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comparison graal/com.oracle.truffle.sl/src/com/oracle/truffle/sl/runtime/SLNull.java @ 13821:b16ec83edc73

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Documentation and more refactoring of Simple Language
author Christian Wimmer <christian.wimmer@oracle.com>
date Wed, 29 Jan 2014 20:45:43 -0800
parents 71991b7a0f14
children 1d3c23e675ed
comparison
equal deleted inserted replaced
13820:20e7727588e8 13821:b16ec83edc73
1 /* 1 /*
2 * Copyright (c) 2012, 2012, Oracle and/or its affiliates. All rights reserved. 2 * Copyright (c) 2012, 2014, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 * 4 *
5 * This code is free software; you can redistribute it and/or modify it 5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as 6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation. 7 * published by the Free Software Foundation.
20 * or visit www.oracle.com if you need additional information or have any 20 * or visit www.oracle.com if you need additional information or have any
21 * questions. 21 * questions.
22 */ 22 */
23 package com.oracle.truffle.sl.runtime; 23 package com.oracle.truffle.sl.runtime;
24 24
25 /**
26 * The SL type for a {@code null} (i.e., undefined) value. In Truffle, it is generally discouraged
27 * to use the Java {@code null} value to represent the guest language {@code null} value. It is not
28 * possible to specialize on Java {@code null} (since you cannot ask it for the Java class), and
29 * there is always the danger of a spurious {@link NullPointerException}. Representing the guest
30 * language {@code null} as a singleton, as in {@link #SINGLETON this class}, is the recommended
31 * practice.
32 * */
25 public final class SLNull { 33 public final class SLNull {
26 34
27 public static final SLNull INSTANCE = new SLNull(); 35 /**
36 * The canonical value to represent {@code null} in SL.
37 */
38 public static final SLNull SINGLETON = new SLNull();
28 39
40 /**
41 * Disallow instantiation from outside to ensure that the {@link #SINGLETON} is the only
42 * instance.
43 */
29 private SLNull() { 44 private SLNull() {
30 } 45 }
31 46
47 /**
48 * This method is, e.g., called when using the {@code null} value in a string concatenation. So
49 * changing it has an effect on SL programs.
50 */
51 @Override
52 public String toString() {
53 return "null";
54 }
32 } 55 }