Re: [Caml-list] Yet another question about insufficient polymorphism

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

On Wed, Mar 12, 2008 at 11:13 PM, Richard Jones <rich@annexia.org> wrote:
> [Hope you don't mind this but I've CC'd back to caml-list ...]
>
>
>  On Wed, Mar 12, 2008 at 10:57:46PM +0100, Olivier Andrieu wrote:
>  > On Wed, Mar 12, 2008 at 9:59 PM, Richard Jones <rich@annexia.org> wrote:
>  > > On Wed, Mar 12, 2008 at 09:19:32PM +0100, Olivier Andrieu wrote:
>  > >  > aren't you missing a variance annotation ? like :
>  > >  >
>  > >  > type -'a component
>  > >
>  > >  I don't know because no one has ever been able to explain covariance &
>  > >  contravariance to me in a comprehensible way !
>  >
>  > there's this post where I tried to explain that to someone:
>  > http://groups.google.com/group/fa.caml/msg/584ea2139b69f0ed
>  >
>  > >  I added the '-' but it doesn't appear to have fixed the problem
>  > >  unfortunately.
>  >
>  > You'll need it anyway I think :)
>  > What's the type of your functions that create components ? (textbox
>  > and button ?)
>
>  val button : int -> int -> string -> [ `Button | `Component ] component
>
>  val textbox : int -> int -> int -> int -> flag list ->
>     [ `Component | `Textbox ] component
>
>
>  > >  Someone mentioned that it might be because when I added the signature,
>  > >  the type became abstract (before, the implementation of the type was
>  > >  exposed).  I can't make that work either though, but maybe because my
>  > >  type is slightly different than it was before ...
>  >
>  > You'll probably have to add some coercions , like:
>  >
>  >   let as_co c = (c :> [`Component] component)
>  >
>  > and then
>  >
>  >   form_add_components form [as_co text; as_co b1; as_co b2];
>
>  Right, this is very confusing.  Adding the -'a annotation:
>
>   type -'a component
>
>  and:
>
>   let as_co c = (c :> [`Component] component) in
>
>   form_add_components form [as_co text; as_co b1; as_co b2];
>
>  works.
>
>  However omitting the minus in "type -'a ..." breaks it,

Right. The '-' annotation says the parameter is contravariant which means that
with a coercion you can "loose" some of the variants. Without the
annotation the coercion would fail.

> and changing the list to:
>
>   ([text; b1; b2] :> [`Component] component list)
>
>  doesn't work either.

Yes because it doesn't even get to the coercion, it fails while trying
to build the list:
text and b1 have incompatible types.

In the end, the easiest way to write this is probably to avoid the
list altogether:

  form_add_component form text ;
  form_add_component form b1 ;
  form_add_component form b2 ;

-- 
  Olivier