Also, why were monad comprehensions removed from Haskell? Was it for efficiency?
No, it was to help beginners. When list comprehensions, and other list operators like ++ and filter were overloaded, then errors involving the class system became much more common, and in particular cropped up for simple list
I now understand the problem. However, I'm not sure that the solution is the right one. I think that it makes more sense to change the way that Haskell deals with the Prelude. I think the way this issue is handled in DrScheme (www.plt-scheme.org) is pretty reasonable. There are a number of different "language levels" available. In the beginner's mode, complex operations are prohibited, and (I think) some operations have restricted semantics. It would seem to make sense for Haskell implementations to provide something similar: a prelude customized for beginners would have no overloaded functions, a prelude for experts would have many of them. Special syntactic forms like comprehensions and do notation can be specialized to the language mode. Examples for beginner's mode: monad comprehensions become list comprehensions do notation is restricted to IO numbers are not overloaded: 4,5,6::Integer and 4.0,5.0,6.0::Float This kind of multi-level system would allow beginner's an easy introduction without forcing people who know what they're doing to remember bizarre names for simple overloaded operators. David Feuer