How to implement a method with a generic return value?

[yuan-y-chang]

The official example says

interface Assignable(T) {
  # A generic interface, with non-generic methods.
  get @0 () -> (value :T);
  set @1 (value :T) -> ();
}

# interface AssignableFactory {
newUnsetAssignable @1 [T] () -> (assignable :Assignable(T));
# Create a new assignable. `get()` on the returned object will
# throw an exception until `set()` has been called at least once.

But how to implement this on the server side?

My schema is

interface Getter {
    # Get a value by its type
    get @0 [T] () -> (value :T);
}

...and the codegen is

impl getter::Server for MyHub {
    async fn get(
        self: Rc<Self>,
        input: getter::GetParams,
        mut output: getter::GetResults,
    ) -> Result<(), capnp::Error> {
        todo!()
    }
}

Can I specify/test the T in the server?

---

I have also tried the polymorphism solution, where generialize the Getter interface.

interface Getter(T) {
    # Get a value by its type
    get @0 () -> (value :T);
}

interface BigInterface extends(
    Getter(Foo),
    Getter(Bar),
) {}

But Getter(Foo) and Getter(Bar) have the same ID, so I need to define the interface alias to specify them on the client side, which has a large amount of duplicate code.

interface FooGetter extends(Getter(Foo)) {}
interface BarGetter extends(Getter(Bar)) {}

interface BigInterface extends(
    FooGetter,
    BarGetter,
) {}

...and the codegen causes duplicate IDs in a match block. (dispatcher type)

warning: unreachable pattern
175 | ...   0x84bf6ef9e29d7118 => crate::utils_capnp::getter::ServerDispatch::<_T,crate::uti...
    |       ------------------ matches all the relevant values
177 | ...   0x84bf6ef9e29d7118 => crate::utils_capnp::getter::ServerDispatch::<_T,crate::uti...
    |       ^^^^^^^^^^^^^^^^^^ no value can reach this
    |
    = note: `#[warn(unreachable_patterns)]` on by default

[yuan-y-chang]

I think the compromise solution is using a type ID and wrap with a generic method to infer the type ID. (capnp::traits::HasTypeId)

interface Getter {
    # Get a value by its type ID
    get @0 [T] (typeId: UInt64) -> (value :T);
}