transcendental-lisp/src/eval/APPLY.java

package eval;

import sexpression.*;

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

    /**
     * Call APPLY with the specified argument list.
     *
     * @param argList
     *            the list of arguments to be sent to APPLY (MUST BE A PROPER LIST)
     * @return the result of evaluating APPLY on <code>argList</code>
     */
    public static SExpression apply(Cons argList) {
        return new APPLY().call(argList);
    }

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

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

            throw new RuntimeException(errMsg);
        }

        SExpression car = argList.getCar(); // function name
        Cons cdr = (Cons) argList.getCdr();
        SExpression cadr = cdr.getCar(); // argument list

        // make sure the second argument is a list
        if (cadr.listp()) {
            LispFunction function = EVAL.lookupFunction(car.toString());

            if (function == null) {
                // check if the car of the list is a lambda expression
                if (car.functionp()) {
                    function = ((LambdaExpression) car).getFunction();
                } else if (LAMBDA.isLambdaExpression(car)) {
                    Cons lexpr = (Cons) car;

                    function = LAMBDA.createFunction(lexpr);
                } else {
                    throw new RuntimeException("undefined function " + car);
                }
            }

            // apply the given function to the given argument list
            return function.call((Cons) cadr);
        }

        // the second argument is not a list
        throw new RuntimeException("APPLY: " + cadr + " is not a list");
    }

}