{-# LANGUAGE CPP #-}
{-# LANGUAGE AutoDeriveTypeable #-}
{-# LANGUAGE DefaultSignatures #-}
{-# LANGUAGE DeriveFoldable #-}
{-# LANGUAGE DeriveFunctor #-}
{-# LANGUAGE DeriveTraversable #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE ImplicitParams #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE RecordWildCards #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE UndecidableInstances #-}

module Control.Monad.Log
       ( -- * Introduction
         -- $intro

         -- * Getting Started
         -- $tutorialIntro

         -- ** Working with @logging-effect@
         -- *** Emitting log messages
         -- $tutorial-monadlog

         -- *** Outputting with 'LoggingT'
         -- $tutorial-loggingt

         -- *** Adapting and composing logging
         -- $tutorial-composing

         -- * @MonadLog@
         logMessage, mapLogMessage, mapLogMessageM,
         MonadLog(..),

         -- * Convenience logging combinators
         -- $convenience
         logDebug, logInfo, logNotice, logWarning, logError, logCritical, logAlert, logEmergency,

         -- * Message transformers
         PP.layoutPretty,
         -- ** Timestamps
         WithTimestamp(..), timestamp, renderWithTimestamp,
         -- ** Severity
         WithSeverity(..), Severity(..), renderWithSeverity,
         -- ** Call stacks
         WithCallStack(..), withCallStack, renderWithCallStack,

         -- * @LoggingT@, a general handler
         LoggingT(..), runLoggingT, mapLoggingT,

         -- ** 'LoggingT' Handlers
         Handler, withFDHandler,

         -- *** Batched handlers
         withBatchedHandler, BatchingOptions(..), defaultBatchingOptions,

         -- * Pure logging
         PureLoggingT(..), runPureLoggingT,

         -- * Discarding logs
         DiscardLoggingT(DiscardLoggingT,discardLogging)

         -- * Aside: An @mtl@ refresher
         -- $tutorialMtl
       ) where

import Prelude hiding (foldMap)
import Control.Applicative
import Control.Concurrent.Async (async, wait)
import Control.Concurrent.STM
import Control.Concurrent.STM.Delay
import Control.Monad (MonadPlus, guard)
import Control.Monad.Base
import Control.Monad.Catch (MonadThrow(..), MonadMask(..), MonadCatch(..), bracket)
import Control.Monad.Cont.Class (MonadCont(..))
import Control.Monad.Error.Class (MonadError(..))
import Control.Monad.Fix
import Control.Monad.Free.Class (MonadFree(..))
import Control.Monad.IO.Class (MonadIO, liftIO)
import Control.Monad.IO.Unlift (MonadUnliftIO(..), UnliftIO(..), withUnliftIO)
import Control.Monad.RWS.Class (MonadRWS)
import Control.Monad.Reader.Class (MonadReader(..))
import Control.Monad.State.Class (MonadState(..))
import Control.Monad.Trans.Class (MonadTrans(..))
import Control.Monad.Trans.Control
import Control.Monad.Trans.Reader (ReaderT(..))
import Control.Monad.Trans.State.Strict (StateT(..))
import Control.Monad.Writer.Class (MonadWriter(..))
import Data.Semigroup ((<>))
import Data.Time (UTCTime, getCurrentTime)
#if !MIN_VERSION_base(4, 9, 0)
import GHC.SrcLoc (SrcLoc, showSrcLoc)
import GHC.Stack
#else
import GHC.Stack (SrcLoc, CallStack, getCallStack, prettySrcLoc)
#endif
import System.IO (Handle, hFlush)
import qualified Data.Text.Lazy as LT
import qualified Data.Text.Prettyprint.Doc as PP
import qualified Data.Text.Prettyprint.Doc.Render.Text as PP
import qualified Data.List.NonEmpty as NEL

-- For 'MonadLog' pass-through instances.
import qualified Control.Monad.Trans.Identity as Identity
import qualified Control.Monad.Trans.Reader as Reader
import qualified Control.Monad.Trans.State.Lazy as LazyState
import qualified Control.Monad.Trans.State.Strict as StrictState
import qualified Control.Monad.Trans.Writer.Lazy as LazyWriter
import qualified Control.Monad.Trans.Writer.Strict as StrictWriter
import qualified Control.Monad.Trans.Maybe as Maybe
import qualified Control.Monad.Trans.Except as Except
import qualified Control.Monad.Trans.Error as Error
import qualified Control.Monad.Trans.RWS.Lazy as LazyRWS
import qualified Control.Monad.Trans.RWS.Strict as StrictRWS
import qualified Control.Monad.Trans.Cont as Cont
import qualified Control.Monad.Trans.List as List
import qualified Control.Monad.Trans.Free as Free
import qualified Control.Monad.Trans.Free.Church as Free
import qualified Control.Monad.Catch.Pure as Exceptions

