The big-picture view is here: https://gitlab.com/owl-lisp/owl/-/blob/master/doc/manual.md

Key points include:

- 100% immutable datastructures

- Immutability is leveraged to make a lot of core operations concurrent

- Continuation-based threading model and Actor-based concurrency

- Fun little VM implemented behind the scenes

That being said, the documentation strongly contradicts the title!

> The goal has not at any point been to become an ultimate Lisp and take over the world

I'm wondering if a language could optimize immutable data structures to use mutable, in place semantics instead of duplication or structural sharing when it is possible. When an immutable data structure only has one consumer (only has one descendant in the flow graph), then it can easily be turned into a mutable version. Generalizing, any linear subgraph in the program's flow graph could be made to use in place semantics on the same mutable variable.

The challenge I guess is figuring out how variables captures by lambdas should be dealt with.