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-- These extensions are required so that we can define a class instance for "String".
{-# LANGUAGE TypeSynonymInstances, FlexibleInstances #-}
module Main ( main ) where
import System.Posix.Syslog
import Data.ByteString.Char8 ( ByteString, pack )
import Data.ByteString.Unsafe ( unsafeUseAsCStringLen )
import Foreign.C.String ( CStringLen, withCStringLen )
-- This class allows us to log normal Strings, ByteStrings, and pretty much any
-- other type to syslog without any explicit conversions. It abstracts the
-- information of how to convert the given type into a CStringLen that can be
-- passed to syslog.
class LogMessage m where
toCStringLen :: m -> (CStringLen -> IO a) -> IO a
instance LogMessage String where
toCStringLen = withCStringLen
instance LogMessage ByteString where
toCStringLen = unsafeUseAsCStringLen
-- This simplified syslog interface can deal efficiently with any LogMessage.
-- It relies on the default 'Facility' to be configured globally.
write :: LogMessage a => Priority -> a -> IO ()
write pri msg = toCStringLen msg (syslog Nothing pri)
-- Now write a couple of String and ByteString messages. On my system, the log
-- file shows the following output:
--
-- May 12 19:49:18 myhost example[26995]: Hello, World.
-- May 12 19:49:18 myhost example[26995]: Default logging mask is [Emergency,Alert,Critical,Error,Warning,Notice,Info,Debug]
main :: IO ()
main =
withSyslog "example" [LogPID, Console] User $ do
write Info "Hello, World."
lm <- setlogmask [Debug]
write Info (pack "This message does not show up.")
write Debug ("Default logging mask is " ++ show lm)
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