Purely Functional Events

SubjWorldValue.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace SubjWorldValue {
    class Program {
        static void Main() {
            var uim = new UIM();
            //var uim2 = uim.AddListenerx(w => w.BPM, (w, i) => w.Set(w1 => w1.Resp, i));
            var uim2 = uim.AddBPMListener((w, i) => w.SetResp(i));
            var uim3 = uim2.Set(w => w.BPM, 5);
            var uim4 = uim.Set(w => w.BPM, 5);
            Console.WriteLine(uim3.Resp.Value);
            Console.WriteLine(uim4.Resp.Value);
        }
    }

    public class UIM {
        public readonly Subj<UIM, int> BPM;
        public readonly Subj<UIM, int> Resp;

        public UIM() {
            BPM = new Subj<UIM, int>(0, (bpm, uim) => new UIM(bpm, Resp));
            Resp = new Subj<UIM, int>(0, (resp, uim) => new UIM(BPM, resp));
        }

        UIM(Subj<UIM, int> bpm, Subj<UIM, int> resp) {
            BPM = bpm;
            Resp = resp;
        }

        public UIM AddBPMListener(Func<UIM, int, UIM> setter) {
            return BPM.AddListener(uim => setter(uim, uim.BPM.Value), this);
        }

        public UIM SetResp(int i) {
            return Resp.SetValue(i, this);
        }
    }

    public static class WorldEx {
        public static TWorld Set<TWorld, TValue>(this TWorld world, Func<TWorld, Subj<TWorld, TValue>> fSubj, TValue value) {
            return fSubj(world).SetValue(value, world);
        }

        public static TWorld AddListenerx<TWorld, TValue>(this TWorld world, Func<TWorld, Subj<TWorld, TValue>> fSubj, Func<TWorld, TWorld> listener) {
            return fSubj(world).AddListener(listener, world);
        }

        public static TWorld AddListenerx<TWorld, TValue>(this TWorld world, Func<TWorld, Subj<TWorld, TValue>> fSubj, Func<TWorld, TValue, TWorld> setter) {
            return world.AddListenerx(fSubj, w => setter(w, fSubj(w).Value));
        }
    }

    public class Subj<TWorld, TValue> {
        public readonly TValue Value;
        readonly IEnumerable<Func<TWorld, TWorld>> _listeners;
        readonly Func<Subj<TWorld, TValue>, TWorld, TWorld> _updater;

        public Subj(TValue value, IEnumerable<Func<TWorld, TWorld>> listeners, Func<Subj<TWorld, TValue>, TWorld, TWorld> updater) {
            Value = value;
            _updater = updater;
            _listeners = listeners;
        }

        public Subj(TValue value, Func<Subj<TWorld, TValue>, TWorld, TWorld> updater)
            : this(value, Enumerable.Empty<Func<TWorld, TWorld>>(), updater) { }

        public TWorld SetValue(TValue value, TWorld world) {
            return _listeners.Aggregate(_updater(new Subj<TWorld, TValue>(value, _listeners, _updater), world), (world1, listener) => listener(world1));
        }

        public TWorld AddListener(Func<TWorld, TWorld> listener, TWorld world) {
            return _updater(new Subj<TWorld, TValue>(Value, _listeners.Concat(new[] { listener }), _updater), world);
        }
    }
}