We’re pleased to announce the fifth release of the Haskell Platform: a single, standard Haskell distribution for everyone.
The specification, along with installers (including Windows, Apple and
Unix installers for a full Haskell environment) are available.
The Haskell Platform is a single, standard Haskell distribution for every system, in the form of a blessed library and tool suite for Haskell distilled from the thousands of libraries on Hackage, along with installers for a wide variety of systems. It saves developers work picking and choosing the best Haskell libraries and tools to use for a task.
When you install the Haskell Platform, you get the latest stable compiler, an expanded set of core libraries, additional development tools, and cabal-install – so you can download anything else you need from Hackage.
What you get is specified here.
— The Platform Infrastructure Team
Control.Monad.Writer 
Can you something about the future plans? I’d hoped that a lot of useful and/or essential libraries would be included soon.
Currently i can only see the “binary”-proposal:
http://trac.haskell.org/haskell-platform/query?status=new&status=assigned&status=reopened&group=milestone&component=Package%20Addition&order=priority
The current proposals are: add text, add transformers.