data-has

A simple extensible product system, a typical usage is to free you from considering how to layer your monad stack, because your can now extend your monad in one layer:

 {-# LANGUAGE FlexibleContexts #-}

 -- in some library code
 ...
 logInAnyReaderHasLogger :: (Has Logger r, MonadReader r m) => LogString -> m ()
 logInAnyReaderHasLogger s = asks getter >>= logWithLogger s

 queryInAnyReaderHasSQL :: (Has SqlBackEnd r, MonadReader r m) => Query -> m a
 queryInAnyReaderHasSQL q = asks getter >>= queryWithSQL q
 ...

 -- now you want to use these effects together
 ...
 logger <- initLogger  ...
 sql <- initSqlBackEnd ...

 (`runReader` (logger, sql)) $ do
       ...
       logInAnyReaderHasLogger ...
       ...
       x <- queryInAnyReaderHasSQL ...
       ...
...