Speaking of monads, one of the themes in Prof. Sussman's "Adventures in advanced symbolic programming" course was that many metacircular evaluator experiments c

Speaking of monads, one of the themes in Prof. Sussman's "Adventures in advanced symbolic programming" course was that many metacircular evaluator experiments can be set up by using mostly the same core evaluator but swapping out the underlying monad. I'm pretty sure I implemented the "ambivalent operator" that way, using the list monad (in Scheme), but when we lifted amb to the host Scheme, it needed call/cc to be able to use the built-in control flow. One thing I like about Haskell is that it's designed so that adding new control flow is not a big deal -- just design a monad. In Scheme it seems to be more work to get it all right.