Nested Interfaces in C#

Question

I am trying to implement an IFunctor interface [Haskell's Fuctor class] and a Maybe interface to create two classes: Just and Nothing.

So far I have:

public interface IFunctor<A> {
    IFunctor<B> fmap<B> (Func<A,B> f);
}

public interface Maybe<A> : IFunctor<A> {
    Maybe<B> fmap<B> (Func<A,B> f);
}

public class Nothing<A> : Maybe<A> {

    public Maybe<B> fmap<B>( Func<A,B> f ){
        return new Nothing<B>();
    }
}

However I get

`Pr.Nothing<A>' does not implement interface member `IFunctor<A>.fmap<B>
(System.Func<A,B>)' and the best implementing candidate `Pr.Nothing<A>.fmap<B>
(System.Func<A,B>)' return type `Pr.Maybe<B>' does not match interface member return
type `IFunctor<B>'

Isn't Maybe<B> a member of IFunctor<B>?

Solution

I ended writing

public interface IFunctor<A> {
    IFunctor<B> fmap<B> (Func<A,B> f);
}

public interface Maybe<A> : IFunctor<A> {
    //Some stuff but not fmap
}

public class Nothing<A> : Maybe<A> {

    public IFunctor<B> fmap<B>( Func<A,B> f ){
        return new Nothing<B>();
    }
}

all your interface have function fmap<B> (Func<A,B> f) so you need implement this function in derived class for each interface, otherwise you can get error when try (IFunctor<A>)Nothing<A> — Grundy
– Grundy, Commented Jun 20, 2014 at 5:32

xenolightning · Accepted Answer · 2014-06-20 05:34:45Z

7

Maybe<A>.fmap() does not override IFunctor<A>.fmap(). Any type that implements Maybe<A> will need to need to implement both Maybe<A> and IFunctor<A>.

public interface IFunctor<A>
{
    IFunctor<B> fmap<B>(Func<A, B> f);
}

public interface Maybe<A> : IFunctor<A>
{
    Maybe<B> fmap<B>(Func<A, B> f);
}

public class Nothing<A> : Maybe<A>
{

    public Maybe<B> fmap<B>(Func<A, B> f)
    {
        return new Nothing<B>();
    }

    //This is the explicit implementation of IFunctor<A>.fmap<B>
    //which in turn invokes method above.
    IFunctor<B> IFunctor<A>.fmap<B>(Func<A, B> f)
    {
        return this.fmap(f);
    }
}

answered Jun 20, 2014 at 5:34

xenolightning

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2 Comments

Cristian Garcia Over a year ago

If I where to override, because that the intention, I would override on Maybe?

xenolightning Over a year ago

You cannot override method definitions in a interface. An interface is a contract, and therefore a way to assume functionality about the implementer. If one were to override methods it could result in undefined behaviour.

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