LibJS: Start building a JavaScript engine for SerenityOS :^)

I always tell people to start building things by working on the thing
that seems the most interesting right now. The most interesting thing
here was an AST + simple interpreter, so that's where we start!

There is no lexer or parser yet, we build an AST directly and then
execute it in the interpreter, producing a return value.

This seems like the start of something interesting. :^)

Libraries/LibJS/AST.h

@@ -0,0 +1,188 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+#include <AK/NonnullOwnPtrVector.h>
+#include <AK/String.h>
+#include <LibJS/Forward.h>
+#include <LibJS/Value.h>
+
+namespace JS {
+
+class ASTNode {
+public:
+    virtual ~ASTNode() {}
+    virtual const char* class_name() const = 0;
+    virtual Value execute(Interpreter&) const = 0;
+    virtual void dump(int indent) const;
+
+protected:
+    ASTNode() {}
+
+private:
+};
+
+class ScopeNode : public ASTNode {
+public:
+    template<typename T, typename... Args>
+    T& append(Args&&... args)
+    {
+        auto child = make<T>(forward<Args>(args)...);
+        m_children.append(move(child));
+        return static_cast<T&>(m_children.last());
+    }
+
+    const NonnullOwnPtrVector<ASTNode>& children() const { return m_children; }
+    virtual Value execute(Interpreter&) const override;
+    virtual void dump(int indent) const override;
+
+protected:
+    ScopeNode() {}
+
+private:
+    NonnullOwnPtrVector<ASTNode> m_children;
+};
+
+class Program : public ScopeNode {
+public:
+    Program() {}
+
+private:
+    virtual const char* class_name() const override { return "Program"; }
+};
+
+class BlockStatement : public ScopeNode {
+public:
+    BlockStatement() {}
+
+private:
+    virtual const char* class_name() const override { return "BlockStatement"; }
+};
+
+class FunctionDeclaration : public ASTNode {
+public:
+    FunctionDeclaration(String name, NonnullOwnPtr<ScopeNode> body)
+        : m_name(move(name))
+        , m_body(move(body))
+    {
+    }
+
+    String name() const { return m_name; }
+    const ScopeNode& body() const { return *m_body; }
+
+    virtual Value execute(Interpreter&) const override;
+    virtual void dump(int indent) const override;
+
+private:
+    virtual const char* class_name() const override { return "FunctionDeclaration"; }
+
+    String m_name;
+    NonnullOwnPtr<ScopeNode> m_body;
+};
+
+class Expression : public ASTNode {
+public:
+};
+
+class ReturnStatement : public ASTNode {
+public:
+    explicit ReturnStatement(NonnullOwnPtr<Expression> argument)
+        : m_argument(move(argument))
+    {
+    }
+
+    const Expression& argument() const { return *m_argument; }
+
+    virtual Value execute(Interpreter&) const override;
+    virtual void dump(int indent) const override;
+
+private:
+    virtual const char* class_name() const override { return "ReturnStatement"; }
+
+    NonnullOwnPtr<Expression> m_argument;
+};
+
+enum class BinaryOp {
+    Plus,
+    Minus,
+};
+
+class BinaryExpression : public Expression {
+public:
+    BinaryExpression(BinaryOp op, NonnullOwnPtr<Expression> lhs, NonnullOwnPtr<Expression> rhs)
+        : m_op(op)
+        , m_lhs(move(lhs))
+        , m_rhs(move(rhs))
+    {
+    }
+
+    virtual Value execute(Interpreter&) const override;
+    virtual void dump(int indent) const override;
+
+private:
+    virtual const char* class_name() const override { return "BinaryExpression"; }
+
+    BinaryOp m_op;
+    NonnullOwnPtr<Expression> m_lhs;
+    NonnullOwnPtr<Expression> m_rhs;
+};
+
+class Literal : public Expression {
+public:
+    explicit Literal(Value value)
+        : m_value(move(value))
+    {
+    }
+
+    virtual Value execute(Interpreter&) const override { return m_value; }
+    virtual void dump(int indent) const override;
+
+private:
+    virtual const char* class_name() const override { return "Literal"; }
+
+    Value m_value;
+};
+
+class CallExpression : public Expression {
+public:
+    explicit CallExpression(String name)
+        : m_name(move(name))
+    {
+    }
+
+    virtual Value execute(Interpreter&) const override;
+    virtual void dump(int indent) const override;
+
+    const String& name() const { return m_name; }
+
+private:
+    virtual const char* class_name() const override { return "CallExpression"; }
+
+    String m_name;
+};
+
+}