[yohanatan]

> F# has a number of these specific implementations but they're kind of "hard-coded" in the language rather than fully extensible.

That's not true. You can extend the language yourself with "computation expressions" (and that's in fact how the 'async', 'let!' et al keywords are implemented).

[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