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
> 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> {
> � � ...
> }
>
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> Verweise
>
> 1. https://capnproto.org/language.html#generic-types
> 2. mailto:capnproto+unsubscr...@googlegroups.com
> 3.
> https://groups.google.com/d/msgid/capnproto/5515b78c-8da3-4fc8-9fb3-93ff7c21ea26%40googlegroups.com?utm_medium=email&utm_source=footer
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