I just refactored my type and transform system prototype (introduced in [1] but changed since then) from using mtlx [2] (type-indexed monad transformers described in [3]) to mtl using RWST. mtlx allowed me to cleanly separate the various monadic components in a convenient way. Unfortunately, I found it to be too slow. The refactoring was an experiment to see how slow. I was rather surprised: Running time of a compiled main with a list of tests: mtlx (7 transformers): 2 min 52 sec mtl (RWST): 0 min 13 sec It's frustrating to see such a huge performance gap for a better design. Regards, Sean [1] http://splonderzoek.blogspot.com/2011/03/draft-type-changing-program-improve... [2] http://hackage.haskell.org/package/mtlx [3] http://www.ittc.ku.edu/~marks/cgi-bin/pubs/monadfactory.pdf