Match on a specific union value only

[domn1995]

Some times we just want to do something given a specific union value, rather than handling all cases explicitly. This is most useful on larger unions.

For example consider the following code:

[Union]
public partial record struct MathResult<T>
    : where T : INumber<T>
{
    public partial record struct Infinity;
    public partial record struct Success(T Value);
    public partial record struct NaN;
    public partial record struct Undefined;
}

MathResult<double> result = // Some complex math calculation.

Some ways we might want to interact with result

• With a dedicated Match method for each case?

// A `MatchX()` is generated for each member `X` of the union. You must provide an else case.
var value = result.MatchSuccess(
    // If `Infinity, NaN, or Undefined` return 0.
    () => 0
);

• Perhaps a more imperative, idiomatic C# way?

// A `TryGetX` is generated for each member `X` of the union.
if (result.TryGetSuccess(out var success))
{
    Console.WriteLine(success.Value);
}