Add compositional delegates example

FunctionalProject/Features/CompositionalDelegates/FilmsModule.cs

@@ -0,0 +1,91 @@
+namespace FunctionalProject.Features.CompositionalDelegates
+{
+    using System;
+    using System.Threading.Tasks;
+    using Botwin;
+    using Botwin.ModelBinding;
+    using Botwin.Response;
+    using FunctionalProject.Features.NamedDelegatesFilms.Films;
+    using FunctionalProject.Features.NamedDelegatesFilms.Films.Permissions;
+    using Microsoft.AspNetCore.Http;
+    using Models;
+
+    public class FilmsModule : BotwinModule
+    {
+        private readonly Films.CreateFilm createFilm;
+
+        public FilmsModule() : base("/api/delegate-coposition/films")
+        {
+            // If the framework lets you, pass the wired-up functions into the constructor instead for testability
+            this.createFilm = ApplicationServices.Default.createFilm;
+
+            this.Post("/", this.CreateFilm);
+        }
+
+        private async Task CreateFilm(HttpContext context)
+        {
+            var result = context.Request.BindAndValidate<Film>();
+
+            if (!result.ValidationResult.IsValid)
+            {
+                context.Response.StatusCode = 422;
+                await context.Response.Negotiate(result.ValidationResult.GetFormattedErrors());
+                return;
+            }
+
+            try
+            {
+                createFilm(result.Data);
+                context.Response.StatusCode = 201;
+            }
+            catch (Exception)
+            {
+                context.Response.StatusCode = 403;
+            }
+        }
+    }
+
+    // This class composes all of the functions together with their dependencies. It is the composition root of the app
+    // You would call this once when the application starts and then thread the functions into the controllers that need them
+    public class ApplicationServices
+    {
+        public Films.CreateFilm createFilm;
+
+        public ApplicationServices()
+        {
+            ValidUserDelegate vud = () => ValidUserQuery.Execute();
+
+            // Use currying to supply 'constructor' dependencies
+            createFilm = Films.Create(vud);
+        }
+
+        public static ApplicationServices Default => new ApplicationServices();
+
+        // As this class grows large, you can chop it up into multiple cohesive classes e.g. FilmService
+    }
+
+    public static class Films
+    {
+        public delegate void CreateFilm(Film film);
+
+        // This is effectively an object constructor
+        public static CreateFilm Create(ValidUserDelegate validUserQuery)
+        {
+            // C# local method syntax makes it easy to return a function from a function
+            // This function is basically the wired up object
+            void func(Film film)
+            {
+                if (!validUserQuery())
+                {
+                    throw new InvalidOperationException();
+                }
+
+                //Do some special MEGA CORP business validation
+
+                //Save to database by writing SQL here
+            }
+
+            return func;
+        }
+    }
+}