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

[Gabriel Scherer <gabriel.scherer@gmail.com>]

What you observe is the so-called "strengthening" of type equalities
in functor applications. See papers 5 or 6 in this list:
  http://caml.inria.fr/about/papers.en.html

It is not a bug, but a feature: you can write functors F such that
applying F(X) twice yields compatible, rather than incompatible,
types. If you want to recover incompatible types, you can seal the
functor result as you did in your workaround, or pass a non-path
functor expression (that behave in a more generative way): F(struct
include X end).

On your more general code:
- I do not understand why you specify the abstract type ('a t) to be
contravariant, and I suspect this will be unsound (is an (< m : int >
t) also an (< m : int; s : string > t)?)
- I am not sure using (Obj.magic (ref 0)) is safe wrt. types whose
representation is not always a pointer (ie. floats)

On Sun, May 6, 2012 at 2:53 PM, Goswin von Brederlow <goswin-v-b@web.de> wrote:
> 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))
>
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