Cache.hs

{-# LANGUAGE LambdaCase #-}
module MBug.Cache (cachedIO, cachedIO_) where
import qualified Data.ByteString.Lazy           as BL
import           Data.Text                      (Text)
import qualified Data.Text                      as T
import           Data.Time.Clock                (diffUTCTime, getCurrentTime)
import           System.Directory
  ( createDirectoryIfMissing
  , doesFileExist
  , getModificationTime
  )
import           System.Environment.XDG.BaseDir
  ( getUserCacheDir
  , getUserCacheFile
  )

-- | Cache file is stale if is does not exist or if its modification
-- time is more than 15 minutes from now in past.
isCacheFileStale :: FilePath -> IO Bool
isCacheFileStale path =
  doesFileExist path >>= \case
    False -> pure False
    True  -> do
      now   <- getCurrentTime
      mtime <- getModificationTime path
      pure $ diffUTCTime now mtime < 900

-- | Perform IO action labeled with some 'Text' value, if no actions
-- with same label were executed for last 15 minutes; return path to
-- file, containing cached value and value itself.
cachedIO :: Text -> IO BL.ByteString -> IO (FilePath, BL.ByteString)
cachedIO label action = do
  getUserCacheDir "mbug" >>= createDirectoryIfMissing True
  cache <- getUserCacheFile "mbug" (T.unpack label)
  isCacheFileStale cache >>= \case
    True  -> (,) <$> pure cache <*> BL.readFile cache
    False -> action >>= \output -> do BL.writeFile cache output
                                      pure (cache, output)

-- | Same as 'cachedIO', but ignore information about cache file.
cachedIO_ :: Text -> IO BL.ByteString -> IO BL.ByteString
cachedIO_ label action = fmap snd $ cachedIO label action