--------------------------------------------------------------------------------
-- | The class of monads that support logging.
--
-- Laws:
--
-- /Monoid homomorphism/:
--
-- @
-- 'logMessageFree' a '*>' 'logMessageFree' b = 'logMessageFree' (a '<>' b)
-- @
class Monad m => MonadLog message m | m -> message where
  -- | Fold log messages into this computation. Looking to just log a
  -- message? You probably want 'logMessage'.
  --
  -- The perhaps strange type here allows us to construct a monoid out of /any/
  -- type of log message. You can think of this as the simpler type:
  --
  -- @
  -- logMessageFree :: [message] -> m ()
  -- @
  logMessageFree :: (forall n. Monoid n => (message -> n) -> n) -> m ()
  default logMessageFree :: (m ~ t n, MonadTrans t, MonadLog message n) => (forall mon. Monoid mon => (message -> mon) -> mon) -> m ()
  logMessageFree inj = lift (logMessageFree inj)
  {-# INLINEABLE logMessageFree #-}

-- | Append a message to the log for this computation.
logMessage :: MonadLog message m => message -> m ()
logMessage m = logMessageFree (\inject -> inject m)
{-# INLINEABLE logMessage #-}

-- | Re-interpret the log messages in one computation. This can be useful to
-- embed a computation with one log type in a larger general computation.
mapLogMessage
  :: MonadLog message' m
  => (message -> message') -> LoggingT message m a -> m a
mapLogMessage f m =
  runLoggingT m
              (logMessage . f)
{-# INLINEABLE mapLogMessage #-}

-- | Monadic version of 'mapLogMessage'. This can be used to annotate a
-- message with something that can only be computed in a monad. See e.g.
-- 'timestamp'.
mapLogMessageM
  :: MonadLog message' m
  => (message -> m message') -> LoggingT message m a -> m a
mapLogMessageM f m =
  runLoggingT m ((>>= logMessage) . f)
{-# INLINEABLE mapLogMessageM #-}

instance MonadLog message m => MonadLog message (Identity.IdentityT m)
instance MonadLog message m => MonadLog message (Reader.ReaderT r m)
instance MonadLog message m => MonadLog message (StrictState.StateT s m)
instance MonadLog message m => MonadLog message (LazyState.StateT s m)
instance (Monoid w, MonadLog message m) => MonadLog message (StrictWriter.WriterT w m)
instance (Monoid w, MonadLog message m) => MonadLog message (LazyWriter.WriterT w m)
instance MonadLog message m => MonadLog message (Maybe.MaybeT m)
instance MonadLog message m => MonadLog message (Except.ExceptT e m)
instance (Error.Error e, MonadLog message m) => MonadLog message (Error.ErrorT e m)
instance (Monoid w, MonadLog message m) => MonadLog message (StrictRWS.RWST r w s m)
instance (Monoid w, MonadLog message m) => MonadLog message (LazyRWS.RWST r w s m)
instance MonadLog message m => MonadLog message (Cont.ContT r m)
instance MonadLog message m => MonadLog message (List.ListT m)
instance (Functor f, MonadLog message m) => MonadLog message (Free.FreeT f m)
instance (Functor f, MonadLog message m) => MonadLog message (Free.FT f m)
instance MonadLog message m => MonadLog message (Exceptions.CatchT m)

--------------------------------------------------------------------------------
-- | Add \"Severity\" information to a log message. This is often used to convey
-- how significant a log message is.
data WithSeverity a =
  WithSeverity {msgSeverity :: Severity -- ^ Retrieve the 'Severity' a message.
               ,discardSeverity :: a -- ^ View the underlying message.
               }
  deriving (Eq,Ord,Read,Show,Functor,Traversable,Foldable)

-- | Classes of severity for log messages. These have been chosen to match
-- @syslog@ severity levels
data Severity =
 Emergency -- ^ System is unusable. By @syslog@ convention, this level should not be used by applications.
 | Alert -- ^ Should be corrected immediately.
 | Critical -- ^ Critical conditions.
 | Error -- ^ Error conditions.
 | Warning -- ^ May indicate that an error will occur if action is not taken.
 | Notice -- ^ Events that are unusual, but not error conditions.
 | Informational -- ^ Normal operational messages that require no action.
 | Debug -- ^ Information useful to developers for debugging the application.
  deriving (Eq,Enum,Bounded,Read,Show,Ord)

instance PP.Pretty Severity where
  pretty = PP.pretty . LT.pack . show

-- | Given a way to render the underlying message @a@, render a message with its
-- severity.
--
-- >>> renderWithSeverity id (WithSeverity Informational "Flux capacitor is functional")
-- [Informational] Flux capacitor is functional
renderWithSeverity
  :: (a -> PP.Doc ann) -> (WithSeverity a -> PP.Doc ann)
renderWithSeverity k (WithSeverity u a) =
  PP.brackets (PP.pretty u) PP.<+> PP.align (k a)

