Re: [Caml-list] Objects and polymorphic variants

[Jeremy Yallop <yallop@gmail.com>]

On 2 February 2013 23:18, Reed Wilson <cedilla@gmail.com> wrote:
> What I really want is a signature like this:
> method private method_12 : int -> [ > `One | `Two ]
> method method_123 : int -> [ `One | `Two | `Three ]
> method method_124 : int -> [ `One | `Two | `Four ]
>
> If I replace method_12 with a function outside the class it works fine, but
> for whatever reason method_12 really wants to be the exact same type as
> method_123 and method_124.
>
> Is there any way around this typing requirement for methods?

I think that the problem arises because methods are typed similarly to
mutually-recursive functions.  Unless you give type signatures, both
functions that are marked as mutually recursive and methods are
assumed to be monomorhpic.  For example, in

    let rec f = fun x -> x
        and g = fun x -> f (x + 1)

the types are

    val f : int -> int
    val g : int -> int

i.e. f is assigned the type with which it is used in the body of g.
If you remove the (unnecessary) mutual recursion then the more general
types will be inferred; for example, in

    let f = fun x -> x
    let g = fun x -> f (x + 1)

the types are

    val f : 'a -> 'a
    val g : int -> int

It's also possible to ensure that f is assigned the more general type
by using a type signature:

    let rec f : 'a. 'a -> 'a = fun x -> x
        and g = fun x -> f (x + 1)

With objects, the situation is similar, except that you can't mark
methods non-recursive, so you have to give a type signature to avoid
the monomorphising.  So

   object (self)
     method f = fun x -> x
     method g = fun x -> self#f (x + 1)
   end

receives the type

   < f : int -> int;
     g : int -> int >

whereas

   object (self)
     method f : 'a. 'a -> 'a = fun x -> x
     method g = fun x -> self#f (x + 1)
   end

receives the more general type

   < f : 'a. 'a -> 'a;
     g : int -> int >

In your example you can ensure that the type you want is inferred by
annotating method_12 with a polymorphic signature:

   object (self)
     method private method_12 : 'a. int -> ([> `One | `Two] as 'a) = function
       | 1 -> `One
       | _ -> `Two

     method method_123 = function
      | 3 -> `Three
      | x -> self#method_12 x

     method method_124 = function
      | 4 -> `Four
      | x -> self#method_12 x
   end

Now the inferred types for method_123 and method_125 are distinct:

  < method_123 : int -> [> `One | `Three | `Two ];
    method_124 : int -> [> `Four | `One | `Two ] >

Hope that helps a bit,

Jeremy.