Haskell Platform call for consensus: add transformers and revise the mtl package to depend on it

[Ross Paterson]

On Thu, Nov 04, 2010 at 08:36:24PM -0500, Jeremy Shaw wrote:
> I intend to upload a new version of happstack 0.5 which has the proper
> constraints. And happstack 6 will be migrated to transformers/monads-fd.

mtl-2.0.0.0 is now a copy of what monads-fd was, and monads-fd is now
a deprecated stub re-exporting mtl-2.0.0.0.  So you'll want to migrate
to transformers/mtl-2.

But in your case, on line 578 of src/Happstack/Server/SimpleHTTP.hs:

    getFilter     m = WebT $ ErrorT $ fmap lft $ getFilter (runErrorT $ unWebT m)

If you replace fmap with liftM, it will build with both old and new
versions of the mtl package.