Single Pass Recursion in Rust

This is the third post in a three-post series. In the first post we developed a stack-safe, ergonomic, and concise method for working with recursive data structures (using a simple expression language as an example). In the second post we made it fully generic, providing a set of generic tools for expanding and collapsing any recursive data structure in Rust.

In this post we will see how to combine these two things - expanding a structure and collapsing it at the same time, performing both operations in a single pass. In the process, we will gain the ability to write arbitrary recursive functions over traditional boxed-pointer recursive structures (instead of the novel RecursiveTree type introduced in my previous post) while retaining stack safety.

execution graph for simultaneously expanding and collapsing a simple expression

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Fully generic recursion in Rust

Previously, we introduced a method for writing performant stack safe recursion in Rust for a single recursive data structure. This post uses the same ideas to implement a single recursion backend that can collapse or expand any recursive data structure.
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