[kentonv]

Cap'n Proto Promises are highly analogous to promises in Javascript. Both are derived directly from the E language, which has been around for decades. It makes sense to stick with the terminology used in similar designs, so that people don't have to re-learn the same concept when switching languages.

C++'s standard library for some reason decided -- relatively recently -- to introduce the term "promise" to mean something different (what most people call a "resolver" or a "fulfiller" for a promise), which is unfortunate. It's C++ that is being inconsistent here.

There are also subtle historical differences between the meaning of "promise" and "future". Historically, futures have usually existed in multi-threaded designs rather than event-loop/callback designs; you wait() on a future, blocking the calling thread, whereas with promises you call promise.then() to register a callback to call when a promise completes. The C++ committee again seems to have gotten this confused with their definition of "future", which looks more like a traditional promise.

[sluukkonen]

I have no idea why the C++ designers chose those particular names, but at least Scala uses the same Promise/Future dichotomy, where you resolve (or reject) a Promise into a Future. Perhaps they drew from the same well?

[moosingin3space]

How do Cap'n Proto Promises and Rust futures relate?

[dwrensha]

They have a lot in common!

For a while, capnp-rpc-rust used `gj::Promise`, which is based directly on the C++ Cap'n Proto implementation of promises (i.e. `kj::Promise`). Back in January, capnp-rpc-rust was updated to use `futures::Future` instead, and it was a fairly straightforward transition, as described in this blog post: https://dwrensha.github.io/capnproto-rust/2017/01/04/rpc-fut...

The trickiest part of the transition was dealing with scheduling. The implementation of `kj::Promise` has a built-in scheduling queue that guarantees a certain form of deterministic FIFO semantics, and those semantics are heavily depended upon in the Cap'n Proto RPC implementation. Rust's `future::Future` is less batteries-included, requiring capnp-rpc-rust to explicitly create queues where deterministic scheduling is needed.

Confusing the terminology perhaps even more, in capnproto-rust there is a type `capnp::capability::Promise` that implements `futures::Future`.