**Packages**- mtl
- transformers

A monad transformer that combines ReaderT, WriterT and StateT. This version is lazy; for a strict version, see Control.Monad.Trans.RWS.Strict, which has the same interface.

Lazy state monads, passing an updatable state through a computation. See below for examples.
In this version, sequencing of computations is lazy. For a strict version, see Control.Monad.Trans.State.Strict, which has the same interface.
Some computations may not require the full power of state transformers:
* For a read-only state, see Control.Monad.Trans.Reader.
* To accumulate a value without using it on the way, see Control.Monad.Trans.Writer.

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.