monad-log (hackage): This package provide a mtl style MonadLog class and a concrete monad transformer LogT, the main difference between this package and monad-logger are:  Base monad has to be an instance of MonadIO.  Parametrized logging environment for extensibility.  Basic logging environment type(Label,Loc,NameSpace,ThreadId) are included, and you can easily make your own.  JSON logging built-in.  default to fast-logger backend, with good stdout and file support.  If you are an application author, you can use LogT transformer, it's just a specialized reader monad to inject `Logger env`.  If you are a library author, you should do following two things:  make your monad stack an instance of MonadLog, usually you can do this by embedding `Logger env` into your monad's reader part.  provide a default formatter, and API to run with customized formatter.
Keywords: development|library|mit