Maybe class

Maybe.cs

namespace System.Monads
{
    public class Maybe<TA>
    {
        private readonly TA _value;

        private bool IsPresent => _value != null;

        private Maybe() { _value = default(TA); }
        private Maybe(TA value) { _value = value; }

        public static Maybe<T> Just<T>(T x) => new Maybe<T>(x);
        public static Maybe<TA> Nothing => new Maybe<TA>();

        public Maybe<TB> SelectMany<TB>(Func<TA, Maybe<TB>> f) => IsPresent ? f(_value) : Maybe<TB>.Nothing;
        public Maybe<TA> Where(Func<TA, bool> f) => IsPresent && f(_value) ? this : Nothing;
        public Maybe<TB> Select<TB>(Func<TA, TB> f) => IsPresent ? Maybe<TB>.Just(f(_value)) : Maybe<TB>.Nothing;

        public TA GetOrElse(TA otherwise) => IsPresent ? _value : otherwise;
        public TA GetOrElse(Lazy<TA> otherwise) => IsPresent ? _value : otherwise.Value;
        public Maybe<TA> OrElse(Maybe<TA> other) => IsPresent ? this : other;

        public Maybe<TA> Do(Action<TA> f)
        {
            if (IsPresent) { f(_value); }
            return this;
        }
    }

    public static class MaybeExtensions
    {
        public static Maybe<TA> Flatten<TA>(this Maybe<Maybe<TA>> x) => x.GetOrElse(Maybe<TA>.Nothing);
    }
}