I see your overall point which is a good one and I know this is a nitpick, but I thought Rich’s core solution to the cost of immutable data structures was to find a way to get the cost down by extending some existing research by Phil Bagwell.

From https://dl.acm.org/doi/pdf/10.1145/3386321

“I then set out to find a treelike implementation for hash maps which would be amenable to the path-copying with structural sharing approach for persistence. I found what I wanted in hash array mapped tries (HAMTs) [Bagwell 2001]. I built (in Java) a persistent implementation of HAMTs with branching factor of 32, using Java’s fast System.arrayCopy during path copying. The node arrays are freshly allocated and imperatively manipulated during node construction, and never mutated afterwards. Thus the implementation is not purely functional but the resulting data structures are immutable after construction. I designed and built persistent vectors on similar 32-way branching trees, with the path copying strategy. Performance was excellent, more akin to O(1) than the theoretical bounds of O(logN). This was the breakthrough moment for Clojure. Only after this did I feel like Clojure could be practical, and I moved forward with enthusiasm to release it later that year (2007).”

Or you tell me what I’m missing. Big fan of your work in core.async if this is the same halgari.