[Dn_Ab]

Your code is not doing the same thing. A few minor bits; env has no 'lines' in its definition and String.sub does not allocate objects: it's better to think of String.sub, String.length, etc. as functions packaged in a module of helpful combinators.

OCaml is a language with currying and higher order functions, all that power is wasted if instead of composition you `.Objected` into everything. The advantages of piping, currying and composition on first class functions simply cannot be overstated.

Okay, so the next thing, from String.sub line 0 3 it's clear that you need to know the string length, hence the > 4 can't be avoided. You could pack it away into a function but why waste time if you're not going to write this 3 or more times?

The function `start` takes an env, a string list and returns unit. It's a procedure. The function looks for a string that starts with "+++" and either fails to find and returning () else passes to `modified` (also a procedure, tail recursive) which goes through the list (line :: lines returns the head and the tail of the list) applying modifications to env by tracking which lines were modified. `Lines` is an integer list and `result` is an integer pair list, so hopefully it's clearer now why your code is doing something completely different.