Genric Enum ic Cap'p Proto

[Mostafa Sedaghat joo]

AS Cap'p Scheme Language says: "A struct or interface type may be parameterized, making it “generic”."

I am wondering is it possible (technically) to have Generic Enums?

In Rust, this is very common that a method can return "Result" Generic Enum, that can be an error or a generic value:

enum Result<T, E> {
 Ok(T),
 Err(E),
}

If technically it is not possible to define a generic enum, what is the bests practice of define this function in Cap'n Proto:

fn foo() -> Result<String, Error> {
    ...
}

[Ian Denhardt]

The closest analogue to Rust's enums is a union. You could define
`Result` like:

struct Result(T, E) {
union {
ok @0 :T;
err @1 :E;
}
}

Note however that rust-style error checking has problems in an rpc
context, in particular using this as a return value will defeat promise
pipelining, so if you intend on calling methods on the value wrapped in
`ok`, you may not want to do this if latency is a concern, using
capnproto exceptions instead. The rust implementation maps these to rust
`Result`s when you wait for a promise anyway.

One downside of using capnproto exceptions is that you don't have
the ability to define custom error types; there are a fixed set defined
by the RPC protocol and you can attach a string to them, but you can't
really do "structured" errors.

-Ian

Quoting Mostafa Sedaghat joo (2020-05-10 11:35:29)
> AS [1]Cap'p Scheme Language says: "A struct or interface type may be

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