Re: [Caml-list] Signature substitution and type parameters

[Jeremy Yallop <yallop@gmail.com>]

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 (:=).