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For a NOSQL layer -- I'm looking for the answer to that same question myself!  We've been experimenting with Cassandra (used via the hscassandra package based in turn on cassandra-thrift).  Already it's clear that there are many areas that need work.  The Haskell code generated by Thrift itself has a lot of room for improvement (for the intrepid hacker: cycles there would be well-spent).