Environment.java

package jscheme;

/** Environments allow you to look up the value of a variable, given
 * its name.  Keep a list of variables and values, and a pointer to
 * the parent environment.  If a variable list ends in a symbol rather
 * than null, it means that symbol is bound to the remainder of the
 * values list. 
 * @author Peter Norvig, peter@norvig.com http://www.norvig.com 
 * Copyright 1998 Peter Norvig, see http://www.norvig.com/license.html */

public class Environment extends SchemeUtils {
	public Object vars;

	public Object vals;

	public Environment parent;

	/** A constructor to extend an environment with var/val pairs. */
	public Environment(Object vars, Object vals, Environment parent) {
		this.vars = vars;
		this.vals = vals;
		this.parent = parent;
		if (!numberArgsOK(vars, vals))
			warn("wrong number of arguments: expected " + vars + " got " + vals);
	}

	/** Construct an empty environment: no bindings. **/
	public Environment() {
	}

	/** Find the value of a symbol, in this environment or a parent. */
	public Object lookup(String symbol) {
		Object varList = vars, valList = vals;
		// See if the symbol is bound locally
		while (varList != null) {
			if (first(varList) == symbol) {
				return first(valList);
			} else if (varList == symbol) {
				return valList;
			} else {
				varList = rest(varList);
				valList = rest(valList);
			}
		}
		// If not, try to look for the parent
		if (parent != null)
			return parent.lookup(symbol);
		else
			return error("Unbound variable: " + symbol);
	}

	/** Add a new variable,value pair to this environment. */
	public Object define(Object var, Object val) {
		vars = cons(var, vars);
		vals = cons(val, vals);
		if (val instanceof Procedure
				&& ((Procedure) val).name.equals("anonymous procedure"))
			((Procedure) val).name = var.toString();
		return var;
	}

	/** Set the value of an existing variable **/
	public Object set(Object var, Object val) {
		if (!(var instanceof String))
			return error("Attempt to set a non-symbol: " + stringify(var));
		;
		String symbol = (String) var;
		Object varList = vars, valList = vals;
		// See if the symbol is bound locally
		while (varList != null) {
			if (first(varList) == symbol) {
				return setFirst(valList, val);
			} else if (rest(varList) == symbol) {
				return setRest(valList, val);
			} else {
				varList = rest(varList);
				valList = rest(valList);
			}
		}
		// If not, try to look for the parent
		if (parent != null)
			return parent.set(symbol, val);
		else
			return error("Unbound variable: " + symbol);
	}

	public Environment defPrim(String name, int id, int minArgs) {
		define(name, new Primitive(id, minArgs, minArgs));
		return this;
	}

	public Environment defPrim(String name, int id, int minArgs, int maxArgs) {
		define(name, new Primitive(id, minArgs, maxArgs));
		return this;
	}

	/** See if there is an appropriate number of vals for these vars. **/
	boolean numberArgsOK(Object vars, Object vals) {
		return ((vars == null && vals == null) || (vars instanceof String) || (vars instanceof Pair
				&& vals instanceof Pair && numberArgsOK(((Pair) vars).rest,
				((Pair) vals).rest)));
	}

}