Journal.hs

-----------------------------------------------------------------------------
--
-- Module      :  Data.Journal
-- Copyright   :  (c) 2017 Brian W Bush
-- License     :  MIT
--
-- Maintainer  :  Brian W Bush <consult@brianwbush.info>
-- Stability   :  Stable
-- Portability :  Portable
--
-- | Simple logging to a journal.
--
-----------------------------------------------------------------------------


{-# LANGUAGE FlexibleContexts #-}


module Data.Journal (
-- * Types
  Key
, Entry
, Journal(..)
) where


import Control.Monad.Except (MonadError, MonadIO)
import Data.ByteString.Char8 (ByteString)


-- | Journal entries are keyed by byte strings.
type Key = ByteString


-- | Journal entries are byte strings.
type Entry = ByteString


-- | Type class for logging to a journal.
class Journal a where

  -- | Append a log entry.
  append :: (MonadIO m, MonadError String m)
         => a            -- ^ The journal.
         -> (Key, Entry) -- ^ The key and value to be logged.
         -> m ()         -- ^ An action to peform the logging.

  -- | Erase a log entry.
  erase :: (MonadIO m, MonadError String m)
        => a    -- ^ The journal.
        -> Key  -- ^ The key for the entry to be erased.
        -> m () -- ^ An action to erase the log entry.

  -- | Replay the journal to extract all log entries.
  replay :: (MonadIO m, MonadError String m)
         => Bool             -- ^ Whether to compress the journal, erasing obsolete entries.
         -> a                -- ^ The journal.
         -> m [(Key, Entry)] -- ^ An action listing all log entries.

  -- | Erase all log entries from the journal.
  clear :: (MonadIO m, MonadError String m)
        => a    -- ^ The journal.
        -> m () -- ^ An action to erase all log entries.

  -- | Close a journal.
  close :: (MonadIO m, MonadError String m)
        => a    -- ^ The journal.
        -> m () -- ^ An action to close the journal.