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Shared types: dependency in modules with polymorphic type
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Date: -- (:)
From: Peng Zang <peng.zang@g...>
Subject: Re: [Caml-list] Shared types: circular dependency in modules with polymorphic type
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I think this is because UI.t is abstract and is in fact, never created.  You 
need to tell the old caml that UI.t is actuall an int VI.t.  Then it should 
work.

module Make_P (VI : VB)
              (UI : U with type t = int VI.t)
              (RI : R with type t = UI.t and type instance = UI.instance ) =
struct ... end

module P1 = Make_P ( VB1 ) ( U1 ) ( R1 )

Works like a charm for me,

Peng


On Thursday 14 May 2009 07:41:24 am Hugo Ferreira wrote:
> Hello,
>
> I have again encountered a problem with sharing types.
> However this time I really have a set of interfaces
> which need to be "stitched" together via type sharing.
>
> Problem is I am dealing with a circular polymorphic
> type dependency. Or at least seems so. I have added the
> very simple code for testing at the end. But the short
> of it is: with the code below
>
> module Make_P (VI : VB)
>                (UI : U)
>                (RI : R with type t = UI.t )
> = struct
>
>    type instance = UI.instance
>    type t = UI.t
>
>    let do_something_nice null =
>      let vb0 = VI.empty in
>      let vb1 = VI.add vb0 UI.zero in
>      RI.do_something_else vb1 UI.one
>
> end
>
> I get the error:
>
> This expression (vb1) has type UI.instance VI.t but is here used with type
>    RI.t = UI.t
> vb1: UI.instance VI.t
>
>
> The problem is that VI.t is polymorphic.
> And UI uses VI.t setting the polymorphic
> type in the process.
> I want to make it "UI.instance VI.t."
> How can I do that. I have tried:
>
>
> module Make_P (VI : VB)
>                (UI : U with type t = U.instance VI.t)
>                (RI : R with type t = UI.t )
> and
>
> module Make_P (UI : U)
>                (VI : VB with type 'a t = UI.t)
>                (RI : R with type t = UI.t )
>
> with no success.
>
>
>
> --------------------- Code ---------------
>
>
>
> module type VB = sig
>
>    type 'a t
>
>    val empty : 'a t
>    val add : 'a t -> 'a -> 'a t
> end
>
>
> module VB1 : VB
> = struct
>
>    type 'a t = 'a list
>
>    let empty = []
>    let add l e = e :: l
> end
>
>
> module type U =
>    sig
>
>      type instance = int
>      type t
>
>      val zero : instance
>      val one : instance
>
>      val empty : t
>      val do_something : t -> instance -> t
> end
>
>
> module Make_U (Vb : VB)
> = struct
>
>    type instance = int
>    type t = instance Vb.t
>
>    let zero = 0
>    let one = 1
>
>    let empty = Vb.empty
>    let do_something ts inst = Vb.add ts inst
>
> end
>
>
> module U1 = Make_U ( VB1 )
>
> let _ =
>
>    let vb0 = VB1.empty in
>    let vb1 = VB1.add vb0 U1.zero in
>    let vb2 = U1.do_something vb1 U1.one in
>    let _ = VB1.add vb2 U1.zero in
>    ()
>
>
> module type R =
>    sig
>
>      type instance = int
>      type t
>
>      val do_something_else : t -> instance -> t
> end
>
> module Make_R (VI : VB)
>                (UI : U)
> = struct
>
>    type instance = UI.instance
>    type t = UI.t
>
>    let do_something_else ts inst = UI.do_something ts inst
>
> end
>
> module R1 = Make_R ( VB1 ) ( U1 )
>
>
> let _ =
>
>    let vb0 = VB1.empty in
>    let vb1 = VB1.add vb0 U1.zero in
>    let vb2 = R1.do_something_else vb1 U1.one in
>    let _ = VB1.add vb2 U1.zero in
>    ()
>
>
> module Make_P (VI : VB)
>                (UI : U)
>                (RI : R with type t = UI.t )
> = struct
>
>    type instance = UI.instance
>    type t = UI.t
>
>    let do_something_nice null =
>      let vb0 = VI.empty in
>      let vb1 = VI.add vb0 UI.zero in
>      RI.do_something_else vb1 UI.one
>
> end
>
> module P1 = Make_P ( U1 ) ( VB1 ) ( R1 )
>
> or
>
> module P1 = Make_P ( VB1 ) ( U1 ) ( R1 )
>
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