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From: Jacques Garrigue <garrigue@math.nagoya-u.ac.jp>
In-Reply-To: <20110118121036.GK323@melkinpaasi.cs.helsinki.fi>
Date: Tue, 18 Jan 2011 23:03:05 +0900
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References: <20110118020018.GI323@melkinpaasi.cs.helsinki.fi> <7f9a42a3cac8ea539f9f48432cd8a5af.squirrel@mail.mpi-sws.org> <20110118121036.GK323@melkinpaasi.cs.helsinki.fi>
To: Lauri Alanko <la@iki.fi>
X-Mailer: Apple Mail (2.1082)
Subject: Re: [Caml-list] Signature monomorphism in functors

On 2011/01/18, at 11:00, Lauri Alanko wrote:

> I guess what I want is "bounded signature polymorphism" of some sort:
> 
> module F(A : sig module type S <: S1 module M : S end) 
>  : S3 with module M = A.M = 
> struct
>  module M = A.M
> end
> 
> module M3B : S3 with module M = M2 = 
>  F(struct module type S = S2 module M = M2 end)
> 
> Alas, there are no signature bounds in ocaml and I cannot offhand
> think of a way to encode them.

You can always encode it by using an abstract module type:

  module type S1'  = sig type t  module type X  module X : X end

This way you can put anything you wish in X.
But this is going to be pretty heavy to use...

Alternatively, if you don't need all the functionality of modules,
objects let you do exactly that. A function from objects to objects may
be polymorphic in the available methods.

Jacques Garrigue