Examples of JS-Like higher-order functions in C# , C#

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            int[] data = new int[] {0, 1, 2, 3};
            Console.WriteLine(data.CollStr());
            Console.WriteLine(data.Map(x => x + 1).CollStr());    //print array human-readable
            Console.WriteLine(data.Filter(x => x > 1).CollStr()); //+1 to all
            Console.WriteLine(data.Fold(0, (i, o) => i + o));     //sum
        }
        
    }
    static class Extn
    {
        public static IEnumerable<Out> Map<In,Out>(this IEnumerable<In> a, Func<In,Out> f)
        {
            foreach(In i in a) yield return f(i);
        }
        public static Out Fold<In,Out>(this IEnumerable<In> a, Out initial, Func<In,Out,Out> f)
        {
            Out tor = initial;
            foreach(In i in a) tor = f(i, tor);
            return tor;
        }
        public static IEnumerable<T> Filter<T>(this IEnumerable<T> a, Func<T, bool> f)
        {
            foreach(T t in a) if(f(t)) yield return t;
        }
        public static string JoinToString<T>(this IEnumerable<T> a, string joiner = "")
        {
            string result = a.Fold("", (i, o) => o + i.ToString() + joiner);
            return joiner.Length == 0 ? result : result.Remove(result.Length - joiner.Length);
        }
        public static string CollStr<T>(this IEnumerable<T> coll)
        {
            string res = coll.Fold("[", (i, o) => o + i.ToString() + ",");
            return res.Remove(res.Length - 1) + "]";
        }
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0,17 sec, absolute running time: 0,08 sec, cpu time: 0,08 sec, average memory usage: 13 Mb, average nr of threads: 3