[lmm]

True, but what you can't do is abstract over them; you can't write a function that operates on "some generic computation expression" (you can't even write that type). So you can't write "sequence" or "traverseM" or more complicated/specific things you build on top of these.

[yohanatan]

There is an encoding supporting that level of abstraction (perhaps though not as straightforward as you'd prefer); namely using interfaces (which are first-class) to encode module signatures. See: https://gist.github.com/t0yv0/192353