 'MonadLog'.
dumpLogSeq :: (MonadLog m) => LogSeq -> m ()
dumpLogSeq = F.mapM_ (uncurry logAt)

-- | Runs a 'WriterLog', and when it finishes, resends all log messages
-- to the a monad that implements 'MonadLog'.
execWriterLog :: (MonadLog m) => WriterLog a -> m a
execWriterLog k = do
  let (r, msgs) = runWriterLog k
  dumpLogSeq msgs
  return r

instance (Monad m) => MonadLog (WriterLogT m) where
  logAt = curry (WriterLogT . tell . singleton)

instance MonadTransControl WriterLogT where
#if MIN_VERSION_monad_control(1,0,0)
-- Needs Undecidable instances
    type StT WriterLogT a = (a, LogSeq)
    liftWith f = WriterLogT . WriterT $ liftM (\x -> (x, mempty))
                              (f runWriterLogT)
    restoreT = WriterLogT . WriterT
#else
    newtype StT WriterLogT a =
      StWriterLog { unStWriterLog :: (a, LogSeq) }
    liftWith f = WriterLogT . WriterT $ liftM (\x -> (x, mempty))
                              (f $ liftM StWriterLog . runWriterLogT)
    restoreT = WriterLogT . WriterT . liftM unStWriterLog
#endif
    {-# INLINE liftWith #-}
    {-# INLINE restoreT #-}

instance (MonadBaseControl IO m)
         => MonadBaseControl IO (WriterLogT m) where
#if MIN_VERSION_monad_control(1,0,0)
-- Needs Undecidable instances
  type StM (WriterLogT m) a
    = ComposeSt WriterLogT m a
  liftBaseWith = defaultLiftBaseWith
  restoreM = defaultRestoreM
#else
  newtype StM (WriterLogT m) a
    = StMWriterLog { runStMWriterLog :: ComposeSt WriterLogT m a }
  liftBaseWith = defaultLiftBaseWith StMWriterLog
  restoreM = defaultRestoreM runStMWriterLog
#endif
  {-# INLINE liftBaseWith #-}
  {-# INLINE restoreM #-}