-- | @
-- 'logDebug' = 'logMessage' . 'WithSeverity' 'Debug'
-- @
logDebug :: MonadLog (WithSeverity a) m => a -> m ()
logDebug = logMessage . WithSeverity Debug
{-# INLINEABLE logDebug #-}

-- | @
-- 'logInfo' = 'logMessage' . 'WithSeverity' 'Informational'
-- @
logInfo :: MonadLog (WithSeverity a) m => a -> m ()
logInfo      = logMessage . WithSeverity Informational
{-# INLINEABLE logInfo #-}

-- | @
-- 'logNotice' = 'logMessage' . 'WithSeverity' 'Notice'
-- @
logNotice :: MonadLog (WithSeverity a) m => a -> m ()
logNotice    = logMessage . WithSeverity Notice
{-# INLINEABLE logNotice #-}

-- | @
-- 'logWarning' = 'logMessage' . 'WithSeverity' 'Warning'
-- @
logWarning :: MonadLog (WithSeverity a) m => a -> m ()
logWarning   = logMessage . WithSeverity Warning
{-# INLINEABLE logWarning #-}

-- | @
-- 'logError' = 'logMessage' . 'WithSeverity' 'Error'
-- @
logError :: MonadLog (WithSeverity a) m => a -> m ()
logError     = logMessage . WithSeverity Error
{-# INLINEABLE logError #-}

-- | @
-- 'logCritical' = 'logMessage' . 'WithSeverity' 'Critical'
-- @
logCritical :: MonadLog (WithSeverity a) m => a -> m ()
logCritical  = logMessage . WithSeverity Critical
{-# INLINEABLE logCritical #-}

-- | @
-- 'logAlert' = 'logMessage' . 'WithSeverity' 'Alert'
-- @
logAlert :: MonadLog (WithSeverity a) m => a -> m ()
logAlert     = logMessage . WithSeverity Alert
{-# INLINEABLE logAlert #-}

-- | @
-- 'logEmergency' = 'logMessage' . 'WithSeverity' 'Emergency'
-- @
logEmergency :: MonadLog (WithSeverity a) m => a -> m ()
logEmergency = logMessage . WithSeverity Emergency
{-# INLINEABLE logEmergency #-}

--------------------------------------------------------------------------------
-- | Add a timestamp to log messages.
--
-- Note that while most log message transformers are designed to be used at the
-- point of logging, this transformer is best applied within the handler.
-- This is advised as timestamps are generally applied uniformly, so doing it
-- in the handler is fine (no extra information or context of the program is
-- required). The other reason is that logging with a timestamp requires
-- 'MonadIO' - while the rest of your computation is free to use 'MonadIO',
-- it's best to avoid incurring this constraint as much as possible, as it is
-- generally untestable.
data WithTimestamp a =
  WithTimestamp {discardTimestamp :: a  -- ^ View the underlying message.
                ,msgTimestamp :: UTCTime -- ^ Retireve the time a message was logged.
                }
  deriving (Eq,Ord,Read,Show,Functor,Traversable,Foldable)

-- | Given a way to render the underlying message @a@ and a way to format
-- 'UTCTime', render a message with its timestamp.
--
-- >>> renderWithTimestamp (formatTime defaultTimeLocale rfc822DateFormat) id timestamppedLogMessage
-- [Tue, 19 Jan 2016 11:29:42 UTC] Setting target speed to plaid
renderWithTimestamp :: (UTCTime -> String)
                       -- ^ How to format the timestamp.
                    -> (a -> PP.Doc ann)
                       -- ^ How to render the rest of the message.
                    -> (WithTimestamp a -> PP.Doc ann)
renderWithTimestamp formatter k (WithTimestamp a t) =
  PP.brackets (PP.pretty (LT.pack (formatter t))) PP.<+> PP.align (k a)

-- | Add the current time as a timestamp to a message.
timestamp :: (MonadIO m) => a -> m (WithTimestamp a)
timestamp msg = do
       now <- liftIO getCurrentTime
       pure (WithTimestamp msg now)
{-# INLINEABLE timestamp #-}

--------------------------------------------------------------------------------
-- | Add call stack information to log lines.
--
-- This functional requires that you pass around the call stack via implicit
-- parameters. For more information, see the GHC manual (section 9.14.4.5).
data WithCallStack a = WithCallStack { msgCallStack :: CallStack
                                     , discardCallStack :: a }
  deriving (Functor,Traversable,Foldable,Show)

