[Haskell-cafe] new version of the mtl package

[Ross Paterson]

After lengthy discussion on the libraries list, the mtl package has been
updated to depend on the transformers package, from which mtl re-exports
the monad transformers and the MonadTrans and MonadIO classes.  This makes
the two packages compatible.  The monads-fd package is now a deprecated
stub re-exporting the mtl modules.

Ths transformers/mtl-2 combination is under consideration for inclusion
in the Haskell Platform:

    http://trac.haskell.org/haskell-platform/wiki/Proposals/transformers

Discussion is on the libraries list.  So far there have been no objections.

The new version has a few incompatibilities with the old, which will break
a few packages, though the great majority build unchanged.  The above
web page lists the incompatibilities and suggested fixes.  You might
also consider whether the portable transformers package suffices for
your package.

As packages move to the new version, there will be increasing dependency
clashes with packages that currently specify a bound on their mtl
dependency excluding the new version.  In some cases the dependency can
be simply relaxed with no other change, or with small changes obtain
compatibility with both versions.  In other cases one must choose a
version, or use cpp hackery to support both.