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diff --git a/tiger-compiler/src/ast/visitor.hh b/tiger-compiler/src/ast/visitor.hh
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+/**
+ ** \file ast/visitor.hh
+ ** \brief Definition of ast::Visitor.
+ */
+
+#pragma once
+
+#include <functional>
+#include <ast/fwd.hh>
+#include <misc/select-const.hh>
+
+namespace ast
+{
+  /** \brief Root class of all Ast visitors.
+   **
+   ** GenVisitor<CONSTIFY> is the root class of all Ast visitors. */
+  template <template <typename> class Const> class GenVisitor
+  {
+    /** \name Ctor & dtor.
+     ** \{ */
+  public:
+    /// Convenient abbreviation.
+    template <typename Type> using const_t = typename Const<Type>::type;
+
+    /// Destroy a GenVisitor.
+    virtual ~GenVisitor();
+    /** \} */
+
+    /// The entry point: visit \a e.
+    virtual void operator()(const_t<Ast>& e);
+    virtual void operator()(const_t<ArrayExp>&) = 0;
+    virtual void operator()(const_t<ArrayTy>&) = 0;
+    virtual void operator()(const_t<AssignExp>&) = 0;
+    virtual void operator()(const_t<BreakExp>&) = 0;
+    virtual void operator()(const_t<CallExp>&) = 0;
+    virtual void operator()(const_t<CastExp>&) = 0;
+    virtual void operator()(const_t<ChunkList>&) = 0;
+    virtual void operator()(const_t<ClassTy>&) = 0;
+    virtual void operator()(const_t<Field>&) = 0;
+    virtual void operator()(const_t<FieldInit>&) = 0;
+    virtual void operator()(const_t<FieldVar>&) = 0;
+    virtual void operator()(const_t<ForExp>&) = 0;
+    virtual void operator()(const_t<FunctionDec>&) = 0;
+    virtual void operator()(const_t<IfExp>&) = 0;
+    virtual void operator()(const_t<IntExp>&) = 0;
+    virtual void operator()(const_t<LetExp>&) = 0;
+    virtual void operator()(const_t<MethodCallExp>&) = 0;
+    virtual void operator()(const_t<MethodDec>&) = 0;
+    virtual void operator()(const_t<NameTy>&) = 0;
+    virtual void operator()(const_t<NilExp>&) = 0;
+    virtual void operator()(const_t<ObjectExp>&) = 0;
+    virtual void operator()(const_t<OpExp>&) = 0;
+    virtual void operator()(const_t<RecordExp>&) = 0;
+    virtual void operator()(const_t<RecordTy>&) = 0;
+    virtual void operator()(const_t<SeqExp>&) = 0;
+    virtual void operator()(const_t<SimpleVar>&) = 0;
+    virtual void operator()(const_t<StringExp>&) = 0;
+    virtual void operator()(const_t<SubscriptVar>&) = 0;
+    virtual void operator()(const_t<TypeDec>&) = 0;
+    virtual void operator()(const_t<VarDec>&) = 0;
+    virtual void operator()(const_t<WhileExp>&) = 0;
+
+    virtual void operator()(const_t<FunctionChunk>&) = 0;
+    virtual void operator()(const_t<MethodChunk>&) = 0;
+    virtual void operator()(const_t<TypeChunk>&) = 0;
+    virtual void operator()(const_t<VarChunk>&) = 0;
+
+    virtual void operator()(const_t<AssertExp>&) = 0;
+
+    /// Helper to visit nodes manipulated via a pointer.
+    template <class E> void operator()(E* e);
+
+  protected:
+    /** A convenient shortcut for recurring code like this:
+
+        \code
+        if (e)
+          e->accept(*this);
+        \endcode
+
+        However, the drawback of this approach is that it doesn't take
+        care of the constness, and any \a const violation will be
+        reported \em within the body of this method, not at its
+        corresponding call site.
+
+        We cannot use the misc/select_const.hh approach here, since
+        the compiler cannot resolve a function overloaded or
+        specialized on an associated type of a template.  E.g., writing
+        \a accept like this:
+
+        \code
+        template <typename E>
+        void accept(const_t<E>* e);
+        \endcode
+
+        won't work directly.  Of course, one can help the compiler,
+        providing it with \a E
+
+        \code
+        accept<ast::NameTy>(e.result_get());
+        \endcode
+
+        but this is painful.  */
+    template <typename E> void accept(E* e);
+  };
+
+  /// Shorthand for a const visitor.
+  using ConstVisitor = GenVisitor<misc::constify_traits>;
+  /// Shorthand for a non const visitor.
+  using Visitor = GenVisitor<misc::id_traits>;
+
+} // namespace ast
+
+#include <ast/visitor.hxx>