using System;
using System.Collections.Generic;

public struct Either<TLeft, TRight> {
    readonly TLeft left;
    readonly TRight right;
    readonly bool isRight;

    Either(TLeft left, TRight right, bool isRight) {
        this.left = left;
        this.right = right;
        this.isRight = isRight;
    }

    public bool IsLeft => !isRight;
    public bool IsRight => isRight;

    public TLeft Left {
        get {
            if (IsRight) throw new InvalidOperationException("No Left value present.");
            return left;
        }
    }

    public TRight Right {
        get {
            if (IsLeft) throw new InvalidOperationException("No Right value present.");
            return right;
        }
    }

    public static Either<TLeft, TRight> FromLeft(TLeft left) => new (left, default, false);
    public static Either<TLeft, TRight> FromRight(TRight right) => new (default, right, true);

    public TResult Match<TResult>(Func<TLeft, TResult> leftFunc, Func<TRight, TResult> rightFunc) {
        return IsRight ? rightFunc(right) : leftFunc(left);
    }

    public Either<TLeft, TResult> Select<TResult>(Func<TRight, TResult> map) {
        return IsRight ? Either<TLeft, TResult>.FromRight(map(right)) : Either<TLeft, TResult>.FromLeft(left);
    }

    public Either<TLeft, TResult> SelectMany<TResult>(Func<TRight, Either<TLeft, TResult>> bind) {
        return IsRight ? bind(right) : Either<TLeft, TResult>.FromLeft(left);
    }

    public static implicit operator Either<TLeft, TRight>(TLeft left) => FromLeft(left);
    public static implicit operator Either<TLeft, TRight>(TRight right) => FromRight(right);

    public override string ToString() => IsRight ? $"Right({right})" : $"Left({left})";
}