It's not particularly worse than monad transformers with respect to non-commutative effects. With monad transformers, one can trip up just as happily on excepti

It's not particularly worse than monad transformers with respect to non-commutative effects. With monad transformers, one can trip up just as happily on exceptions, and mtl-style stack-polymorphic code can be run with various effect orderings with possibly varying semantics. I failed to mention though that Eff would also benefit from some boost to type inference for open sums. Currently it's throwing up hands for component effects with polymorphic inner type parameters, like `State s` or `Reader r`, so we can't do things like `put 0`, but only `put (0 :: Int)` (in the absence of other type annotations in outer scopes). It's fortunately not an impossible problem; we just need to allow for unique projections from sums in our type-level machinery (this requires some annoying boilerplate currently, but maybe GHC 8.0 will make it easier with the new kind system).