phantom type thing question

phantom type thing question

[micha]

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;;

make_symbol : params -> [`Symbol] sym;;
make_image: params -> [`Image] sym;;

or should it be:
make_image: params -> [`Symbol | `Image] sym;;
make_xpm; params -> [`Symbol | `Image | `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 :-)


 Michael

Re: [Caml-list] phantom type thing question

[Olivier Andrieu]

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

Re: [Caml-list] phantom type thing question

[micha]

Am Thu, 1 Mar 2007 14:47:47 +0100
schrieb "Olivier Andrieu" <oandrieu@gmail.com>:

> On 3/1/07, micha <micha-1@fantasymail.de> wrote:
> >
> 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

o.k., but after reading section 6.8.1 of the manual I don't see why it
needs to be -'a and not +'a


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

thanks, that helped,


 Michael 

Re: [Caml-list] phantom type thing question

[Olivier Andrieu]

On 3/1/07, micha <micha-1@fantasymail.de> wrote:
> Am Thu, 1 Mar 2007 14:47:47 +0100
> schrieb "Olivier Andrieu" <oandrieu@gmail.com>:
>
> > On 3/1/07, micha <micha-1@fantasymail.de> wrote:
> > >
> > 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
>
> o.k., but after reading section 6.8.1 of the manual I don't see why it
> needs to be -'a and not +'a
>
>
> > that should be:
> > val get_image_width : [> xpm] sym -> int

your parameter encodes some "capabilities": you want the function
get_image_width to accept values that have the tag `xpm and possibly
more tags, hence the [> ] in the type of the function.
In other words, you want the type xpm sym to be a subtype of
[xpm|`xpm8] sym (for instance), so that get_image_width accept boths.
Since [xpm|`xpm8] is a subtype of xpm sym (the other way around), that
means the parameter needs to be contravariant.

Re: [Caml-list] phantom type thing question

[micha]

thanks for the examples, I indeed thought the other way round :-)
It's clear now,

 Michael