public abstract class Maybe<T>
     {
     	public abstract T Value { get; }
     
     	public virtual string Failure => null;
     	
     	public SelectMany<U, V>(Func<T, Maybe<U>> f, Func<T, U, V> g)
     	{
     		var that = f(this.Value);
     		
     		if(that.Failure != null)
     			return new Failed(that.Failure);
     			
     		return new Success(g(this.Value, that.Value));		
     	}
     	
     	public sealed class Success : Maybe<T>
     	{
     		public override Value { get; }
     		
     		public Success(T value) => this.Value = value;
     	}
     	
     	public sealed class Failed : Maybe<T>
     	{
     		public override Failure { get; }
     		
     		public override Value => throw new InvalidOperationException();
     		
     		public Failed(string s) => this.Failure = s;
     	}
     }
     
     Maybe<string> UpdateCustomerWithErrorHandling()
     {
     	return 
     		from req in receiveRequest();
     		from v in validateRequest(req)
     		from c in canicalizeEmail(req)
     		from d in db.updateDbFromRequest(req)
     		from e in smptServer.sendEmail(req.Email)
     		
     		select "OK";
     
     }