*single*recursion backend that can collapse or expand any recursive data structure.

# 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

# Stack Machines for Free

Previously we saw how to expand seed values into recursive structures given a function that expands a single layer of structure, and how to collapse recursive structures into a single value given a function that consumes a single layer of structure. Here we'll see how to fuse those two steps, to generate a stack machine that simultaneously expands and collapses recursive structures, given just a function for expanding layers and a function for collapsing layers.