-- | Given a way to render the underlying message @a@ render a message with a
-- callstack.
--
-- The callstack will be pretty-printed underneath the log message itself.
renderWithCallStack :: (a -> PP.Doc ann) -> WithCallStack a -> PP.Doc ann
renderWithCallStack k (WithCallStack stack msg) =
  k msg <> PP.line <> PP.indent 2 (prettyCallStack (getCallStack stack))

#if MIN_VERSION_base(4, 9, 0)
showSrcLoc :: SrcLoc -> String
showSrcLoc = prettySrcLoc
#endif

prettyCallStack :: [(String,SrcLoc)] -> PP.Doc ann
prettyCallStack [] = "empty callstack"
prettyCallStack (root:rest) =
  prettyCallSite root <> PP.line <> PP.indent 2 (PP.vsep (map prettyCallSite rest))
  where prettyCallSite (f,loc) =
          PP.pretty (LT.pack f) <> ", called at " <>
          PP.pretty (LT.pack (showSrcLoc loc))

-- | Construct a 'WithCallStack' log message.
--
-- This should normally be preferred over just using 'WithCallStack' as it will
-- append a new entry to the stack - pointing to this exact log line. However,
-- if you are creating a combinator (such as a wrapper that logs and throws
-- an exception), you may be better manually capturing the 'CallStack' and
-- using 'WithCallStack'.
withCallStack :: (?stack :: CallStack) => a -> WithCallStack a
withCallStack = WithCallStack ?stack

--------------------------------------------------------------------------------
-- | 'LoggingT' is a very general handler for the 'MonadLog' effect. Whenever a
-- log entry is emitted, the given 'Handler' is invoked, producing some
-- side-effect (such as writing to @stdout@, or appending a database table).
newtype LoggingT message m a =
  LoggingT (ReaderT (Handler m message) m a)
  deriving (Monad,Applicative,Functor,MonadFix,Alternative,MonadPlus,MonadIO,MonadWriter w,MonadCont,MonadError e,MonadMask,MonadCatch,MonadThrow,MonadState s)

instance MonadBase b m => MonadBase b (LoggingT message m) where
  liftBase = lift . liftBase

instance MonadBaseControl b m => MonadBaseControl b (LoggingT message m) where
  type StM (LoggingT message m) a = StM m a
  liftBaseWith runInBase =
    LoggingT (ReaderT (\handler ->
                         liftBaseWith
                           (\runInReader ->
                              runInBase (\(LoggingT (ReaderT m)) ->
                                           runInReader (m handler)))))
  restoreM st = LoggingT (ReaderT (\_ -> restoreM st))

instance MonadUnliftIO m => MonadUnliftIO (LoggingT msg m) where
  askUnliftIO =
    LoggingT . ReaderT $ \h ->
      withUnliftIO $ \u ->
        return (UnliftIO (unliftIO u . flip runLoggingT h))

-- | Given a 'Handler' for a given @message@, interleave this 'Handler' into the
-- underlying @m@ computation whenever 'logMessage' is called.
runLoggingT
  :: LoggingT message m a -> Handler m message -> m a
runLoggingT (LoggingT (ReaderT m)) handler = m handler
{-# INLINEABLE runLoggingT #-}

instance MonadTrans (LoggingT message) where
  lift = LoggingT . ReaderT . const
  {-# INLINEABLE lift #-}

instance MonadReader r m => MonadReader r (LoggingT message m) where
  ask = lift ask
  {-# INLINEABLE ask #-}
  local f (LoggingT (ReaderT m)) = LoggingT (ReaderT (local f . m))
  {-# INLINEABLE local #-}
  reader f = lift (reader f)
  {-# INLINEABLE reader #-}

newtype Ap m = Ap { runAp :: m () }

instance Applicative m => Semigroup (Ap m) where
  Ap l <> Ap r = Ap (l *> r)
  {-# INLINEABLE (<>) #-}

instance Applicative m => Monoid (Ap m) where
  mempty = Ap (pure ())
  {-# INLINEABLE mempty #-}
#if !(MIN_VERSION_base(4,11,0))
  Ap l `mappend` Ap r = Ap (l *> r)
  {-# INLINEABLE mappend #-}
#endif

-- | The main instance of 'MonadLog', which replaces calls to 'logMessage' with calls to a 'Handler'.
instance Monad m => MonadLog message (LoggingT message m) where
  logMessageFree foldMap = LoggingT (ReaderT (\handler -> runAp (foldMap (Ap . handler))))
  {-# INLINEABLE logMessageFree #-}

instance MonadRWS r w s m => MonadRWS r w s (LoggingT message m)

instance (Functor f,MonadFree f m) => MonadFree f (LoggingT message m)

