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Truffle SL is an example language implemented using the Truffle API.
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/*
 * Copyright (c) 2012, 2018, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * The Universal Permissive License (UPL), Version 1.0
 *
 * Subject to the condition set forth below, permission is hereby granted to any
 * person obtaining a copy of this software, associated documentation and/or
 * data (collectively the "Software"), free of charge and under any and all
 * copyright rights in the Software, and any and all patent rights owned or
 * freely licensable by each licensor hereunder covering either (i) the
 * unmodified Software as contributed to or provided by such licensor, or (ii)
 * the Larger Works (as defined below), to deal in both
 *
 * (a) the Software, and
 *
 * (b) any piece of software and/or hardware listed in the lrgrwrks.txt file if
 * one is included with the Software each a "Larger Work" to which the Software
 * is contributed by such licensors),
 *
 * without restriction, including without limitation the rights to copy, create
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 * Software and the Larger Work(s), and to sublicense the foregoing rights on
 * either these or other terms.
 *
 * This license is subject to the following condition:
 *
 * The above copyright notice and either this complete permission notice or at a
 * minimum a reference to the UPL must be included in all copies or substantial
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 */
package com.oracle.truffle.sl.runtime;

import com.oracle.truffle.api.interop.InteropLibrary;
import com.oracle.truffle.api.interop.TruffleObject;
import com.oracle.truffle.api.library.ExportLibrary;
import com.oracle.truffle.api.library.ExportMessage;

/**
 * The SL type for a {@code null} (i.e., undefined) value. In Truffle, it is generally discouraged
 * to use the Java {@code null} value to represent the guest language {@code null} value. It is not
 * possible to specialize on Java {@code null} (since you cannot ask it for the Java class), and
 * there is always the danger of a spurious {@link NullPointerException}. Representing the guest
 * language {@code null} as a singleton, as in {@link #SINGLETON this class}, is the recommended
 * practice.
 */
@ExportLibrary(InteropLibrary.class)
@SuppressWarnings("static-method")
public final class SLNull implements TruffleObject {

    /**
     * The canonical value to represent {@code null} in SL.
     */
    public static final SLNull SINGLETON = new SLNull();

    /**
     * Disallow instantiation from outside to ensure that the {@link #SINGLETON} is the only
     * instance.
     */
    private SLNull() {
    }

    /**
     * This method is, e.g., called when using the {@code null} value in a string concatenation. So
     * changing it has an effect on SL programs.
     */
    @Override
    public String toString() {
        return "NULL";
    }

    /**
     * {@link SLNull} values are interpreted as null values by other languages.
     */
    @ExportMessage
    boolean isNull() {
        return true;
    }
}