Re: [Caml-list] Question on private type abbreviations

[Jacques Garrigue <garrigue@math.nagoya-u.ac.jp>]

On 2015/07/24 23:04, immanuel litzroth wrote:
> 
> I have a question related to private type abbreviations
> I'm interfacing C++ and ocaml and I want to make sure that the ranges of integer types are correct and reflect them in the ocaml interface.
> 
> So I define
> type uint8 = private int
> and 
> type int8 = private int
> same for the other sizes/signedness
> and the appropriate functions to do range checking (those are external and use 
> std::numeric limits)
> external uint8 : int -> uint8 = "make_uint8"
> ... 
> this gives typesafety and avoids boxing/unboxing and makes sure that the user can
> only pass values that are range checked at the earliest opportunity.
> 
> Now I wanna check my code
> for all the types I wanna use 1 checking function something like this:
> 
> let test_conversions   (the_fun : int -> 't)  (the_val : int) =
>   try
>     let the_t = the_fun the_val in
>     Printf.printf "Numbers are %d\n" (the_t : 't :> int)
>   with
>   | Invalid_argument str -> Printf.printf "Error: %s" str
> 
> let () = test_conversions uint8 1 -> will work
> ..
> let () = test_conversions uint64  (-1) -> will print Error...
> 
> Now this doesn't typecheck because the type  var 't in the signature is too general.
> what I need to put there is "a type coercible to int"
> Is that possible? Is there some way to achieve this?

I see no way to do that implicitly.
Namely, subtyping is only checked for coercions, so if you don’t write a coercion for
each of your types, this won’t work.
This means you need to add another parameter:

let test_conversions  (coerce : ’t -> int)  (the_fun : int -> 't)  (the_val : int) =
  try
    let the_t = the_fun the_val in
    Printf.printf "Numbers are %d\n” (coerce the_t)
  with
  | Invalid_argument str -> Printf.printf "Error: %s” str

let from_uint8 x : uint8 :> int = x
let from_uint64 x : uint64 :> int = x

let () = test_conversions from_uint8 uint8 1
..
let () = test_conversions from_uint64 uint64  (-1) 

Jacques Garrigue