-- | 'LoggingT' unfortunately does admit an instance of the @MFunctor@ type
-- class, which provides the @hoist@ method to change the monad underneath
-- a monad transformer. However, it is possible to do this with 'LoggingT'
-- provided that you have a way to re-interpret a log handler in the
-- original monad.
mapLoggingT :: (forall x. (Handler m message -> m x) -> (Handler n message' -> n x))
            -> LoggingT message m a
            -> LoggingT message' n a
mapLoggingT eta (LoggingT (ReaderT f)) = LoggingT (ReaderT (eta f))
{-# INLINEABLE mapLoggingT #-}

--------------------------------------------------------------------------------
-- | Handlers are mechanisms to interpret the meaning of logging as an action
-- in the underlying monad. They are simply functions from log messages to
-- @m@-actions.
type Handler m message = message -> m ()

-- | Options that be used to configure 'withBatchingHandler'.
data BatchingOptions =
  BatchingOptions {flushMaxDelay :: Int -- ^ The maximum amount of time to wait between flushes
                  ,flushMaxQueueSize :: Int -- ^ The maximum amount of messages to hold in memory between flushes}
                  ,blockWhenFull :: Bool -- ^ If the 'Handler' becomes full, 'logMessage' will block until the queue is flushed if 'blockWhenFull' is 'True', otherwise it will drop that message and continue.
                  }
  deriving (Eq,Ord,Read,Show)

-- | Defaults for 'BatchingOptions'
--
-- @
-- 'defaultBatchingOptions' = 'BatchingOptions' {'flushMaxDelay' = 1000000
--                                          ,'flushMaxQueueSize' = 100
--                                          ,'blockWhenFull' = 'True'}
-- @
defaultBatchingOptions :: BatchingOptions
defaultBatchingOptions = BatchingOptions 1000000 100 True

-- | Create a new batched handler. Batched handlers take batches of messages to
-- log at once, which can be more performant than logging each individual
-- message.
--
-- A batched handler flushes under three criteria:
--
--   1. The flush interval has elapsed and the queue is not empty.
--   2. The queue has become full and needs to be flushed.
--   3. The scope of 'withBatchedHandler' is exited.
--
-- Batched handlers queue size and flush period can be configured via
-- 'BatchingOptions'.
withBatchedHandler :: (MonadIO io,MonadMask io)
                   => BatchingOptions
                   -> (NEL.NonEmpty message -> IO ())
                   -> (Handler io message -> io a)
                   -> io a
withBatchedHandler BatchingOptions{..} flush k =
  do closed <- liftIO (newTVarIO False)
     channel <- liftIO (newTBQueueIO (fromIntegral flushMaxQueueSize))
     bracket (liftIO (async (repeatWhileTrue (publish closed channel))))
             (\publisher ->
                do liftIO (do atomically (writeTVar closed True)
                              wait publisher))
             (\_ ->
                k (\msg ->
                     liftIO (atomically
                               (writeTBQueue channel msg <|>
                                check (not blockWhenFull)))))
  where repeatWhileTrue m =
          do again <- m
             if again
                then repeatWhileTrue m
                else return ()
        publish closed channel =
          do flushAlarm <- newDelay flushMaxDelay
             (messages,stillOpen) <-
               atomically
                 (do messages <-
                       flushAfter flushAlarm <|> flushFull <|> flushOnClose
                     stillOpen <- fmap not (readTVar closed)
                     return (messages,stillOpen))
             mapM_ flush (NEL.nonEmpty messages)
             pure stillOpen
          where flushAfter flushAlarm =
                  do waitDelay flushAlarm
                     isEmptyTBQueue channel >>= guard . not
                     emptyTBQueue channel
                flushFull =
                  do isFullTBQueue channel >>= guard
                     emptyTBQueue channel
                flushOnClose =
                  do readTVar closed >>= guard
                     emptyTBQueue channel
        emptyTBQueue q =
          do mx <- tryReadTBQueue q
             case mx of
               Nothing -> return []
               Just x -> fmap (x :) (emptyTBQueue q)

