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Emulating width subtyping with 1st-class modules
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Date: -- (:)
From: Dario Teixeira <darioteixeira@y...>
Subject: Emulating width subtyping with 1st-class modules

I have a problem where some form of width subtyping for records would be
useful.  At the present I'm taking advantage of the structural subtyping nature
of Ocaml's object system to emulate the width subtyping.  This works and is
reasonably compact, but I'm still open to other approaches.  It has occurred
to me that 3.12's modules-as-first-class-values provide yet another solution:

module type BRIEF =
	val a: int
	val b: string

module type FULL =
	include BRIEF
	val c: float

let print_brief m =
	let module M = (val m: BRIEF) in
	Printf.printf "A: %d, B: %s\n" M.a M.b

let print_full m =
	let module M = (val m: FULL) in
	Printf.printf "A: %d, B: %s, C: %f\n" M.a M.b M.c

module Full =
	let a = 1
	let b = "full"
	let c = 0.5

module Brief =
	let a = 0
	let b = "short"

let () =
	print_brief (module Brief : BRIEF);
	print_brief (module Full : BRIEF);
	print_full (module Full : FULL)

While this approach seems awfully verbose, I reckon it could be made much
more palatable via some Camlp4 sugaring.  Nevertheless, I have a question:
just how heavy would this approach be when compared to the object one?
And how would it fare in comparison to regular records?

Thanks for your attention!
Best regards,
Dario Teixeira