Daniel Fischer wrote: Thanks for your quick reply.
Rebuild the libraries that depend on mtl against mtl-2.
$ ghc-pkg unregister mtl-1.1.0.2
should tell you which packages are affected, unregister those first (or later after unregistering mtl-1.1.0.2 with --force) and reinstall them against mtl-2. That doesn't work however, if any of them have a dependency bound mtl < 2, so check that first (and even if they have no upper bound on mtl, they may use the State or Writer, ... data constructors which aren't available anymore since State etc. are now type synonyms).
I had already tried that and I knew I had a lot of packages depending on mtl-1.
Or upgrade to ghc-7.0.3 and install everything fresh.
I feared this might be the answer :-) I'm about to buy a new computer so this is probably the way I will go. Many thanks for your help, Dominic.