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Truffle SL is an example language implemented using the Truffle API.
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/*
 * Copyright (c) 2014, 2019, 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
 * derivative works of, display, perform, and distribute the Software and make,
 * use, sell, offer for sale, import, export, have made, and have sold the
 * 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
 * portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 */
package com.oracle.truffle.sl.builtins;

import java.io.BufferedReader;
import java.io.IOException;

import com.oracle.truffle.api.CompilerDirectives.TruffleBoundary;
import com.oracle.truffle.api.dsl.Specialization;
import com.oracle.truffle.api.nodes.NodeInfo;
import com.oracle.truffle.sl.SLException;
import com.oracle.truffle.sl.runtime.SLContext;

/**
 * Builtin function that reads a String from the {@link SLContext#getInput() standard input}.
 */
@NodeInfo(shortName = "readln")
public abstract class SLReadlnBuiltin extends SLBuiltinNode {

    @Specialization
    public String readln() {
        String result = doRead(SLContext.get(this).getInput());
        if (result == null) {
            /*
             * We do not have a sophisticated end of file handling, so returning an empty string is
             * a reasonable alternative. Note that the Java null value should never be used, since
             * it can interfere with the specialization logic in generated source code.
             */
            result = "";
        }
        return result;
    }

    @TruffleBoundary
    private String doRead(BufferedReader in) {
        try {
            return in.readLine();
        } catch (IOException ex) {
            throw new SLException(ex.getMessage(), this);
        }
    }
}