/*
 * Name: Mike Cifelli
 * Course: CIS 443 - Programming Languages
 * Assignment: Lisp Interpreter 1
 */

package eval;

import parser.*;

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

    // The number of arguments that CONS takes.
    private static final int NUM_ARGS = 2;

    public Cons call(Cons argList) {
        // retrieve the number of arguments passed to CONS
        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("CONS"), argList);
            String errMsg = "too " +
                            ((argListLength > NUM_ARGS) ? "many" : "few") +
                            " arguments given to CONS: " + originalSExpr;

            throw new RuntimeException(errMsg);
        }

        // the car of the CONS cell we are going to create
        SExpression argOne = argList.getCar();

        Cons cdr = (Cons) argList.getCdr();

        // the cdr of the CONS cell we are going to create
        SExpression argTwo = cdr.getCar();

        return new Cons(argOne, argTwo);
    }

}