using System;

using System.Collections.Generic;

using System.Diagnostics;

using System.Diagnostics.Contracts;

using System.Linq;

 // Functional Programming in C# with Categories 

 //https://leanpub.com/functional-programming-in-cSharp-with-categories

namespace Foldables.List.Compose

​

{

​

    public class Demo

    {

        public static void Main()

        {           

           var compositeList = new List<List<int>>() { new List<int>() { 1, 2 }, new List<int>() { 3, 4 } ,new List<int>() {5, 10 }};

            var compositeFoldResult = compositeList.FoldT(monoid: (() => 0, (x, y) => x + y));

            Console.WriteLine( "total sum:"+compositeFoldResult);

​

​

        }

​

    }

    public static partial class funcExtensions

    {       

        public static T1 MatchWith<T, T1>(this List<T> @this, (Func<T1> Empty, Func<T, List<T>, T1> Cons) algebra) =>

                (@this.Count == 0) ?

                algebra.Empty() :

                algebra.Cons(@this[0], @this.GetRange(1, @this.Count - 1));

        [Pure]

        public static T Fold<T>(this List<T> @this, (Func<T> empty, Func<T, T, T> concat) monoid) =>

            @this.MatchWith(algebra: (

                Empty: monoid.empty,

                Cons: (v, r) => monoid.concat(v, r.Fold(monoid))

            ));

        public static T FoldT<T>(this List<List<T>> @this, (Func<T> empty, Func<T, T, T> concat) monoid)

           => @this.Select(x => x.Fold(monoid)).ToList().Fold(monoid);

​

    }

}