Without knowing how exactly capnproto promise pipelining works, when I thought about it, I was concerned about cases like reading a directory and stating everything in it, or getting back two response values and wanting to pass only one to the next call. The latter could be made to work, I guess, but the former depends on eg the number of values in the result list.
In the actual implementation, when making a pipelined call on a result X, you actually say something like "Call X.foo.bar.baz()" -- that is, you can specify a nested property of the results which names the object that you actually want to operate on.
At present, the only operation allowed here is reading a nested property, and that seems to solve 99% of use cases. But one could imagine allowing other operations, like "take the Nth element of this array" or even "apply this call to all elements in the array, returning an array of results".
