/interpreter/tags/at_build150307/src/edu/vub/at/objects/natives/grammar/AGSymbol.java

  1/**
  2 * AmbientTalk/2 Project
  3 * AGSymbol.java created on 26-jul-2006 at 16:21:55
  4 * (c) Programming Technology Lab, 2006 - 2007
  5 * Authors: Tom Van Cutsem & Stijn Mostinckx
  6 * 
  7 * Permission is hereby granted, free of charge, to any person
  8 * obtaining a copy of this software and associated documentation
  9 * files (the "Software"), to deal in the Software without
 10 * restriction, including without limitation the rights to use,
 11 * copy, modify, merge, publish, distribute, sublicense, and/or
 12 * sell copies of the Software, and to permit persons to whom the
 13 * Software is furnished to do so, subject to the following
 14 * conditions:
 15 *
 16 * The above copyright notice and this permission notice shall be
 17 * included in all copies or substantial portions of the Software.
 18 *
 19 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
 20 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
 21 * OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
 22 * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
 23 * HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
 24 * WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 25 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
 26 * OTHER DEALINGS IN THE SOFTWARE.
 27 */
 28package edu.vub.at.objects.natives.grammar;
 29
 30import edu.vub.at.exceptions.InterpreterException;
 31import edu.vub.at.objects.ATContext;
 32import edu.vub.at.objects.ATObject;
 33import edu.vub.at.objects.ATText;
 34import edu.vub.at.objects.grammar.ATSymbol;
 35import edu.vub.at.objects.natives.NATText;
 36
 37import java.util.HashMap;
 38
 39/**
 40 * @author tvcutsem
 41 *
 42 * The native implementation of a symbol AG element.
 43 * Symbols should only be created via a call to AGSymbol.alloc
 44 * This ensures that symbols remain unique within one AmbientTalk VM.
 45 */
 46public class AGSymbol extends AGExpression implements ATSymbol {
 47
 48	/** hashmap shared by ALL actors/VMs, hence, thread access should be explicitly synchronized */
 49	private static final HashMap _STRINGPOOL_ = new HashMap();
 50
 51	private final String txt_;
 52	
 53	protected AGSymbol(String txt) {
 54		txt_ = txt;
 55	}
 56	
 57	public static final AGSymbol jAlloc(String name) {
 58		synchronized (_STRINGPOOL_) {
 59			AGSymbol existing = (AGSymbol) _STRINGPOOL_.get(name);
 60			if (existing == null) {
 61				existing = new AGSymbol(name);
 62				_STRINGPOOL_.put(name, existing);
 63			}
 64			return existing;	
 65		}
 66	}
 67	
 68	public static final AGSymbol alloc(NATText txt) {
 69		return jAlloc(txt.javaValue);
 70	}
 71	
 72	public ATText base_getText() { return NATText.atValue(txt_); }
 73	
 74	/**
 75	 * To evaluate a symbol reference, look up the symbol in the lexical scope.
 76	 * 
 77	 * AGSYM(txt).eval(ctx) = ctx.scope.lookup(AGSYM(txt))
 78	 * 
 79	 * @return the value bound to this symbol in the lexical environment
 80	 */
 81	public ATObject meta_eval(ATContext ctx) throws InterpreterException {
 82		return ctx.base_getLexicalScope().meta_lookup(this);
 83	}
 84
 85	/**
 86	 * Quoting a symbol results in the same symbol.
 87	 * 
 88	 * sym.quote(ctx) = sym
 89	 */
 90	public ATObject meta_quote(ATContext ctx) throws InterpreterException {
 91		return this;
 92	}
 93	
 94	public NATText meta_print() throws InterpreterException {
 95		return NATText.atValue(txt_);
 96	}
 97	
 98	public boolean isSymbol() {
 99		return true;
100	}
101	
102	public ATSymbol asSymbol() {
103		return this;
104	}
105	
106	// comparison and identity operations
107	
108	public boolean equals(Object other) {
109		// pointer equality is valid for symbols as they are pooled
110		return this == other;
111	}
112	
113	// important as AGSymbols are often used as keys in hash tables
114	
115	public int hashCode() {
116		return this.txt_.hashCode();
117	}
118	
119	public String toString() {
120		return txt_;
121	}
122	
123	/**
124	 * After deserialization, ensure that the symbol remains unique.
125	 */
126	public ATObject meta_resolve() throws InterpreterException {
127		return jAlloc(txt_);
128	}
129
130}