What goes in a Stack package.yaml file?

Question

I notice that Stack supports using a package.yaml file that it will use to generate a .cabal file. For example, yi-core/package.yaml. Its structure

Answer 1

This is hpack, available here: https://github.com/sol/hpack

It's a slightly different package format, and, as you say, can be used to generate cabal files. It isn't directly tied to stack.