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diff --git a/src/expr.h b/src/expr.h
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+// Copyright 2015 Google Inc. All rights reserved
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef EXPR_H_
+#define EXPR_H_
+
+#include <string>
+#include <vector>
+
+#include "string_piece.h"
+
+using namespace std;
+
+class Evaluator;
+struct Loc;
+
+class Evaluable {
+ public:
+  virtual void Eval(Evaluator* ev, string* s) const = 0;
+  string Eval(Evaluator*) const;
+
+ protected:
+  Evaluable();
+  virtual ~Evaluable();
+};
+
+class Value : public Evaluable {
+ public:
+  // All NewExpr calls take ownership of the Value instances.
+  static Value* NewExpr(Value* v1, Value* v2);
+  static Value* NewExpr(Value* v1, Value* v2, Value* v3);
+  static Value* NewExpr(vector<Value*>* values);
+
+  static Value* NewLiteral(StringPiece s);
+  virtual ~Value();
+  virtual bool IsLiteral() const { return false; }
+  // Only safe after IsLiteral() returns true.
+  virtual StringPiece GetLiteralValueUnsafe() const { return ""; }
+
+  static string DebugString(const Value*);
+
+ protected:
+  Value();
+  virtual string DebugString_() const = 0;
+};
+
+enum struct ParseExprOpt {
+  NORMAL = 0,
+  DEFINE,
+  COMMAND,
+  FUNC,
+};
+
+Value* ParseExprImpl(const Loc& loc,
+                     StringPiece s,
+                     const char* terms,
+                     ParseExprOpt opt,
+                     size_t* index_out,
+                     bool trim_right_space = false);
+Value* ParseExpr(const Loc& loc,
+                 StringPiece s,
+                 ParseExprOpt opt = ParseExprOpt::NORMAL);
+
+string JoinValues(const vector<Value*>& vals, const char* sep);
+
+#endif  // EXPR_H_