RPG EDSL

Program.cs

// アイテムやスキルの効果を、プログラム上のデータとして表現し、それを解釈・実行することで各パラメータを算出する。
// EDSL (Embedded Domain Specific Language 組み込みDSL) として知られているテクニック。

namespace Edsl
{
    // Stats represents parameters of the character.
    // Stats can only be calculated by Effect.execute() method.
    record Stats(string Name, int HP, int MP, int ATK, int DEF);

    // Condition represents the condition of the game.
    // Condition can be calculated by Effect.execute() method.
    // Condition also can be constructed by whole game routine.
    abstract record Condition
    {
        abstract public bool Satisfies(Condition given);
        public record Grabbed(Stats Enemy) : Condition
        {
            public override bool Satisfies(Condition given)
            {
                return given switch
                {
                    Grabbed grabbed => Enemy.Name == grabbed.Enemy.Name,
                    _ => false
                };
            }
        }
    }

    // Effect represents the effect of items or skills.
    abstract record Effect
    {
        // execute() method calculates the stats and condition.
        abstract public (Stats, Condition) Execute(Stats stats, Condition given);

        // Damage represents the damage to stats.
        public record Damage(int Value) : Effect
        {
            public override (Stats, Condition) Execute(Stats stats, Condition given)
            {
                return (stats with { HP = stats.HP - (Value - stats.DEF) }, given);
            }
        }
        public record Heal(int Value) : Effect
        {
            public override (Stats, Condition) Execute(Stats stats, Condition given)
            {
                return (stats with { HP = stats.HP + Value }, given);
            }
        }
        public record Charge(int Value) : Effect
        {
            public override (Stats, Condition) Execute(Stats stats, Condition given)
            {
                return (stats with { MP = stats.MP + Value }, given);
            }
        }
        public record Buff(int Value) : Effect
        {
            public override (Stats, Condition) Execute(Stats stats, Condition given)
            {
                return (stats with { ATK = stats.ATK + Value }, given);
            }
        }
        public record Guard(int Value) : Effect
        {
            public override (Stats, Condition) Execute(Stats stats, Condition given)
            {
                return (stats with { DEF = stats.DEF + Value }, given);
            }
        }
        public record When(Condition Want, Effect Effect) : Effect
        {
            public override (Stats, Condition) Execute(Stats stats, Condition given)
            {
                if (given.Satisfies(Want))
                {
                    return Effect.Execute(stats, given);
                }
                else
                {
                    return (stats, given);
                }
            }
        }
        public record Then(Effect First, Effect Second) : Effect
        {
            public override (Stats, Condition) Execute(Stats stats, Condition given)
            {
                var (stats1, given1) = First.Execute(stats, given);
                return Second.Execute(stats1, given1);
            }
        }

    }
}

namespace Game
{
    class Program
    {
        static void Main(string[] args)
        {
            var player = new Edsl.Stats("Player", 100, 100, 20, 0);
            Console.WriteLine($"Player: {player}");
            var enemy = new Edsl.Stats("Enemy", 45, 0, 20, 0);
            Console.WriteLine($"Enemy: {enemy}");

            var buffItem = new Edsl.Effect.Buff(10);
            var guardItem = new Edsl.Effect.Guard(10);

            // attack 10 damage, then attack 20 damage if grabbed
            var grabAttack = (Edsl.Stats player) => new Edsl.Effect.Then(
                new Edsl.Effect.Damage(player.ATK),
                new Edsl.Effect.When(
                    new Edsl.Condition.Grabbed(enemy),
                    new Edsl.Effect.Damage(player.ATK * 2)
                )
            );

            // enemy is grabbed
            var condition = new Edsl.Condition.Grabbed(enemy);

            // player use buffItem
            var (player1, condition1) = buffItem.Execute(player, condition);
            Console.WriteLine($"Player use buffItem: {player1}");

            // enemy use guardItem
            var (enemy1, condition2) = guardItem.Execute(enemy, condition1);
            Console.WriteLine($"Enemy use guardItem: {enemy1}");

            // enemy attacked by player with grabAttack
            // damage is (30 - 10) + (30 * 2 - 10) = 20 + 50 = 70
            var (enemy2, _) = grabAttack(player1).Execute(enemy1, condition2);
            Console.WriteLine($"Enemy attacked: {enemy2}");
        }
    }
}