Re: [Caml-list] phantom type thing question

["Olivier Andrieu" <oandrieu@nerim.net>]

On 3/1/07, micha <micha-1@fantasymail.de> wrote:
>
> I'm getting confused while trying to implement some subtyping hierachy
> with polymorphic variants.
> say I have the hierachy:
>
> symbol -> image -> xpmimage
>
> and types like that:
>
> type 'a sym;;

if you want subtyping, you'll need to add a variance annotation to
your type parameter. Otherwise since your type sym is abstract, you'll
get no subtyping at all. In this case you probably want a
contravariant type parameter:

type -'a sym

> make_image: params -> [`Symbol | `Image] sym;;
> make_xpm; params -> [`Symbol | `Image | `Xpm ] sym;;

you can define type names, this gives shorter definitions for deep hierarchies:

type symbol = [`Symbol]
type image = [symbol | `Image]
type xpm   = [image | `Xpm]

make_xpm : params -> xpm sym

> so that some functions work only on some symbols:
>
> val get_image_width: [< `Image | `XpmImage] sym -> int;;
>
> I think I mix up the [< ..] and [> ... ] type constructs, allthough I
> thought I understood it :-)

that should be:
val get_image_width : [> xpm] sym -> int

--
  Olivier