Majorly improved support for Windows platforms. Binary builds are now entirely freestanding. There is no longer any need to install Cygwin or Mingwin to use it. It's a one-click-install-and-off-you-go story now.
Several small changes to bring GHC into line with the newest Haskell 98 report.
RTS options can now be specified using the environment variable GHCRTS.
The maximum heap size, set with +RTS -Msize, now defaults to unlimited. We recommend however that you set a maximum heap size appropriate for your machine using the GHCRTS variable.
Thanks to the heroic efforts of Ken Shan <email@example.com>, GHC now works again on the Alpha architecture (just Tru64 at the moment), and many 64-bit bugs have been shaken out. Interactive mode and the native code generator do not currently work, sorry.
New option: -fno-code which causes GHC to omit code generation and all future compilation stages (see Section 4.12.5).
New option -package-conf file. This makes GHC read additional package descriptions from file. See Section 4.10.2.
+RTS -iseconds flag reinstated for heap profiling. See Section 5.5.
-funbox-strict-fields now works correctly. See Section 4.11.2.
Many bug fixes related to implicit-parameter support.
Data declarations with no constructors are allowed. This facilitates types whose only value is bottom, and which have no representation.
You can disconnect numeric syntax from the Prelude syntax. Doing this means you can define your own arithmetic. The relevant flag is -fno-implicit-prelude. See Section 7.1.
GHCi now works on Windows.
Partial FFI support in GHCi. At the moment, foreign import (static and dynamic) are supported on x86 and sparc platforms.
New interpreter command :add, to add modules to the current bunch. See Section 3.6.
New interpreter command :info, to provide information on values, types and classes, a la Hugs. See Section 3.6.
Multiple target modules are supported. You may specify multiple modules for the :add and :load commands. From the command line, you may specify multiple modules when starting GHCi and when compiling in --make mode. See Section 3.6.
New flags -ignore-dot-ghci and -read-dot-ghci. See Section 3.8.
When starting GHCi from the command line, you may now use the flags -Llibrary-path and -llibrary-name to specify libraries to be loaded into the interactive session. See Section 3.5.
The SocketPrim and BSD modules in the net package have been re-written to use the FFI, be more portable, and be more thread-safe. There are minor user-visible changes: the functions writeSocket, readSocket and readSocketAll have been removed (use socketToHandle and Handle operations instead).
New function: Exception.throwDynTo. See Section 4.12.5 in Haskell Libraries.
BlockedOnDeadMVar and ThreadKilled exceptions no longer generate any output by default for forked threads.
Weak.addForeignFinalizer is deprecated; use the ForeignPtr library instead.
The I/O library has been completely rewritten, using the new FFI libraries and hsc2hs. The main improvement is to the way streams are handled: simultaneous read and write with multiple threads using a socket or FIFO is now possible, whereas before it was necessary to use two separate handles.
As a result of the I/O rewrite, the extensions IOExts.hConnectTo and IOExts.withHandleFor have been removed. Also, hGetBuf and hPutBuf have been removed, and hGetBufFull and hPutBufFull have been renamed to hGetBuf and hPutBuf respectively (similary for the BA versions of these functions).
System.exitWith now throws ExitException rather than causing the program to exit immediately. ExitException is by default caught by the top level exception handler in the main thread, where it causes the program to exit. Hence, calling System.exitWith in GHCi no longer causes GHCi itself to exit.
New function: MVar.addMVarFinalizer (see Section 2.4.3 in Haskell Libraries).
New module SystemExts for useful system-ish functionality (see Section 4.37 in Haskell Libraries).
SocketPrim: added instances of Eq and Show for the Socket type.
The implementation of MarsalAlloc.allocaBytes now uses GHC's internal allocator and is much faster than before. As a result, the UnsafeCString type and functions are no longer necessary and have been removed from CString.
Partial support for hierarchical module names.
Mucho hacking on the .NET code generator, including some FFI extensions for .NET interop. It's still severely b0rk3d, so won't do anything useful. Yet.
Emission of external Core format. The goal is for other tools to be able to grab the Core resulting from GHC's front end manglings and optimisations. Core format is formally defined by the document An External Representation of the GHC Core Language in the GHC repository. We also have sample tools for reading, writing and typechecking external Core. Ultimately we plan that Core files can also be read by GHC. The relevant flag is -fext-core.
A compacting garbage collector has been added. It isn't on by default, because it is somewhat slower than the existing copying collector, but can be automatically enabled when memory gets tight by setting the maximum heap size (see Section 4.16.2). Compaction works together with the existing generational scheme: it is only used on the oldest generation. Despite the "experimental" classification, we've jumped up and down quite significantly on this code, and it seems stable.
The implementation of the Array, IArray and MArray libraries have been rewritten so that bulk operations like listArray and elems do less range checking and hence go faster.
The CPUTime library has been replaced by one written using the FFI. No externally visible changes.
The compiler has a new strictness-and-absence analyser, which is alleged to help it generate better code.