> The first thing to note is that there is no indication that foo or bar can fail.

I don't see how this is different from traditional programming.

> I do think this is a really neat concept, but I have major reservations about the readability/debuggability of the resulting code.

I don't think that readability will be harmed, but I can understand your concerns about debugging. I feel that cluttering the code with consistency/error checks everywhere actually harms much more readability.