I like to match a value against null and convert it to an option type.

var item = selectedItem.Match()
    .With(si => string.IsNullOrEmpty(si), new None<Fruit>())
    .With(si => si == "Banana", (new Banana()).ToOption())
    .With(si => si == "Apple", (new Apple()).ToOption())
    .Else(new None<Fruit>()); 

unfortunately this does not work when 'selectedItem' is null in the current version.

FunctionalSharp should be ported to dotnetstandard. Fody.NullGuard is not available, so there need to be some additional checks.

Create code comments and a wiki on the usage of the Discriminated union list

Create code ocmments and wiki documentation for Document Discriminated Extensions

I am missing the following method in PatternMatchSingleContext<T>:

public PatternMatchSingleAction<T, TResult> With<TResult>(T valueToMatch, Action<TResult> action)
{
    return With(this.value, (value) => value.Equals(valueToMatch), _ => action());
}

This also requires an implementation of the class PatternMatchSingleAction<T, TResult> and likely an implementation of other appropriate methods.

Use case

public enum VerticalPosition{
   Top, Center, Bottom
}

private double CalculateRefY(VerticalPosition position)
{
    return position.Match<VerticalPosition, double>()
        .With<double>(VerticalPosition.Top, CalculateRefYTop)
        .With<double>(VerticalPosition.Bottom, CalculateRefYBottom)
        .Else<double>(CalculateRefYCenter);
}

private double CalculateRefYTop() { return 0d; }
private double CalculateRefYBottom() { return Height; }
private double CalculateRefYCenter() { return Height / 2d; }

Workaround
Thankfully it is about the same amount of coding to use an switch statement. I just don't like switch as much:

private double CalculateRefY(VerticalPosition position)
{
    switch(position)
    {
        case VerticalPosition.Top: return CalculateRefYTop();
        case VerticalPosition.Bottom: return CalculateRefYBottom();
        default: return CalculateRefYCenter();
    }
}

When trying to navigate to the documentation page: http://pvanlohuizen.nl/FunctionalSharp/ there is a "There isn't a GitHub Pages site here." 404 page.

I regularily have the case that I wast to use the option type instead of nullables because of the added savety. unfortunately it is rather cruel to cast otructure types to a nullable before making them an option type and then unpacking them.

instead of:

var optionalYear = year == 0 ? new None<DateTime?> : ((DateTime?)new DateTime(year,1,1)).ToOption(); 
var unpackedYear = optionalYear.Match(y => y.HasValue ? y.Value.Year : DateTime.MinValue.Year, DateTime.MinValue.Year); 

i want to simply:

var optionalYear = year == 0 ? new None<DateTime> : (new DateTime(year,1,1)).ToOption(); 
var unpackedYear = optionalYear.Match(y => y.Year, DateTime.MinValue.Year); 

which is much simpler to use.

Sort of similar focus:
https://github.com/wallymathieu/with

Is there anything you can use from that lib? Or anything that could inspire other ideas?

FunctionalSharp.PatternMatching should work with Action instead of Action<T> too:

value.Match()
    .With(v => v == "foo", DoSomething())
    .Else(DoSomethingElse()); 

instead of:

 var iDoNotNeedThis = value.Match()
      .With(v => v == "foo", v => { DoSomething(); return 0; })
      .Else(v => { DoSomethingElse(); return 0; }); 

Create code comments and a wiki on how to use the Discriminated union.

create a T4 template where a named class inherrits from a DiscruminatedUnion/withBase/List that automatically implements the constructors and allows for addition (make partial)

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Consolidate everything in a single project allowing for easier publishing and usage

have a look into migrating the generation code to scripty

There seems to be an issue with the tag numbering. Instead of tag=0; tag=1; tag=2; it reads tag=1; tag=1; tag=2;:

if(value.GetType() == typeof(Type1))
{
	Item1 = (Type1)value; 
	tag = 1; // ERROR: should be 0
}
else if(value.GetType() == typeof(Type2))
{
	Item2 = (Type2)value; 
	tag = 1; 
}