Is it possible to define an exception inside a function

问题

One way to implement "early returns" in OCaml is via exceptions:

exception Exit

let myfunc () = 
    try
      for i = 0 to .... do
        if .. then raise Exit
      done; false
    with Exit -> true

However, is there a way to declare this Exit exception in the body of the function, so its name is not visible to other functions in the module?

(* I would like to do this, but it gives a syntax error *)
let myfunc () = 
    exception Exit
    try
      for i = 0 to .... do
        if .. then raise Exit
      done; false
    with Exit -> true

回答1:

Yes, what you want is possible by using a local module:

let myfunc () = 
  let module M = struct exception Exit end in
  try
    for i = 0 to 3 do
      if true then raise M.Exit
    done; false
  with M.Exit -> true

That style is not particularly pleasant to read, though, so I wouldn't recommend it. It is enough to omit showing Exit at the next module interface if you want to hide it from most of the rest of the program.

来源：https://stackoverflow.com/questions/33312718/is-it-possible-to-define-an-exception-inside-a-function