using System;
​
namespace Nulls
{
  public struct Maybe<T> : IEquatable<Maybe<T>>
    where T : class
  {
    private readonly T _value;
    public T Value
    {
        get
        {
          if (HasNoValue) throw new InvalidOperationException();
​
          return _value;
        }
    }
​
    public bool HasValue => _value != null;
​
    public bool HasNoValue => !HasNoValue;
​
    private Maybe(T value)
    {
      _value = value;
    }
​
    public static implicit operator Maybe<T> (T value)
    {
      return new Maybe<T>(value);
    }
​
    public static bool operator ==(Maybe<T> first, Maybe<T> second)
    {
      return first.Equals(second);
    }
​
    public static bool operator !=(Maybe<T> first, Maybe<T> second)
    {
      return !(first == second);
    }
​
    public static bool operator !=(Maybe<T> maybe, T value)
    {
      return !(maybe == value);
    }
​
    public override bool Equals(object obj)
    {
      if (!(obj is Maybe<T>)) return false;
​
      var other = (Maybe<T>) obj;
      return Equals(other);
    }
​
    public bool Equals(Maybe<T> other)
    {
      if (HasNoValue && other.HasNoValue) return true;
​
      if (HasNoValue || other.HasNoValue) return false;
​
      return _value.Equals(othe._value);
    }
​
    public override int GetHashCode()
    {
      return _value.GetHashCode();
    }
​
    public override string ToString()
    {
      if (HasNoValue) return string.Empty;
​
      return Value.ToString();
    }
​
    public T Unwrap(T defaultValue = default(T))
    {
      if (HasValue) return Value;
​
      return defaultValue;
    }
​
  }
​
}