Removing the old notation was certainly not solely motivated by wanting to free up the keyword, but the reason that it remains reserved is in anticipation of future use, rather than simple negligence (though of course this decision could be reversed before 1.0).

Right, I guess I meant that it's not just for HKT, but for something useful.

Anyway, none of this particularly matters.

F# doesn't have HKT, but still has a variant of do syntax via "computation expressions" http://msdn.microsoft.com/en-us/library/dd233182.aspx. It's less elegant than HKT because you have to name the monad used for the expression i.e. io { exprs }, and also involves additional boilerplate code to define them - but accomplishes many of the same objectives.

The problem is that without HKT you can't abstract over these things; you can't write useful functions like "sequence", and so code that uses these expressions becomes a kind of second-class citizen that can't be refactored the way you'd do with normal code.

Of course, HKT is preferable, just in the error reporting context (i.e. this particular example) I'm not sure much the addt'l abstraction (functions like sequence) further solves the problem, seems like the expressions described may be sufficient.

But I agree, in general, you absolutely want HKT for the reasons you mentioned.

Said another way, if HKT in rust is doable, let's do that - but if that turns out not to be the case, there are some nice conpromises such as this example, which I think, at least, is better than the proposed ? Operator, because it is a bit more general/versatile.