transcendental-lisp/eval/EVAL.java

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

package eval;

import parser.*;
import java.util.HashMap;

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

    // A table to contain all the built-in and user-defined Lisp functions.
    private static HashMap<String, LispFunction> functionTable =
            new HashMap<String, LispFunction>();

    static {
        // place all of the built-in functions into the function table
        functionTable.put("*", new MULTIPLY());
        functionTable.put("+", new PLUS());
        functionTable.put("-", new MINUS());
        functionTable.put("/", new DIVIDE());
        functionTable.put("<", new LESSP());
        functionTable.put("=", new EQUALSP());
        functionTable.put(">", new GREATERP());
        functionTable.put("APPLY", new APPLY());
        functionTable.put("ATOM", new ATOM());
        functionTable.put("CAR", new CAR());
        functionTable.put("CDR", new CDR());
        functionTable.put("COND", new COND());
        functionTable.put("CONS", new CONS());
        functionTable.put("DEFUN", new DEFUN());
        functionTable.put("EQ", new EQ());
        functionTable.put("EQUAL", new EQUAL());
        functionTable.put("EVAL", new EVAL());
        functionTable.put("EXIT", new EXIT());
        functionTable.put("FIRST", new CAR());
        functionTable.put("FUNCALL", new FUNCALL());
        functionTable.put("GREATERP", new GREATERP());
        functionTable.put("LAMBDA", new LAMBDA());
        functionTable.put("LENGTH", new LENGTH());
        functionTable.put("LET", new LET());
        functionTable.put("LIST", new LIST());
        functionTable.put("LISTP", new LISTP());
        functionTable.put("LOAD", new LOAD());
        functionTable.put("NULL", new NULL());
        functionTable.put("PRINT", new PRINT());
        functionTable.put("QUOTE", new QUOTE());
        functionTable.put("REST", new CDR());
        functionTable.put("SETF", new SETF());
        functionTable.put("SYMBOL-FUNCTION", new SYMBOL_FUNCTION());
    }

    /**
     * Retrieve the function table.
     *
     * @return
     *  the function table
     */
    public static HashMap<String, LispFunction> getFunctionTable() {
        return functionTable;
    }

    /**
     * Look up a function by its name.
     *
     * @param functionName
     *  the name of the function to look up
     * @return
     *  the function with the name <code>functionName</code> if it exists; null
     *  otherwise
     */
    public static LispFunction lookupFunction(String functionName) {
        return functionTable.get(functionName);
    }

    /**
     * Look up a symbol's value using its name.
     *
     * @param symbolName
     *  the name of the symbol to look up (must not be null)
     * @return
     *  the value of <code>symbolName</code> if it has one; null otherwise
     */
    public static SExpression lookupSymbol(String symbolName) {
        if (symbolName.equals("NIL")) {
            return Nil.getUniqueInstance();
        } else if (symbolName.equals("T")) {
            return Symbol.T;
        } else if (symbolName.startsWith(":")) {
            return new Symbol(symbolName);
        }

        return SETF.lookup(symbolName);
    }

    /**
     * Determine if the given list is dotted.
     *
     * @param list
     *  the list to be tested (must not be null)
     * @return
     *  <code>true</code> if <code>list</code> is dotted; <code>false</code>
     *  otherwise
     */
    public static boolean isDotted(Cons list) {
        if (list.nullp()) {
            return false;
        }

        SExpression cdr = list.getCdr();

        if (cdr.listp()) {
            return isDotted((Cons) cdr);
        }

        // the cdr of 'list' is not a list, therefore it is dotted
        return true;
    }

    /**
     * Evaluate the given S-expression.
     *
     * @param sexpr
     *  the S-expression to evaluate
     * @return
     *  the value of <code>sexpr</code>
     */
    public static SExpression eval(SExpression sexpr) {
        Cons expList = LIST.makeList(sexpr);
        EVAL evalFunction = new EVAL();

        return evalFunction.call(expList);
    }

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

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

            throw new RuntimeException(errMsg);
        }

        SExpression arg = argList.getCar();

        if (arg.listp()) {
            if (arg.consp()) {
                return evaluateList((Cons) arg);
            }

            return arg;  // 'arg' is NIL
        }

        if (arg.symbolp()) {
            SExpression symbolValue = lookupSymbol(arg.toString());

            if (symbolValue != null) {
                return symbolValue;
            }

            throw new RuntimeException("variable " + arg + " has no value");
        }

        return arg;  // 'arg' is a NUMBER or a STRING
    }

    // Evaluate the specified list.
    //
    // Parameters: list - the list to evaluate
    // Returns: the value of 'list'
    // Precondition: 'list' must not be null.
    private SExpression evaluateList(Cons list) {
        SExpression car = list.getCar();
        SExpression cdr = list.getCdr();

        LispFunction function = 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);
            }
        }

        // make sure the list of arguments for 'function' is a list
        if (cdr.listp()) {
            Cons args = (Cons) cdr;

            // make sure the list of arguments is not dotted
            if (isDotted(args)) {
                throw new RuntimeException("argument list given to " + car +
                                           " is dotted: " + list);
            }

            // determine if we should evaluate the arguments that will be
            // passed to 'function'
            if (function.evaluateArguments()) {
                args = evaluateArgList(args);
            }

            return function.call(args);
        }

        // the list of arguments is not a list!
        throw new RuntimeException("argument list given to " + car +
                                   " is dotted: " + list);
    }

    // Evaluate a list of arguments for a function.
    //
    // Parameters: arguments - a list of arguments for a function
    // Returns: a list consisting of the values of the S-expressions found in
    //          'arguments'
    // Precondition: 'arguments' must not be null.
    private Cons evaluateArgList(Cons arguments) {
        if (arguments.nullp()) {
            return Nil.getUniqueInstance();
        }

        SExpression car = eval(arguments.getCar());
        SExpression cdr = arguments.getCdr();

        if (cdr.listp()) {
            return new Cons(car, evaluateArgList((Cons) cdr));
        }

        // remove any parameters found after a dot (put here in case the check
        // for a dotted parameter list is not done prior to this call)
        return new Cons(car, Nil.getUniqueInstance());
    }

}