-- | 'withFDHandler' creates a new 'Handler' that will append a given file
-- descriptor (or 'Handle', as it is known in the "base" library). Note that
-- this 'Handler' requires log messages to be of type 'PP.Doc'. This abstractly
-- specifies a pretty-printing for log lines. The two arguments two
-- 'withFDHandler' determine how this pretty-printing should be realised
-- when outputting log lines.
--
-- These 'Handler's asynchronously log messages to the given file descriptor,
-- rather than blocking.
withFDHandler
  :: (MonadIO io,MonadMask io)
  => BatchingOptions
  -> Handle -- ^ The 'Handle' to write log messages to.
  -> Double -- ^ The @ribbonFrac@ parameter to 'PP.renderPretty'
  -> Int -- ^ The amount of characters per line. Lines longer than this will be pretty-printed across multiple lines if possible.
  -> (Handler io (PP.Doc ann) -> io a)
  -> io a
withFDHandler options fd ribbonFrac width = withBatchedHandler options flush
  where
    flush messages = do
      PP.renderIO
        fd
        (PP.layoutPretty
           (PP.LayoutOptions (PP.AvailablePerLine width ribbonFrac))
           (PP.vsep (NEL.toList messages) <> PP.line'))
      hFlush fd

--------------------------------------------------------------------------------
-- | A 'MonadLog' handler optimised for pure usage. Log messages are accumulated
-- strictly, given that messasges form a 'Monoid'.
newtype PureLoggingT log m a = MkPureLoggingT (StateT log m a)
  deriving (Functor,Applicative,Monad,MonadFix,MonadCatch,MonadThrow,MonadIO,MonadMask,MonadReader r,MonadWriter w,MonadCont,MonadError e,Alternative,MonadPlus)

instance MonadBase b m => MonadBase b (PureLoggingT message m) where
  liftBase = lift . liftBase

instance MonadTransControl (PureLoggingT message) where
    type StT (PureLoggingT message) a = StT (StateT message) a
    liftWith = defaultLiftWith MkPureLoggingT (\(MkPureLoggingT m) -> m)
    restoreT = defaultRestoreT MkPureLoggingT

instance MonadBaseControl b m => MonadBaseControl b (PureLoggingT message m) where
  type StM (PureLoggingT message m) a = ComposeSt (PureLoggingT message) m a
  liftBaseWith     = defaultLiftBaseWith
  restoreM         = defaultRestoreM

-- | Run a computation with access to logging by accumulating a log under its
-- 'Monoid' instance.
runPureLoggingT
  :: Monoid log
  => PureLoggingT log m a -> m (a,log)
runPureLoggingT (MkPureLoggingT (StateT m)) = m mempty
{-# INLINEABLE runPureLoggingT #-}

mkPureLoggingT
  :: (Monad m,Monoid log)
  => m (a,log) -> PureLoggingT log m a
mkPureLoggingT m =
  MkPureLoggingT
    (StateT (\s ->
               do (a,l) <- m
                  return (a,mappend s l)))
{-# INLINEABLE mkPureLoggingT #-}

instance MonadTrans (PureLoggingT log) where
  lift = MkPureLoggingT . lift
  {-# INLINEABLE lift #-}

instance (Functor f, MonadFree f m) => MonadFree f (PureLoggingT log m)

-- | A pure handler of 'MonadLog' that accumulates log messages under the structure of their 'Monoid' instance.
instance (Monad m, Monoid log) => MonadLog log (PureLoggingT log m) where
  logMessageFree foldMap = mkPureLoggingT (return ((), foldMap id))
  {-# INLINEABLE logMessageFree #-}

instance MonadRWS r w s m => MonadRWS r w s (PureLoggingT message m)

instance MonadState s m => MonadState s (PureLoggingT log m) where
  state f = lift (state f)
  {-# INLINEABLE state #-}
  get = lift get
  {-# INLINEABLE get #-}
  put = lift . put
  {-# INLINEABLE put #-}

--------------------------------------------------------------------------------
-- | A 'MonadLog' handler that throws messages away.
--
-- The documentation may appear a bit confusing, but note that the full type of
-- 'discardLogging' is:
--
-- @
-- 'discardLogging' :: 'DiscardLoggingT' message m a -> m a
-- @
newtype DiscardLoggingT message m a =
  DiscardLoggingT {discardLogging :: m a -- ^ Run a 'MonadLog' computation by throwing away all log requests.
                  }
  deriving (Functor,Applicative,Monad,MonadFix,MonadCatch,MonadThrow,MonadIO,MonadMask,MonadReader r,MonadWriter w,MonadCont,MonadError e,Alternative,MonadPlus,MonadState s,MonadRWS r w s,MonadBase b)

instance MonadBaseControl b m => MonadBaseControl b (DiscardLoggingT message m) where
  type StM (DiscardLoggingT message m) a = StM m a
  liftBaseWith runInBase = lift (liftBaseWith (\runInOrig -> runInBase (runInOrig . discardLogging)))
  restoreM = lift . restoreM

instance MonadTrans (DiscardLoggingT message) where
  lift = DiscardLoggingT
  {-# INLINEABLE lift #-}

instance (Functor f,MonadFree f m) => MonadFree f (DiscardLoggingT message m)

-- | The trivial instance of 'MonadLog' that simply discards all messages logged.
instance Monad m => MonadLog message (DiscardLoggingT message m) where
  logMessageFree _ = return ()
  {-# INLINEABLE logMessageFree #-}

{- $intro

@logging-effect@ provides a toolkit for general logging in Haskell programs
and libraries. The library consists of the type class 'MonadLog' to add log
output to computations, and this library comes with a set of instances to help
you decide how this logging should be performed. There are predefined handlers
to write to file handles, to accumulate logs purely, or to discard logging
entirely.

