Problem Statement

In many languages, functions can be nested, resulting in outer functions and inner functions. The inner function can access variables from the outer function. In most languages, the inner function can also modify variables in the outer function.

Write a program consisting of two nested functions that prints the following text. The outer function (called MakeList or equivalent) is responsible for creating the list as a whole and is given the separator ". " as argument. It also defines a counter variable to keep track of the item number. This demonstrates how the inner function can influence the variables in the outer function. The inner function (called MakeItem or equivalent) is responsible for creating a list item. It accesses the separator from the outer function and modifies the counter.

Let's start with the solution:

let make_list separator =
  let counter = ref 1 in

  let make_item item =
    let result = string_of_int !counter ^ separator ^ item ^ "\n" in
    incr counter;
    result
  in

  make_item "first" ^ make_item "second" ^ make_item "third"

let () =
  print_string (make_list ". ")