monadinstances.cs

typeclassinstance Monad<Nullable> {
    Nullable<b> >>= <a,b>(Nullable<a> x, Func<a, Nullable<b>> f) {
        if (x.HasValue) {
            return f(x.Value);
        } else {
            return null;
        }
    }

    Nullable<a> return_ <a>(x) {
        return new Nullable<a>(x);
    }
}

typeclassinstance Monad<List> {
    List<b> >>= <a, b>(List<a> xs, Func<a, List<b>> f) {
        List<b> ret = new List<b>();
        foreach (a x in xs) {
            ret.AddRange(f(x));
        }

        return ret;
    }

    List<a> return_ <a>(a x) {
        return new List<a> {x};
    }
}