Ropes are a scalable string implementation: they are designed for efficient operation that involve the string as a whole such as concatenation and substring. This library implements ropes for OCaml (it is rich enough to replace strings).
The easier way to install this library is to use opam:
opam install rope
To compile the development version, you will need to install jbuilder and then issue
jbuilder build @install
To run the tests, install the module Benchmark and do