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Truffle SL is an example language implemented using the Truffle API.
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/*
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 * 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
 * portions of the Software.
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 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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package com.oracle.truffle.sl.nodes.controlflow;

import com.oracle.truffle.api.frame.VirtualFrame;
import com.oracle.truffle.api.nodes.NodeInfo;
import com.oracle.truffle.api.nodes.UnexpectedResultException;
import com.oracle.truffle.api.profiles.ConditionProfile;
import com.oracle.truffle.sl.SLException;
import com.oracle.truffle.sl.nodes.SLExpressionNode;
import com.oracle.truffle.sl.nodes.SLStatementNode;
import com.oracle.truffle.sl.nodes.util.SLUnboxNodeGen;

@NodeInfo(shortName = "if", description = "The node implementing a condional statement")
public final class SLIfNode extends SLStatementNode {

    /**
     * The condition of the {@code if}. This in a {@link SLExpressionNode} because we require a
     * result value. We do not have a node type that can only return a {@code boolean} value, so
     * {@link #evaluateCondition executing the condition} can lead to a type error.
     */
    @Child private SLExpressionNode conditionNode;

    /** Statement (or {@link SLBlockNode block}) executed when the condition is true. */
    @Child private SLStatementNode thenPartNode;

    /** Statement (or {@link SLBlockNode block}) executed when the condition is false. */
    @Child private SLStatementNode elsePartNode;

    /**
     * Profiling information, collected by the interpreter, capturing the profiling information of
     * the condition. This allows the compiler to generate better code for conditions that are
     * always true or always false. Additionally the {@link CountingConditionProfile} implementation
     * (as opposed to {@link BinaryConditionProfile} implementation) transmits the probability of
     * the condition to be true to the compiler.
     */
    private final ConditionProfile condition = ConditionProfile.createCountingProfile();

    public SLIfNode(SLExpressionNode conditionNode, SLStatementNode thenPartNode, SLStatementNode elsePartNode) {
        this.conditionNode = SLUnboxNodeGen.create(conditionNode);
        this.thenPartNode = thenPartNode;
        this.elsePartNode = elsePartNode;
    }

    @Override
    public void executeVoid(VirtualFrame frame) {
        /*
         * In the interpreter, record profiling information that the condition was executed and with
         * which outcome.
         */
        if (condition.profile(evaluateCondition(frame))) {
            /* Execute the then-branch. */
            thenPartNode.executeVoid(frame);
        } else {
            /* Execute the else-branch (which is optional according to the SL syntax). */
            if (elsePartNode != null) {
                elsePartNode.executeVoid(frame);
            }
        }
    }

    private boolean evaluateCondition(VirtualFrame frame) {
        try {
            /*
             * The condition must evaluate to a boolean value, so we call the boolean-specialized
             * execute method.
             */
            return conditionNode.executeBoolean(frame);
        } catch (UnexpectedResultException ex) {
            /*
             * The condition evaluated to a non-boolean result. This is a type error in the SL
             * program.
             */
            throw SLException.typeError(this, ex.getResult());
        }
    }
}