Comment by TUSF

Zig already has an Allocator interface that gets passed around, and the convention is that libraries don't select an Allocator. Only provide APIs that accept allocators. If there's a certain process that works best with an Arena, then the API may wrap a provided function in an Arena, but not decide on their own underlying allocator for the user.

For Zig users, adopting this same mindset for Io is not really anything new. It's just another parameter that occasionally needs to be passed into an API.