定义抽象语法树，也就是各种表达式的数据结构。
class BaseAST {
public:
virtual ~BaseAST();
};
class VariableAST : public BaseAST {
std::string var_name_;
public:
VariableAST(std::string &name) : var_name_(name) {}
};
class NumericAST : public BaseAST {
int numeric_val_;
public:
NumericAST(int val) : numeric_val_(val) {}
};
class BinaryAST : public BaseAST {
std::string operator_;
BaseAST *lhs_, *rhs_;
public:
BinaryAST(std::string op, BaseAST* lhs, BaseAST* rhs) :
operator_(op), lhs_(lhs), rhs_(rhs) {}
};
class FunctionDeclAST {
std::string func_name_;
std::vector<std::string> args_;
public:
FunctionDeclAST(const std::string &name, const std::vector<std::string> &args) :
func_name_(name), args_(args) {}
};
class FunctionDefnAST {
FunctionDeclAST *func_decl_;
BaseAST* body_;
public:
FunctionDefnAST(FunctionDeclAST *proto, BaseAST *body) :
func_decl_(proto), body_(body) {}
};
class FunctionCallAST : public BaseAST {
std::string callee_;
std::vector<BaseAST*> args_;
public:
FunctionCallAST(const std::string &callee, const std::vector<BaseAST*> args) :
callee_(callee), args_(args) {}
};
