Fork combinator revisited - supporting multiple part functions in C#

This article shows an example of a Fork combinator or 'monad' that will allow you to specify a join function that operates on all the part results and allow you to specify multiple part functions to be operated in sequence. First off, we define a simple map monad to map a value to another value of possibly other type. Then we define the fork combinator. The code below is very simple and short, it uses LINQ functionality to combine the results via the Select method, Linq is also functional so this is how we build up functional monads in C#, using Linq and Func and generics (and pattern matching and more).

Combinators.cs

public static class Combinators {
	
	public static TOut Map<TIn, TOut>(this TIn @this, Func<TIn, TOut> f) => f(@this);
	
	public static TOut Fork<TIn, TMiddle, TOut>(this TIn @this, Func<IEnumerable<TMiddle>, TOut> joinFunc,
		params Func<TIn, TMiddle>[] partFuncs) => partFuncs.Select(pf => pf(@this)).Map(joinFunc);
	
}

Let's look at a simple demo how to use this

Program.cs

public static class Program {
	
	string hello = "hhhhhheeeeeeeelllllllllllooooo";
	
	int sumOfLettersToLookFor = hello.Fork(results => (int)results.Sum(), 
				x => (double)x.Count(l => l == 'h'),
				x => (double) x.Count(l => l == 'e'),
				x => (double) x.Count(l => l == 'l'),
				x => (double) x.Count(l => l == 'o'));
	
	sumOfLettersToLookFor.Dump();
	
}

Functional programming has many of these monads that are very short and allows you to do combinations that would be lengthy and stateful in the procedural / object oriented way but elegant and short in the functional world. Finally a screenshot from Linqpad 7 showing the code above works : (A reference to Jerry Seinfeld to the right for those who know Seinfeld episodes)