Question

Well in Ocaml you can do this at compile-time :

let handle item = match item with
   | 1 -> "Do this";
   | 2 -> "Do that";
   | n -> "Do Nothing";
;;

Is there a way to realize it at runtime ? Like some kind of chain of responsibility pattern ?

Answer 1

Yes, there is such a pattern.

Let’s assume that you’re dealing with a function that must accept an integer and return a string, and that by default it will return "Do nothing" for every integer :

let func : (int -> string) ref = ref (fun _ -> "Do nothing") 

let _ = (!func) 1
- : string = "Do nothing"

If you wish to say that "Do this" should be returned when the argument is 1, you can do:

let () = 
  let old = !func in 
  func := (function 1 -> "Do this" | n -> old n)

let _ = (!func) 1
- : string = "Do this"