using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var nums = new[] { 1, 2, 3, 6, 7, 12, 21, 55 };

        var sum = nums.Reduce(

            //x => (double)x,

            x => y => (double)x + y);

        Console.WriteLine(sum);

    }

}

​

public static class Extensions

{

    public static Func<TResult, TResult> ReduceFromSeed<T, TResult>(

        this IEnumerable<T> source,

        Func<T, Func<TResult, TResult>> reduce)

    {

        return (TResult _seed) =>

        {

            var result = _seed;

            foreach(var item in source)

            {

                Console.WriteLine(result);

                result = reduce(item)(result);

            }

            return result;

        };

    }

    public static TResult Reduce<T, TResult>(

        this IEnumerable<T> source,

        Func<T, Func<TResult, TResult>> reduce)

    {

        return source.Skip(1).ReduceFromSeed(reduce)(source.First());

    }

    public static TResult Reduce2<T, TResult>(

        this IEnumerable<T> source,

        Func<T, TResult> convert,

        Func<T, Func<TResult, TResult>> reduce)

    {

        return source.Skip(1).ReduceFromSeed(reduce)(convert(source.First()));

    }

}