Re: Type constraint to explain that a polymorphic variants is included into another

[Sylvain Le Gall <sylvain@le-gall.net>]

On 08-10-2010, Jake Donham <jake@donham.org> wrote:
> On Fri, Oct 8, 2010 at 10:13 AM, Sylvain Le Gall <sylvain@le-gall.net> wrote:
>> This code doesn't compile because I see no way to explain that F.kind is
>> included into plugin_kind.
>
> As you have written it, F.kind is of course completely abstract. I am
> not sure where you need F.kind to be a strict subtype of plugin_kind,
> but you could say type kind = plugin_kind (this seems a bit useless
> however).
>
> I don't think there is a way to use subtyping implicitly when applying
> a functor, but you can always do it explicitly by interposing a module
> of signature PLUGIN_FAMILY which embeds the specific kind in
> plugin_kind and passes the other components through.
>
> You could also have a general and a specific type in the plugin
> signature, and use the general one for general operations (e.g. help)
> but the specific one wherever that is needed. I am not sure I
> understand what you're trying to achieve however.
>

My goal is that the compiler prevents me to do 
Build.act pkg.plugin_install 
-> because plugin_install is of type [`Install] plugin and Build.act
   needs [`Build] plugin

but allow me to do 

help pkg.plugin_install 
-> because help needs [`Build | `Install] plugin.

But maybe I am missing something here and try to overengineer something
simple.

Regards,
Sylvain Le Gall