Cabal does not install the version 4.7.0.0 of base

Question

I need the new function "isLeft" of Data.Either v. 4.7.0.0, but cabal does not install the new library.

Microsoft Windows [Version 6.1.7601]
Copyright (c) 2009 Microsoft Corporation.  All rights reserved.

C:\Users\Alberto>cabal update
Downloading the latest package list from hackage.haskell.org
Note: there is a new version of cabal-install available.
To upgrade, run: cabal install cabal-install

C:\Users\Alberto>cabal install base
Resolving dependencies...
All the requested packages are already installed:
base-4.6.0.1
Use --reinstall if you want to reinstall anyway.

C:\Users\Alberto>date
The current date is: 19/04/2014

Why this happens?

What to do to upgrade the base package to version 4.7 ?

Answer 1

base is a special package. You cannot upgrade it. If you want base-4.7, you'll have to use GHC 7.8.