Add sum-types about imtools HOT 6 CLOSED

Would Opt be replaced

Update:
Opt is similar to Nullable.

from imtools.

C# interfaces with implementations may help to eliminate

           public bool Equals(Case1 other) => EqualityComparer<T1>.Default.Equals(Value, other.Value);
            public override bool Equals(object obj) => obj is Case1 c && Equals(c);

from imtools.

Sum could support structs like this (no copying):

    public interface ICase<T> where T: readonly struct
    {
        ref T Value { get; }
    }

dotnet/csharplang#1620

from imtools.

Another optional https://github.com/AArnott/ImmutableObjectGraph/blob/24eb8b7144201b91e04ea59fcbc9e216eff8bfa5/src/ImmutableObjectGraph/Optional%601.cs

from imtools.

This optinal was considered http://ecma-international.org/publications/files/ECMA-TR/ECMA%20TR-089.pdf
This was done https://github.com/IllidanS4/SharpUtils/blob/master/ObjectModel/Optional.cs

I guess for either Opt or Optional:Union most these fancy API should be done.

from imtools.

Done, added initial version to build upon.

from imtools.