A Monad Transformer for explicitly typed checked exceptions, described in detail by:

• Jose Iborra, "Explicitly Typed Exceptions for Haskell", PADL'10, January 2010, http://dl.dropbox.com/s/lgm12trtl0swtra/PADL10.pdf?dl=1

Control.Monad.Exception.Pure provides the classic, Either based monad, whereas Control.Monad.Exception.IO provides a more advanced IO based monad with the ability to handle asynchronous exceptions as well.