[Caml-list] Implementation for a (nearly) typesafe shallow option type and a compiler bug?

[Goswin von Brederlow <goswin-v-b@web.de>]

Hi,

as mentioned earlier I wanted to have a

    type 'a shallow = NULL | 'a

that is like an 'a option but without the indirection.

The first idea was that no ocaml value can be the C NULL pointer (usualy
all bits set to 0) and the C NULL pointer points outside the ocaml heap
so the GC is fine with that too. So NULL could be encoded as C NULL
pointer and 'a used as is.

First problem is that this does not work for an 'a shallow shallow. And
there is no way to constrain 'a to not be a 'b shallow in the type
definition.

Second problem is that someone else might define a 'a shallow2 type with
the same idea and 'a shallow2 shallow would also not work. And with
abstract types it would be impossible to test for that even if ocaml
would allow negative constraints.


The common problem here is that NULL is not unique for each type. So I
thought of a way to make it unique. What I came up with is using a 
functor to create a unique NULL value for each instance. Source code at
the end.

Output:
NULL -> None
Some 5 -> Some 5
NULL -> None
Some NULL -> Some None
Some Some 5 -> Some Some 5
1 2 3 4 
a b c d 
NULL -> Some 70221466564508

As you can see from the output this solves the problem of 'a shallow
shallow and would also solve the 'a shallow2 shallow case.

But in the last line a new problem appears. Each instance of the functor
has a unique NULL element. But instances of the functor with the same
type are compatible. So an IntShallow2.t can be used instead of an
IntShallow.t but then the NULL does not match.

To me this looks like a bug in ocaml because the resulting type of the
functor application does not match the type I specified for the functor:

  module type SHALLOW =
    sig
      type -'a t
      val null : 'a t
      val make : 'a -> 'a t
      val as_option : 'a t -> 'a option
    end
  module Make : functor (Type : TYPE) -> SHALLOW

module IntShallow2 :
  sig
    type 'a t = 'a Shallow.Make(Int).t
    val null : 'a t
    val make : 'a -> 'a t
    val as_option : 'a t -> 'a option
  end

The type I specified has 'a t abstract while the IntShallow2 has the
type 'a t more concret.

Restricting the type to what it should already be results in the correct
error:

module IntShallow2 = (Shallow.Make(Int) : Shallow.SHALLOW)
let () =
  Printf.printf "NULL -> %s\n" (to_string (IntShallow2.null))

File "shallow.ml", line 91, characters 42-60:
Error: This expression has type 'a IntShallow2.t
       but an expression was expected of type
         int IntShallow.t = int Shallow.Make(Int).t

Why is that?

MfG
        Goswin

======================================================================
module Shallow : sig
  module type TYPE = sig type 'a t end
  module type SHALLOW =
    sig
      type -'a t
      val null : 'a t
      val make : 'a -> 'a t
      val as_option : 'a t -> 'a option
    end
  module type SHALLOWFUNCTOR = functor (Type : TYPE) -> SHALLOW
  module Make : functor (Type : TYPE) -> SHALLOW
end = struct
  module type TYPE = sig type 'a t end
  module type SHALLOW =
    sig
      type -'a t
      val null : 'a t
      val make : 'a -> 'a t
      val as_option : 'a t -> 'a option
    end
  module type SHALLOWFUNCTOR = functor (Type : TYPE) -> SHALLOW

  module Make_intern =
    functor (Type : TYPE) ->
      struct
	type -'a t
	(* Dummy object unique to the type *)
	let null = Obj.magic (ref 0)
	let make x = Obj.magic x
	let as_option x = if x == null then None else Some (Obj.magic x)
      end
  module Make = (Make_intern :  functor (Type : TYPE) -> SHALLOW)
end

module Int = struct type 'a t = int end
  
module IntShallow = Shallow.Make(Int)
module IntShallowShallow = Shallow.Make(IntShallow)

let to_string x =
  match IntShallow.as_option x with
    | None -> "None"
    | Some x -> Printf.sprintf "Some %d" x

let to_string2 x =
  match IntShallowShallow.as_option x with
    | None -> "None"
    | Some x -> Printf.sprintf "Some %s" (to_string x)

module List = struct
  module Item = struct
    type 'a t = 'a
  end

  module Shallow = Shallow.Make(Item)

  type 'a item = { mutable next : 'a list; data : 'a; }
  and 'a list = 'a item Shallow.t

  let null = Shallow.null
  let cons x y = Shallow.make { next = y; data = x; }
  let rec iter fn x =
    match Shallow.as_option x with
      | None -> ()
      | Some { next; data; } -> fn data; iter fn next
end

let () =
  Printf.printf "NULL -> %s\n" (to_string (IntShallow.null));
  Printf.printf "Some 5 -> %s\n" (to_string (IntShallow.make 5));
  Printf.printf "NULL -> %s\n" (to_string2 (IntShallowShallow.null));
  Printf.printf "Some NULL -> %s\n"
    (to_string2 (IntShallowShallow.make IntShallow.null));
  Printf.printf "Some Some 5 -> %s\n"
    (to_string2 (IntShallowShallow.make (IntShallow.make 5)));
  let x = List.null in
  let x = List.cons 4 x in
  let x = List.cons 3 x in
  let x = List.cons 2 x in
  let x = List.cons 1 x in
  List.iter (Printf.printf "%d ") x;
  print_newline ();
  let y = List.null in
  let y = List.cons "d" y in
  let y = List.cons "c" y in
  let y = List.cons "b" y in
  let y = List.cons "a" y in
  List.iter (Printf.printf "%s ") y;
  print_newline ()

module IntShallow2 = Shallow.Make(Int)

let () =
  Printf.printf "NULL -> %s\n" (to_string (IntShallow2.null))