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In-Reply-To: <20130812074545.GA4646@kerneis.info>
References: <20130812074545.GA4646@kerneis.info>
Date: Mon, 12 Aug 2013 11:20:17 +0100
Message-ID: <CAAxsn=GzCM=9ZmvaQj+iZkmGy=ba6hJfQ+p4DuRpne96Bhv6og@mail.gmail.com>
From: Jeremy Yallop <yallop@gmail.com>
To: Gabriel Kerneis <gabriel@kerneis.info>
Cc: caml-list@yquem.inria.fr
Content-Type: text/plain; charset=UTF-8
Subject: Re: [Caml-list] Signature substitution and type parameters

On 12 August 2013 08:45, Gabriel Kerneis <gabriel@kerneis.info> wrote:
> Is there a concise way to instanciate a module containing a parametric
> type for a particular value of this type parameter?
>
>   module type Poly = sig type 'a t val f : 'a t -> 'a t end ;;
>   module type Mono = sig type t val f : t -> t end ;;
>
>   module PolyM : Poly = struct type 'a t let f x = x end ;;
>
> The following works but is tedious when Poly contains many functions
> instead of only one:
>
>   module MonoM : Mono = struct
>     open PolyM
>     type t = int PolyM.t
>     let f = PolyM.f
>   end ;;

If you use 'include' instead of 'let' it generalizes nicely to the
many-function case:

  module MonoM : Mono = struct
    type t = int PolyM.t
    include (PolyM : Mono with type t := t)
  end

A slight variation makes it possible to achieve a similar effect
without using the Mono signature:

  type _ s = int PolyM.t
  module MonoM = struct
    type t = unit s
    include (PolyM : Poly with type 'a t := 'a s)
  end

Your other approach can be made to work, too.  The key is to define a
type alias with no parameters (type t = int PolyM.t), then use
destructive substitution (:=).