LINQ for Option

OptionLinq.cs

namespace CSharpTests;

// I know this is ugly and bad encoding of Option
// but it's OK for this demo
public interface Option<T> {}
public record None<T>() : Option<T>;
public record Some<T>(T Value) : Option<T>;

// let's make LINQ work
public static class EnvelopeExtensions
{
    public static Option<R> Select<T, R>(this Option<T> source, Func<T, R> mapper) =>
        source switch
        {
            None<T> => new None<R>(),
            Some<T> x => new Some<R>(mapper(x.Value)),
            _ => throw new Exception("Impossible happened, panic")
        };

    public static Option<R> SelectMany<T, I, R>(this Option<T> source, Func<T, Option<I>> bind, Func<T, I, R> proj) =>
        source switch
        {
            None<T> => new None<R>(),
            Some<T> x => bind(x.Value).Select(a => proj(x.Value, a)),
            _ => throw new Exception("Impossible happened, panic")
        };
}


public class UnitTest1
{
    [Fact]
    public void Refl()
    {
        var someResult =
            from x in new Some<bool>(false)
            from y in new Some<bool>(true)
            select (x, y);

        var someExpected = new Some<(bool, bool)>((false, true));
        someResult.Should().Be(someExpected);

        var noneResult =
            from x in new None<bool>()
            from y in new Some<bool>(true)
            select (x, y);
        
        var noneExpected = new None<(bool, bool)>();
        noneResult.Should().Be(noneExpected);
    }
}