Unlike other logging libraries available on Hackage, 'MonadLog' does /not/
assume that you will be logging text information. Instead, the choice of logging
data is up to you. This leads to a highly compositional form of logging, with
the able to reinterpret logs into different formats, and avoid throwing
information away if your final output is structured (such as logging to a
relational database).

-}

{- $tutorialIntro

@logging-effect@ is designed to be used via the 'MonadLog' type class and
encourages an "mtl" style approach to programming. If you're not familiar with
the @mtl@, this approach uses type classes to keep the choice of monad
polymorphic as you program, and you later choose a specific monad transformer
stack when you execute your program. For more information, see
<#tutorialMtl Aside: An mtl refresher>.

-}

{- $tutorialMtl #tutorialMtl#

If you are already familiar with the @mtl@ you can skip this section. This is not
designed to be an exhaustive introduction to the @mtl@ library, but hopefully
via a short example you'll have a basic familarity with the approach.

In this example, we'll write a program with access to state and general 'IO'
actions. One way to do this would be to work with monad transformers, stacking
'StateT' on top of 'IO':

@
import "Control.Monad.Trans.State.Strict" ('StateT', 'get', 'put')
import "Control.Monad.Trans.Class" ('lift')

transformersProgram :: 'StateT' 'Int' 'IO' ()
transformersProgram = do
  stateNow <- 'get'
  'lift' launchMissles
  'put' (stateNow + 42)
@

This is OK, but it's not very flexible. For example, the transformers library
actually provides us with two implementations of state monads - strict and a
lazy variant. In the above approach we have forced the user into a choice (we
chose the strict variant), but this can be undesirable. We could imagine that
in the future there may be even more implementations of state monads (for
example, a state monad that persists state entirely on a remote machine) - if
requirements change we are unable to reuse this program without changing its
type.

With the @mtl@, we instead program to an /abstract specification/ of the effects
we require, and we postpone the choice of handler until the point when the
computation is ran.

Rewriting the @transformersProgram@ using the @mtl@, we have the following:

@
import "Control.Monad.State.Class" ('MonadState'('get', 'put'))
import "Control.Monad.IO.Class" ('MonadIO'('liftIO'))

mtlProgram :: ('MonadState' 'Int' m, 'MonadIO' m) => m ()
mtlProgram = do
  stateNow <- 'get'
  'liftIO' launchMissles
  'put' (stateNow + 42)
@

Notice that @mtlProgram@ doesn't specify a concrete choice of state monad. The
"transformers" library gives us two choices - strict or lazy state monads. We
make the choice of a specific monad stack when we run our program:

@
import "Control.Monad.Trans.State.Strict" ('execStateT')

main :: 'IO' ()
main = 'execStateT' mtlProgram 99
@

Here we chose the strict variant via 'execStateT'. Using 'execStateT'
/eliminates/ the 'MonadState' type class from @mtlProgram@, so now we only have
to fulfill the 'MonadIO' obligation. There is only one way to handle this, and
that's by working in the 'IO' monad. Fortunately we're inside the @main@
function, which is in the 'IO' monad, so we're all good.

-}

{- $tutorial-monadlog

To add logging to your applications, you will need to make two changes.

First, use the 'MonadLog' type class to indicate that a computation has
access to logging. 'MonadLog' is parameterized on the type of messages
that you intend to log. In this example, we will log a 'PP.Doc' that is
wrapped in 'WithSeverity'.

@
testApp :: 'MonadLog' ('WithSeverity' ('PP.Doc' ann)) m => m ()
testApp = do
  logMessage ('WithSeverity' 'Informational' "Don't mind me")
  logMessage ('WithSeverity' 'Error' "But do mind me!")
@

Note that this does /not/ specify where the logs "go", we'll address that when
we run the program.

-}

{- $tutorial-loggingt

Next, we need to run this computation under a 'MonadLog' effect handler. The
most flexible handler is 'LoggingT'. 'LoggingT' runs a 'MonadLog' computation
by providing it with a 'Handler', which is a computation that can be in the
underlying monad.

