Promises/A+ is not a promise library. It's not a spec about "what should a promise library do". It's about interoperability - "how can I exchange data AND promises between libraries without relying on a shared library/on everyone using the same library for flow control?". Also the spec makes error handling non-surprising, I don't need to read through the docs of a library to know how errors propagate. It looks like your promise library would have been easy to make comply with the spec by just calling "onkeep" "then" (and maybe adapting a few minor details).