Improve the handling of Outcome in V8

[martintmk]

Currently, the Outcome<T> is created as:

var outcome = new Outcome<HttpResponseMessage>(new HttpResponseMessage(HttpStatusCode.Ok));

In Polly V8 there are many places where the delegate need a function that returns ValueTask<Outcome>:

var task = new ValueTask<Outcome<HttpResponseMessage>>(new Outcome<HttpResponseMessage>(new HttpResponseMessage(HttpStatusCode.Ok)));

The syntax gets a little obscure so I am thinking how we can simplify this handling.

How about introduce the Outcome static class that will be used to create instances of Outcome<T> ?

public static class Outcome
{
    public static Outcome<TResult> Result<TResult>(TResult value) => new(value);

    public static ValueTask<Outcome<TResult>> ResultTask<TResult>(TResult value) => new(Result(value));

    public static Outcome<TResult> Exception<TResult>(Exception exception) => new(exception);

    public static ValueTask<Outcome<TResult>> ExceptionTask<TResult>(Exception exception) => new(Exception<TResult>(exception));
}

Now the usage becomes:

var outcome = Outcome.Result(new HttpResponseMessage(HttpStatusCode.Ok));
var task = Outcome.ResultTask(new HttpResponseMessage(HttpStatusCode.Ok));

We can take advantage of automatic type resolution. The constructors on Outcome<T> would be hidden and using the Outcome static class would be the only way to create instances of generic outcome.

This could also clean-up the V8 code a bit.

Contributes to #1324 and #1301.

@martincostello , @joelhulen, @PeterCsalaHbo Your thoughts, should I give this a shot?

[martincostello]

Maybe call it FromResult(), like Task.FromResult()? Otherwise seems reasonable to me.

[martintmk]

@martincostello Any suggestions for the method that returns ValueTask based outcome? I don't want to put Async suffix there because then the analyzers recommend it when it's not relevant?

How about this?

public static ValueTask<Outcome<TResult>> FromResultAsTask<TResult>(TResult value) => new(Result(value));

Or maybe allow this syntax?

Outcome.FromResult("some-value").AsValueTask();

[martincostello]

FromResultAsTask is the best option, I think.

[PeterCsalaHbo]

For me the FromResult and FromResultAsTask seem fine.

public static class Outcome
{
    public static Outcome<TResult> FromResult<TResult>(TResult value) => ...;
    public static ValueTask<Outcome<TResult>> FromResultAsTask<TResult>(TResult value) => ...;
    public static Outcome<TResult> FromException<TResult>(Exception exception) => ...;
    public static ValueTask<Outcome<TResult>> FromExceptionAsTask<TResult>(Exception exception) => ...;
}

I think the AsValueTask would make sense if some API would return an Outcome<TResult> but you need to wrap it into a ValueTask to be compatible with other APIs. But as you said the V8 is async by nature so, if there is no API which returns Outcome<TResult> then having a single method, which combines the Outcome and ValueTask wrappings, eases the usage IMHO.