transcendental-lisp/eval/SETF.java

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

package eval;

import parser.*;

/**
 * <code>SETF</code> represents the SETF form in Lisp.
 */
public class SETF extends LispFunction {

    private static SymbolTable environment = new SymbolTable();

    /**
     * Look up the value of a symbol using its name.
     *
     * @param symbolName
     *  the name of the symbol to look up
     * @return
     *  the value of <code>symbolName</code> if it has one; null otherwise
     */
    public static SExpression lookup(String symbolName) {
        SymbolTable current = environment;

        while (current != null) {
            if (current.contains(symbolName)) {
                return current.get(symbolName);
            }

            current = current.getParent();
        }

        return null;
    }

    /**
     * Set the current environment to the specified value.
     *
     * @param newEnvironment
     *  the value to set the environment to
     */
    public static void setEnvironment(SymbolTable newEnvironment) {
        environment = newEnvironment;
    }

    /**
     * Retrieve the current environment.
     *
     * @return
     *  the current environment
     */
    public static SymbolTable getEnvironment() {
        return environment;
    }

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

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

            throw new RuntimeException(errMsg);
        }

        SExpression symbol = argList.getCar();

        // make sure the first argument is a symbol
        if (! symbol.symbolp()) {
            throw new RuntimeException("SETF: " + symbol + " is not a symbol");
        }

        Cons cdr = (Cons) argList.getCdr();
        SExpression value = EVAL.eval(cdr.getCar());

        SymbolTable current = environment;

        // set 'current' to the symbol table that contains 'symbol' or the
        // global symbol table if 'symbol' is not in the environment
        while ((! current.contains(symbol.toString())) &&
               (current.getParent() != null)) {
            current = current.getParent();
        }

        current.put(symbol.toString(), value);

        return value;
    }

    /**
     * Determine if the arguments passed to this Lisp function should be
     * evaluated.
     *
     * @return
     *  <code>false</code>
     */
    public boolean evaluateArguments() {
        return false;
    }

}