speice.io/_drafts/typed-stack.md

layout	title	description	category	tags
post	Representing Hierarchies - The Reference Stack Pattern			rust

Of late, I've been working to add support for Rust to the Kaitai Struct project. The idea is to describe data formats using a YAML schema, and then generate all the code needed for parsing them. Kind of like if you replaced packages like nom with a YAML document instead of macros in code.

While the project specifics aren't incredibly important, it did force me to take a look at how hierarchies are represented in Rust, something that many people struggle with. The basic problem formulation is simple:

• A root/parent object owns some number of child objects
• Each child needs access to all its parents to do some work

The specifics are what make this a bit complicated:

• Each node in this tree can be of a different (though sometimes predictable) type
• If possible, we'd like to avoid Rc (performance, no_std, pick a reason)

This hierarchical or "DOM-like" structure shows up in two places that I'm familiar with, but is generic enough to be used in a broad range of applications. The first example is parser generators (like Kaitai); as an example, describing the Websocket format requires that every dataframe after the initial know the message type of the first (be it text or binary). The second example is in GUIs, where you typically describe an application as a collection of widgets.

We'll develop a toy DOM-like example as motivation, and look at how it can be extended to accommodate more specific situations as necessary.