Result<TSuccess, TFailure> generator #648
Description
This is a proposal to add a Result<TSuccess, TFailure> as a kind of generator for enum with associated values.
Prerequisite: enum with associated values as described in #30 (comment)
Result<TSuccess, TFailure> is an enum with 2 enumerators (Success & Failure). TSuccess can be a single type or a tuple while TFailure must be a single type.
Result<(x: int32, y: string, tag(1) z: string?), MyError> is a short-hand for:
// a checked enum
enum Result<(x: int32, y: string), MyError> { // pseudo Slice since this is not a valid Slice identifier
Success(x: int32, y: string, tag(1) z: string?) // the success tuple as-is
Failure(value: MyError) // MyError can be any type or typealias, but not a tuple; can't be optional, like typealias
}Result<tag(1) string?, int32> is a short-hand for:
// a checked enum
enum Result<tag(1) string?, int32> { // pseudo Slice since this is not a valid Slice identifier
Success(tag(1) value: string?)
Failure(value: int32)
}Then, you can use Result<TSuccess, TFailure> anywhere you can use a type: as a field, as an operation parameter, and more commonly, as a operation return type:
interface Greeter {
greet(name: string) -> Result<string, GreeterError>
}
struct GreeterError {
message: string
errorCode: int32
}At the IceRPC level, this return value is a StatusCode=Success, even if it holds a GreeterError.
stream parameter
The Result success tuple is fed into the Success enumerator's associated value and can't include a stream. If you want a stream parameter, it has to be separate:
interface Example {
op() -> (result: Result<string, string>, stream: Sequence<uint8>)
op() -> stream Result<string, string>
}Comparison with previous Result proposal
In #3571, Result is a special notation that maps (with IceRPC) to StatusCode=Success and StatusCode=ApplicationError, and takes advantage of the error message carried by the icerpc StatusCode=ApplicationError response.
One advantage of the #3571 proposal is:
op() -> string
op() -> Result<string, SomeError>are "on-the-wire" compatible. That's no longer the case with the new proposal, where Result is a short-hand for an enum.
Language mapping
In Rust and Swift, we would naturally map Result to the native Result type.
In C#, we could create our own generic struct or class. We need map enum-with-associated-value in any case, so we can create a similar looking Result generic. An alternative would be to reuse a third-party Result type.