So I had a chance to set down with this again, and I can not for the life
of me figure out how to get the typeId of a capability using the rust
generator. Since I'm new to the language I presume I'm just doing something
fundamentally wrong. Can you provide any guidance?
On Saturday, May 11, 2019 at 2:09:11 PM UTC-3, Kenton Varda wrote:
>
> Hi Alex,
>
> (You didn't say which language you're using so I'll assume C++, but
> roughly the same should apply in other languages.)
>
> You don't need AnyPointer here. Cap'n Proto supports upcasting and
> downcasting capabilities, as any inheritance heirarchy should. E.g. a
> TextNode::Client can implicitly cast to Node::Client. Downcasting is
> explicit: if you have a `Node::Client`, you can call
> `cilent.castAs<TextNode>()` to get a `TextNode::Client`.
>
> However, `castAs` doesn't actually verify the remote type. If you then
> call a `TextNode`-specific method, and the remote object doesn't actually
> implement `TextNode`, then you'll get an "unimplemented" exception back.
>
> So, you probably want some way to determine the type without calling
> methods to find out if they throw. How to do that is really up to you. One
> option would be to have a big enum. Another would be to use type ID's --
> e.g. you can do `capnp::typeId<TextNode>()` to get the `uint64_t` type ID
> for TextNode. (Note `typeId<T>()` is a constant expression -- you can use
> it in in a switch case.)
>
> So you can have a method like:
>
> getType @0 () -> (id :UInt64);
>
> Things get more complicated, of course, if you have a multi-level
> hierarchy, e.g. Node -> Button -> RadioButton. Now you need to send around
> a list of types.
>
> getTypes @0 () -> (ids :List(UInt64));
>
> To avoid incurring a round trip to get the type, you might try to pass the
> type (or list of types) along with the capabilities, like:
>
> interface Node {
> children @0 () -> (list :List(TypedNode));
> }
> struct TypedNode {
> node @0 :Node;
> type @1 :UInt64;
> }
>
> Cap'n Proto does not have any built-in way of communicating the type
> because in the vast majority of use cases, it either isn't needed at all,
> or there is a use-case-specific solution that is cheaper/better than the
> general solution.
>
> -Kenton
>
> On Sat, May 11, 2019 at 9:44 AM <[email protected] <javascript:>> wrote:
>
>> I'm new to statically typed languages in general, I'm doing this as a
>> learning experience.
>>
>> I'm building a toy UI toolkit, and I want to create a tree of widgets.
>> Something like
>>
>> interface Node {
>> children @0 () -> (list :List(Node));
>> }
>>
>> interface TextNode extends(Node) {
>> //some textNode specific stuff
>> }
>>
>> My problem is that I want a node to be able to contain anything that
>> *implements* the node interfaces, even if it's a textNode, or an
>> SVGNode, or whatever. Is there a way to do that without resorting to using
>> AnyPointer or registering all my sub nodes in advance? Some provision for
>> doing structural typing?
>>
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>>
>
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