Maybe.cs

/// <summary>
/// Type to indicate that there might be a value missing.
/// </summary>
/// <typeparam name="TA">The type of a.</typeparam>
public class Maybe<TA>
{
  private TA _value;

  private bool IsPresent => _value != null;

  /// <summary>
  /// Prevents a default instance of the <see cref="Maybe{T}"/> class from being created.
  /// </summary>
  private Maybe() { }

  /// <summary>
  /// Gets nothing.
  /// </summary>
  /// <value>The nothing.</value>
  public static Maybe<TA> Nothing => new Maybe<TA>();

  /// <summary>
  /// Just the specified value.
  /// </summary>
  /// <param name="value">The value.</param>
  /// <returns></returns>
  public static Maybe<TA> Just(TA value) => new Maybe<TA>() {_value = value};

  /// <summary>
  /// Executes the given <paramref name="f"/> when there's a value present for the current <see cref="Maybe{TA}"/>.
  /// </summary>
  /// <typeparam name="TB">The type of the b.</typeparam>
  /// <param name="f">The f.</param>
  /// <returns></returns>
  public Maybe<TB> Bind<TB>(Func<TA, Maybe<TB>> f) => IsPresent ? f(_value) : Maybe<TB>.Nothing;

  /// <summary>
  /// Filter on the wrapped value inside the <see cref="Maybe{TA}"/> by using the given <paramref name="predicate"/>.
  /// </summary>
  /// <param name="predicate">The predicate.</param>
  /// <returns></returns>
  public Maybe<TA> Where(Func<TA, bool> predicate) => Bind(x => predicate(x) ? this : Nothing);

  /// <summary>
  /// Maps the wrapped value inside the <see cref="Maybe{TA}"/> with the given <paramref name="f"/> to a new value <see cref="TB"/> if there's a value present for the current <see cref="Maybe{TA}"/>.
  /// </summary>
  /// <typeparam name="TB">The type of the b.</typeparam>
  /// <param name="f">The f.</param>
  /// <returns></returns>
  public Maybe<TB> Select<TB>(Func<TA, TB> f) => Bind(x => Maybe<TB>.Just(f(_value)));
}