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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
Hi,

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 =
sig
	val a: int
	val b: string
end


module type FULL =
sig
	include BRIEF
	val c: float
end


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 =
struct
	let a = 1
	let b = "full"
	let c = 0.5
end


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


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