/interpreter/tags/at2dist090708/src/edu/vub/at/objects/natives/grammar/AGBegin.java

 1/**
 2 * AmbientTalk/2 Project
 3 * AGBegin.java created on 26-jul-2006 at 12:26:48
 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.eval.Evaluator;
31import edu.vub.at.exceptions.InterpreterException;
32import edu.vub.at.exceptions.XTypeMismatch;
33import edu.vub.at.objects.ATContext;
34import edu.vub.at.objects.ATObject;
35import edu.vub.at.objects.ATTable;
36import edu.vub.at.objects.grammar.ATBegin;
37import edu.vub.at.objects.natives.NATNumber;
38import edu.vub.at.objects.natives.NATText;
39
40/**
41 * AGBegin represents the abstract grammar element of a list of statements.
42 * Examples:
43 *  <tt>a ; b ; c</tt>
44 *  
45 * @author tvcutsem
46 */
47public final class AGBegin extends NATAbstractGrammar implements ATBegin {
48	
49	private final ATTable statements_;
50	
51	public AGBegin(ATTable statements) {
52	  statements_ = statements;
53	}
54	
55	/**
56	 * AGBEGIN encapsulates a sequence of statements and evaluates each of these statements in turn.
57	 * The return value is the value of the last statement.
58	 * 
59	 * AGBEGIN([statement | statements]).eval(ctx) = statement.eval(ctx) ; BEGIN(statements).eval(ctx)
60	 * AGBEGIN([statement]).eval(ctx) = statement.eval(ctx)
61	 * 
62	 * Note that this implementation of begin is *not* tail-recursive.
63	 * Tail-recursion is made impossible because Java will not allow a return
64	 * to the caller until the last expression is also evaluated. Tail-recursion optimalisation
65	 * requires a minimal form of explicit continuations.
66	 */
67	public ATObject meta_eval(ATContext ctx) throws InterpreterException {
68		NATNumber siz = statements_.base_length().asNativeNumber();
69		int lastIdx = siz.javaValue;
70		if (lastIdx == 0) {
71			return Evaluator.getNil();
72		}
73		for (int i = 1; i < lastIdx; i++) {
74			statements_.base_at(NATNumber.atValue(i)).meta_eval(ctx);
75		}
76		return statements_.base_at(NATNumber.atValue(lastIdx)).meta_eval(ctx);
77	}
78
79	/**
80	 * AGBEGIN(statements*).quote(ctx) = AGBEGIN((statements*).quote(ctx))
81	 */
82	public ATObject meta_quote(ATContext ctx) throws InterpreterException {
83		return new AGBegin(statements_.meta_quote(ctx).asTable());
84	}
85	
86	public ATTable base_statements() { return statements_; }
87	
88	public NATText meta_print() throws InterpreterException {
89		return Evaluator.printAsStatements(statements_);
90	}
91	
92	public ATBegin asBegin() throws XTypeMismatch {
93		return this;
94	}
95
96}