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Private and type constraints
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Date: -- (:)
From: Dario Teixeira <darioteixeira@y...>
Subject: Private and type constraints
Hi,

With 'private', you can make sure that users of a module can only use
constructors to create values, while at the same type maintaining the
ability to pattern-match (which abstracting the type makes impossible).
With 3.10, this can also be done with polymorphic variants:

module Foobar:
sig
    type foo_t = [ `A ]
    type bar_t = [ `B ]
    type foobar_t = [ foo_t | bar_t ]
    type t = private [< foobar_t ]

    val make_a: unit -> t
    val make_b: unit -> t
end =
struct
    type foo_t = [ `A ]
    type bar_t = [ `B ]
    type foobar_t = [ foo_t | bar_t ]
    type t = foobar_t

    let make_a () = `A
    let make_b () = `B
end


# open Foobar;;
# let ola = make_a ();;
val ola : Foobar.t = `A
# match ola with `A -> true | `B -> false;;
- : bool = true
# let ola2 : Foobar.t = `A;;
Characters 22-24:
  let ola2 : Foobar.t = `A;;
                        ^^
Error: This expression has type [> `A ] but is here used with type Foobar.t
       Types for tag `A are incompatible


Now, I want to do the same for a type 't' that is declared using a type constraint.
There are however two problems: first, the code below is accepted by the toplevel,
but not by the ocamlc/ocamlopt compilers (I'm running version 3.11+dev15); second,
while as expected it prevents the creation of Foobar2.t values without using the
constructor functions, pattern-matching is unfortunately also disallowed.

module Foobar2:
sig
    type foo_t = [ `A ]
    type bar_t = [ `B ]
    type foobar_t = [ foo_t | bar_t ]
    type 'a t = private 'a constraint 'a = [< foobar_t ]

    val make_a: unit -> foo_t t
    val make_b: unit -> bar_t t
end =
struct
    type foo_t = [ `A ]
    type bar_t = [ `B ]
    type foobar_t = [ foo_t | bar_t ]
    type 'a t = 'a constraint 'a = [< foobar_t ]

    let make_a () = `A
    let make_b () = `B
end


# open Foobar2;;
# let ola = make_a ();;
val ola : Foobar2.foo_t Foobar2.t = `A
# match ola with `A -> true;;
Characters 15-17:
  match ola with `A -> true;;
                 ^^
Error: This pattern matches values of type [< `A ]
       but is here used to match values of type Foobar2.foo_t Foobar2.t
# let ola2 : Foobar2.foo_t Foobar2.t = `A;;
Characters 37-39:
  let ola2 : Foobar2.foo_t Foobar2.t = `A;;
                                       ^^
Error: This expression has type [> `A ] but is here used with type
         Foobar2.foo_t Foobar2.t


This is the error produced by the compiler:

Error: The implementation constraint.ml
       does not match the interface (inferred signature):
       Modules do not match:
         sig
           type foo_t = [ `A ]
           type bar_t = [ `B ]
           type foobar_t = [ `A | `B ]
           type 'a t = 'a Foobar2.t constraint 'a = [< foobar_t ]
           val make_a : unit -> foo_t t
           val make_b : unit -> bar_t t
         end
       is not included in
         sig
           type foo_t = [ `A ]
           type bar_t = [ `B ]
           type foobar_t = [ `A | `B ]
           type 'a t = private 'a constraint 'a = [< foobar_t ]
           val make_a : unit -> foo_t t
           val make_b : unit -> bar_t t
         end
       Type declarations do not match:
         type 'a t = 'a Foobar2.t constraint 'a = [< foobar_t ]
       is not included in
         type 'a t = private 'a constraint 'a = [< foobar_t ]


Any idea on what is going on?  Also, is it at all possible to achieve
the 'private' semantics I am looking for with types declared using
a constraint?  I know I can do it by boxing the type inside a dummy
union or a record, but I would like to avoid that hassle if possible.

Thanks for your time + best regards,
Dario Teixeira



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