/*
 * Name: Mike Cifelli
 * Course: CIS 443 - Programming Languages
 * Assignment: Lisp Interpreter 2
 */

package eval;

import parser.*;

/**
 * <code>NULL</code> represents the NULL function in Lisp.
 */
public class NULL extends LispFunction {

    // The number of arguments that NULL takes.
    private static final int NUM_ARGS = 1;

    public SExpression call(Cons argList) {
        // retrieve the number of arguments passed to NULL
        int argListLength = LENGTH.getLength(argList);

        // make sure we have received the proper number of arguments
        if (argListLength != NUM_ARGS) {
            Cons originalSExpr = new Cons(new Symbol("NULL"), argList);
            String errMsg = "too " +
                            ((argListLength > NUM_ARGS) ? "many" : "few") +
                            " arguments given to NULL: " + originalSExpr;

            throw new RuntimeException(errMsg);
        }

        SExpression arg = argList.getCar();

        return (arg.nullp() ? Symbol.T : Nil.getUniqueInstance());
    }

}