6.6.1 is a bugfix release over 6.6. Most library APIs have not changed, so code that worked with 6.6 should continue to work with 6.6.1.
Many, many bugs have been fixed relative to 6.6. Far too many to list here.
The other changes in this release are:
GHC works on Windows Vista.
GHC can now be used to compile C++ files.
There is an
--install-signal-handlers=<yes|no>
RTS flag. The main use is to stop GHC installing signal
handlers when you are putting your code in a DLL.
Newtypes can now be defined using GADT syntax.
Linear implicit parameters are no longer accepted.
There is a manpage for ghc and ghci.
The building guide has been moved to the wiki.
GHC now comes with the filepath library.
The base
package has various bug fixes, as well as a couple
of interface changes that shouldn't affect most users.
The version number has been bumped from 2.0/2.1 to 2.1.1.
The Cabal package has had various bug
fixes. The version number has been bumped from 1.1.6 to
1.1.6.2.
The Win32 package has various bug fixes.
The version number has been bumped from 2.0/2.1 to 2.1.1.
The regex-base package has a couple of
bug fixes. The version number has been bumped from 0.71
to 0.72.
The pretty-printer in the
template-haskell package has been
improved. The version number has been bumped from 2.0 to 2.1.
The unix package has various bug fixes.
The version number has been bumped from 1.0/2.0 to 2.1.
There are newer versions of all of the extralibs.