In OCaml, scoping determines the visibility and accessibility of variables in different parts of the program. Variables are only accessible within the scope in which they are defined.

Scoping in OCaml is lexical, meaning that the scope of a variable is determined by its position in the source code. Variables defined in a certain block of code are only accessible within that block and its nested blocks.

Here’s an example to illustrate scoping:

let x = 1 in
let y = 2 in
  let z = x + y in (* z is accessible here *)
    (* z is not accessible here *)

In this example, the variable x is defined at the top level and is accessible within the entire function. The variable y is defined at the next level and is accessible within the scope of the function and its nested blocks. The variable z is defined in the innermost block and is only accessible within that block.

Shadowing occurs when a variable is defined with the same name as an existing variable in the current scope. In this case, the new variable “shadows” the existing variable and any access to the variable within the scope of the new variable will refer to the new variable instead of the existing one.

Here’s an example to illustrate shadowing:

let x = 1 in
let x = x + 1 in (* x is shadowed here *)
  print_int x (* prints 2 *)

In this example, the variable x is defined at the top level with the value 1, then it is shadowed by a new variable x which is defined with the value of the previous variable x + 1, therefore the value of the variable x is 2.

It’s worth noting that shadowing is generally considered to be bad practice, as it can make the code harder to read and understand, and can lead to unexpected behavior if not handled carefully.