For the moment I've chosen to do the mdo desugaring manually instead of moving to the CVS ghc (or exclusively using Hugs.) Will mdo be in the next release? (That's 5.04.2, I think -- though I've recently been confused about GHC releases.) The relevant revision to Lex.lhs is pretty recent and doesn't seem to be on ghc-5-04-branch.

The current CVS-head for GHC contains the mdo implementation (thanks to Simon PJ). I'm not sure what they're going to call it (i.e. 5.04.2, or 5.05, etc.), but the next release will have mdo for sure. I don't know when they'll make a new release though, maybe one of the Simon's can comment on that? -Levent.