The code in this repository implements a purely functoinal Rope data structure, using the balancing scheme (with slight modifications to suit Ropes) from Ralf Hinze's 1-2 Brother Trees paper.
The file /lib/tiny_rope.ml is a standard UTF-8 rope with the core insert, delete, substring and to string operations. There are also implementations of some functions from the standard library's String module.
The file at /lib/utf8_rope.ml is exactly the same, but it allows line queries as well (enabled by tracking line break information in an array). It is missing the implementation of functions from the String module.
In the /bin/ folder, all files except dune, utils.ml and main.ml are arrays generated from the sequential edit traces at the linked repository. (The files utils.ml and main.ml just apply the edit traces for benchmarking purposes.)
Both of the mentioned files under the /lib/ directory are under the permissive 0BSD license. Feel free to use them in any way you like.
There are F# and Standard ML ports of tiny_rope.ml which are also under 0BSD.