[noblethrasher]

Equivalent C# code (if you want to appreciate the brevity of F# or just understand what the code is doing):

    public abstract class LogicGate
    {
        public abstract bool Evaluate();
    }

    public abstract class BinaryLogic : LogicGate
    {
        protected LogicGate a, b;
        
        public BinaryLogic(LogicGate a, LogicGate b)
        {
            this.a = a;
            this.b = b;
        }
    }

    public class ON : LogicGate
    {
        public override bool Evaluate() { return true; }
    }

    public class OFF : LogicGate
    {
        public override bool Evaluate() { return false; }
    }

    public class NAND : BinaryLogic
    {
        public NAND(LogicGate a, LogicGate b) : base(a, b) { }       

        public override bool Evaluate() 
        { 
            return !(a.Evaluate() && b.Evaluate()); 
        }
    }

    public class NOT : LogicGate
    {
        LogicGate a;

        public NOT(LogicGate a)
        {
            this.a = a;
        }

        public override bool Evaluate() 
        { 
            return new NAND(a, a).Evaluate(); 
        }
    }

    public class AND : BinaryLogic
    {
        public AND(LogicGate a, LogicGate b) : base(a, b) { }
      
        public override bool Evaluate() 
        { 
            return new NOT(new NAND(a, b)).Evaluate(); 
        }
    }

    public class OR : BinaryLogic
    {
        public OR(LogicGate a, LogicGate b) : base(a, b) { }
      
        public override bool Evaluate() 
        { 
            return new NAND(new NOT(a), new NOT(b)).Evaluate(); 
        }
    }

    public class NOR : BinaryLogic
    {
        public NOR(LogicGate a, LogicGate b) : base(a, b) { }
       
        public override bool Evaluate() 
        { 
            return new NOT(new OR(a, b)).Evaluate(); 
        }
    }

    public class XOR : BinaryLogic
    {
        public XOR(LogicGate a, LogicGate b) : base(a, b) { }
       
        public override bool Evaluate() 
        { 
            return new AND(new NAND(a, b), new OR(a, b)).Evaluate(); 
        }
    }

    public class XNOR : BinaryLogic
    {
        public XNOR(LogicGate a, LogicGate b) : base(a, b) { }     

        public override bool Evaluate() { return new NOT(new XOR(a, b)).Evaluate(); }
    }
   

    class Program
    {
        public static void Main(string[] args)
        {
            var on = new ON();
            var off = new OFF();
            
            var result = new List<LogicGate>() 
            { 
                new NAND(on, off), 
                new NAND(off, on), 
                new NAND(on, off), 
                new NAND(on, on) 
            }
            .Select(x => x.Evaluate());

            Console.WriteLine(result);
        }
        
     }