Re: [Caml-list] Polymorphic Variants

["Jonathan Roewen" <jonathan.roewen@gmail.com>]

There are interesting, and extremely useful use cases for polymorphic
variants. One very cool example (IMO) is an xhtml module which mixes
phantom types with polymorphic variants so that the compiler enforces
creation of valid xhtml documents (with some limitations...).

Also, OCaml has ways to control whether a polymorphic variant argument
to a function is open/extensible or not.

As for the records issue: you can wrap them in sub modules to
workaround the name clash problem by fully qualifying the fields with
their module name.

E.g.

module A = struct
  type t = { x : int; y : int }
end

module B = struct
  type t = { x : float; y : float }
end

open A;;
open B;;

let a = { A.x = 2; A.y = 3 };;
let b = { B.x = 1.; B.y = 2. };;

let x_from_A v = v.A.x;;
let x_from_B v = v.B.x;;

Jonathan

On 1/17/07, Tom <tom.primozic@gmail.com> wrote:
> I have a question... I hope it will not be dismissed right away, thou I
> guess most of you will find it stupid (some might however agree with me...
> hopefully).
>
> Cut the crap!
>
> So... why actually are polymorphic variants useful? Why can't they simply be
> implemented as normal, concrete (or how would you call them? ...) variants?
> Doesn't the use of polymorphic variants just mess up the function type?
>
> I'm not orthogonally against polymorphic variants, it's just that I am
> looking for an alternative concept that could be used instead... Maybe
> subtyped records?
>
> - Tom
>
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