Representing data more compactly but unsafely

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Do you know about separate tagging in OCaml ? That is avoiding boxing by having a tag and its attached payload in separate values. I will show a very simple example, but it is surprising what you can achieve with it.

Here is an example of the different ways of programming a function that take either a float or an int and return the closest int.

type my_adt = TI of int | TF of float

(* Safe but uses unnecessary boxing *)
let f_safe = function TI i -> i | TF f -> int_of_float f

type my_tags = TI | TF

(* very unsafe but no useless boxing *)
let f_unsafe tag n =
  match tag with
  | TI ->
      (Obj.magic n : int)
  | TF ->
      int_of_float (Obj.magic n : float)

type _ my_gadt = TI : int my_gadt | TF : float my_gadt

(* Same runtime behaviour as f_unsafe, but safe. *)
let f_gadt : type t. t my_gadt -> t -> int =
 fun tag n -> 
  match tag with 
  |  TI -> n 
  | TF -> int_of_float n
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