For example, we can easily fulfill the 'MonadLog' type class by just using
'print' as our 'Handler':

>>> runLoggingT testApp print
WithSeverity {msgSeverity = Informational, discardSeverity = "Don't mind me"}
WithSeverity {msgSeverity = Error, discardSeverity = "But do mind me!"}

The log messages are printed according to their 'Show' instances, and - while
this works - it is not particularly user friendly. As 'Handler's are just functions
from log messages to monadic actions, we can easily reformat log messages.
@logging-effect@ comes with a few "log message transformers" (such as
'WithSeverity'), and each of these message transformers has a canonical way to
render in a human-readable format:

>>> runLoggingT testApp (print . renderWithSeverity id)
[Informational] Don't mind me
[Error] But do mind me!

That's looking much more usable - and in fact this approach is probably fine for
command line applications.

However, for longer running high performance applications there is a slight
problem. Remember that 'runLoggingT' simply interleaves the given 'Handler'
whenever 'logMessage' is called. By providing 'print' as a 'Handler', our
application will actually block until the log is complete. This is undesirable
for high performance applications, where it's much better to log asynchronously.

@logging-effect@ comes with "batched handlers" for this problem. Batched handlers
are handlers that log asynchronously, are flushed periodically, and have maximum
memory impact. Batched handlers are created with 'withBatchedHandler', though
if you are just logging to file descriptors you can also use 'withFDHandler'.
We'll use this next to log to @STDOUT@:

@
main :: 'IO' ()
main =
  'withFDHandler' 'defaultBatchingOptions' 'stdout' 0.4 80 $ \\logToStdout ->
  'runLoggingT' testApp ('logToStdout' . 'renderWithSeverity' 'id')
@

Finally, as 'Handler's are just functions (we can't stress this enough!) you
are free to slice-and-dice your log messages however you want. As our log
messages are structured, we can pattern match on the messages and dispatch them
to multiple handlers. In this final example of using 'LoggingT' we'll split
our log messages between @STDOUT@ and @STDERR@, and change the formatting of
error messages:

@
main :: IO ()
main = do
  'withFDHandler' 'defaultBatchingOptions' 'stderr' 0.4 80 $ \\stderrHandler ->
  'withFDHandler' 'defaultBatchingOptions' 'stdout' 0.4 80 $ \\stdoutHandler ->
  'runLoggingT' testApp
              (\\message ->
                 case 'msgSeverity' message of
                   'Error' -> stderrHandler ('discardSeverity' message)
                   _     -> stdoutHandler ('renderWithSeverity' id message))
@

>>> main
[Informational] Don't mind me!
BUT DO MIND ME!

-}

{- $tutorial-composing

So far we've considered very small applications where all log messages fit nicely
into a single type. However, as applications grow and begin to reuse components,
it's unlikely that this approach will scale. @logging-effect@ comes with a
mapping function - 'mapLogMessage' - which allows us to map log messages from one
type to another (just like how we can use 'map' to change elements of a list).

For example, we've already seen the basic @testApp@ computation above that used
'WithSeverity' to add severity information to log messages. Elsewhere we might
have some older code that doesn't yet have any severity information:

@
legacyCode :: 'MonadLog' ('PP.Doc' ann) m => m ()
legacyCode = 'logMessage' "Does anyone even remember writing this function?"
@

Here @legacyCode@ is only logging 'PP.Doc', while our @testApp@ is logging
'WithSeverity' 'PP.Doc'. What happens if we compose these programs?

>>> :t runLoggingT (testApp >> legacyCode) (const (pure ()))
  Couldn't match type ‘WithSeverity (Doc ann1)’ with '(Doc ann0)'

Whoops! 'MonadLog' has /functional dependencies/ on the type class which means
that there can only be a single way to log per monad. One solution might be
to 'lift' one set of logs into the other:

>>> :t runLoggingT (testApp >> lift legacyCode) (const (pure ()))
  :: MonadLog (Doc ann) m => m ()

And indeed, this is /a/ solution, but it's not a particularly nice one.

Instead, we can map both of these computations into a common log format:

>>> :t mapLogMessage Left testApp >> mapLogMessage Right (logMessage "Hello")
  :: (MonadLog (Either (WithSeverity (Doc ann)) (Doc ann)) m) => m ()

This is a trivial way of combining two different types of log message. In larger
applications you will probably want to define a new sum-type that combines all of
your log messages, and generally sticking with a single log message type per
application.

-}

{- $convenience

While @logging-effect@ tries to be as general as possible, there is a fairly
common case of logging, namely basic messages with an indication of severity.
These combinators assume that you will be using 'WithSeverity' at the outer-most
level of your log message stack, though no make no assumptions at what is inside
your log messages. There is a @logX@ combinator for each level in 'Severity'.

-}