Nice! This approach can be used to implement coeffects (as e.g. seen in Hack [1]), by only passing explicit effect-producing objects. Imagine a function that's guaranteed to not write to any files, because it can't access `open()`, or can only access a version of `open()` that only accepts read-only access.

[1]: https://docs.hhvm.com/hack/contexts-and-capabilities/introdu...