KumiScript/interpreter/expression/NilExpression.cs

namespace KumiScript.Interpreter
{
    public class NilExpression : ListExpression
    {
        private static NilExpression? _instance;
        private NilExpression()
        {
        }

        public static NilExpression GetInstance()
        {
            if (_instance is null)
                _instance = new NilExpression();

            return _instance;
        }

        public override Expression Car()
        {
            return this;
        }

        public override ListExpression Cdr()
        {
            return this;
        }

        public override ProperListExpression Cons(Expression expr)
        {
            return new ProperListExpression(new List<Expression> {expr});
        }

        public override bool Equals(Expression expr)
        {
            return expr is NilExpression;
        }

        public override Expression Eval(Environment env)
        {
            return this;
        }

        public override List<Expression> EvalMembers(Environment env)
        {
            return new List<Expression>();
        }

        public override List<Expression> GetMembers()
        {
            return new List<Expression>();
        }

        public override string ToString()
        {
            return "null";
        }
    }
}