Lazy +Control.Monad.Trans.Writer +transformers
The lazy WriterT monad transformer, which adds collection of outputs (such as a count or string output) to a given monad.
This version builds its output lazily; for a strict version, see Control.Monad.Trans.Writer.Strict, which has the same interface.
This monad transformer provides only limited access to the output during the computation. For more general access, use Control.Monad.Trans.State instead.