Cannot convert from 'NotLoaded<T>' to 'Func<Loaded<T>>' when calling OptionT>.Match()

[MrYossu]

I'm sure I'm doing something obviously stupid, but I can't see what.

I have the following code that generates a discriminated union...

  [Union]
  public interface LoaderContainer<T> {
    LoaderContainer<T> Loading();
    LoaderContainer<T> Loaded(T value);
    LoaderContainer<T> NotLoaded();
  }

I want to create an extension method that will convert an Option<T> to Loaded<T> if the Option<T> is Some and to NotLoaded<T> if the Option<T> is None.

Seems simple enough...

   public static LoaderContainer<T> ToLoaderContainer<T>(this Option<T> o) =>
      o.Match(val => new Loaded<T>(val), new NotLoaded<T>());

However, this gives a compiler error on the second parameter to Match() (namespaces removed to make it easier to read)...

Cannot convert from 'NotLoaded' to 'Func<Loaded>'

I tried changing the second parameter to be a Func<T>...

    public static LoaderContainer<T> ToLoaderContainer<T>(this Option<T> o) =>
      o.Match(val => new Loaded<T>(val), () => new NotLoaded<T>());

...but this gave a compiler error on the whole second line...

Cannot implicitly convert type 'LanguageExt.Unit' to 'LoaderContainer'

What am I doing wrong? Thanks

[louthy]

I’m not in front of my computer, so not 100% sure, but it seems the compiler can’t resolve a common type, try constructing them with

LoaderContainer.Loaded(…) and LoaderContainer.NotLoaded(…)

[louthy]

In front of my PC now, my suspicions were correct. These are your options:

Tell C# the common join type, by providing the generic return argument to Match:

    public static LoaderContainer<T> ToLoaderContainer<T>(this Option<T> o) =>
        o.Match<LoaderContainer<T>>(val => new Loaded<T>(val), new NotLoaded<T>());

Tell C# the common join-type by casting one of the branches to the base type:

        public static LoaderContainer<T> ToLoaderContainer<T>(this Option<T> o) =>
        o.Match(val => new Loaded<T>(val) as LoaderContainer<T>, new NotLoaded<T>());

Use the constructor functions, rather than new, to create your cases. The constructor functions return the LoaderContainer<T> base-type, not the more specific case-types. This is using point-free style:

    public static LoaderContainer<T> ToLoaderContainer<T>(this Option<T> o) =>
        o.Match(LoaderContainer.Loaded<T>, LoaderContainer.NotLoaded<T>);