# Log effect

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An extensible log effect using extensible-effects. This library introduces two
new effects to your extensible effects arsenal, `Log` and `LogM`. In short, if
you'd like to add logging to pure code (that is, without Lift effect), `Log` is
your best choice, otherwise go for `LogM`.

## Log

This is the simpler of the two. `Log` allows for logging in pure code, as well
as filtering using `filterLog`. The downside of this effect is that when your
code launches multiple threads using `async` or `forkIO`, messages from every
thread other than the thread where the handler is run will be lost.

## LogM

`LogM` loses the ability to provide logging in pure code, but at the same time
allows to log messages from multiple threads.

## Example

```haskell
import Control.Concurrent.Lifted
import Control.Eff
import Control.Eff.Lift
import Control.Eff.Log

someComp :: ( [ Log String, LogM IO String ] <:: r
            , LiftedBase IO r
            ) => Eff r ()
someComp = do logE "Hello!"
              logM "Greetings from the main thread!"
              
              _ <- fork $ do logM "This is a new thread, and this message is still visible."
                             logE "Unfortunately, this one is not."
              return ()

main :: IO ()
main = runLift $ runLog logger $ runLogM logger $ someComp
  where
    -- Here we have to provide an explicit signature for our logger,
    -- because the compiler is unable to figure it out due to ambiguity.
    logger = stdoutLogger :: Logger IO String
```

## See also

[log-effect-syslog](http://hackage.haskell.org/package/log-effect-syslog)
provides necessary types and functions to work with syslog.