Writer Monad in C#

Writer.cs

using System;
using System.Collections.Generic;
using System.Linq;
 
namespace XSharpx {
 public struct Writer<A, B>
    {
        public readonly A a;
        public readonly B b;
        public readonly Semigroup<A> semigroup;

        public Writer(A a, B b, Semigroup<A> semigroup)
        {
            this.a = a;
            this.b = b;
            this.semigroup = semigroup;
        }

        public Tuple<A, B> Apply()
        {
            return new Tuple<A, B>(a, b);
        }
    }

    public static class WriterExtensions
    {
        public static Writer<A, C> Select<A, B, C>(this Writer<A, B> w, Func<B, C> f)
        {
            var c = f(w.b);
            return new Writer<A, C>(w.a, c, w.semigroup);
        }

        
        public static Writer<A, D> SelectMany<A, B, C, D>(this Writer<A, B> w, Func<B, Writer<A,C>> bind, Func<B, C, D> select)
        {
            return SelectMany(w, b => Select(bind(b), c => select(b, c)));
        }

        public static Writer<A, C> SelectMany<A, B, C>(this Writer<A, B> w, Func<B, Writer<A, C>> f)
        {
            var wb = f(w.b);
            var na = w.semigroup.Apply(w.a, wb.a);
            return new Writer<A, C>(na, wb.b, wb.semigroup);
        }
    }
  }