flex_bison读书笔记

0x00: 起因

工作上的一些原因需要学习一下。做个记录督促自己读书学习,好好学习。

0x01: 一些概念

语句和表达式
  • 表达式(Expression)有值,而语句(Statement)不总有。
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    表达式是可以被求值的代码,而语句是一段可执行代码。
    因为表达式可被求值,所以它可写在赋值语句等号的右侧。
    而语句不一定有值,所以像import、for和break等语句就不能被用于赋值。
    Python的语句分为两大类:简单和复合语句。
    简单语句是指一逻辑行的代码。例如表达式语句、赋值语句和return语句等。
    复合语句是指包含、影响或控制一组语句的代码。例如if、try和class语句等。
    表达式本身可以作为表达式语句,也能作为赋值语句的右值或if语句的条件等,所以表达式可以作为语句的组成部分,但不是必须成分(例如continue语句)。
左递归
一个文法是左递归的,若我们可以找出其中存在某非终端符号A,最终会推导出来的句型(sentential form)里面包含以自己为最左符号(left-symbol)的句型
  1. 直接左递归
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Expr ----> Expr + Term

举个例子:

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A ---> Aa|C
  1. 间接左递归
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A ---> Ba|C
B ---> Ab|D

这种会产生:

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A ---> Ba ---> Aba ---> ...

0x02: 高级计算器的实现

先看语法分析的部分:
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$ cat fb3-2.y
/* Companion source code for "flex & bison", published by O'Reilly
* Media, ISBN 978-0-596-15597-1
* Copyright (c) 2009, Taughannock Networks. All rights reserved.
* See the README file for license conditions and contact info.
* $Header: /home/johnl/flnb/code/RCS/fb3-2.y,v 2.1 2009/11/08 02:53:18 johnl Exp $
*/
/* calculator with AST */
%{
# include <stdio.h>
# include <stdlib.h>
# include "fb3-2.h"
%}
%union {
struct ast *a;
double d;
struct symbol *s; /* which symbol */
struct symlist *sl;
int fn; /* which function */
}
/* declare tokens */ 表明类型
%token <d> NUMBER
%token <s> NAME
%token <fn> FUNC
%token EOL
%token IF THEN ELSE WHILE DO LET
//right、left表明了结合顺序,即优先级
%nonassoc <fn> CMP
%right '='
%left '+' '-'
%left '*' '/'
%nonassoc '|' UMINUS
//把值<a>赋值给了 stmt list explist 三者
%type <a> exp stmt list explist
//同理
%type <sl> symlist
%start calclist
%%
//语句,调用相对应的方法,生成AST
stmt: IF exp THEN list { $$ = newflow('I', $2, $4, NULL); }
| IF exp THEN list ELSE list { $$ = newflow('I', $2, $4, $6); }
| WHILE exp DO list { $$ = newflow('W', $2, $4, NULL); }
| exp
;
//右递归
list: /* nothing */ { $$ = NULL; }
| stmt ';' list { if ($3 == NULL)
$$ = $1;
else
$$ = newast('L', $1, $3);
}
;
//表达式的ast构建
exp: exp CMP exp { $$ = newcmp($2, $1, $3); }
| exp '+' exp { $$ = newast('+', $1,$3); }
| exp '-' exp { $$ = newast('-', $1,$3);}
| exp '*' exp { $$ = newast('*', $1,$3); }
| exp '/' exp { $$ = newast('/', $1,$3); }
| '|' exp { $$ = newast('|', $2, NULL); }
| '(' exp ')' { $$ = $2; }
| '-' exp %prec UMINUS { $$ = newast('M', $2, NULL); }
| NUMBER { $$ = newnum($1); }
| FUNC '(' explist ')' { $$ = newfunc($1, $3); }
| NAME { $$ = newref($1); }
| NAME '=' exp { $$ = newasgn($1, $3); }
| NAME '(' explist ')' { $$ = newcall($1, $3); }
;
//表达式列表
explist: exp
| exp ',' explist { $$ = newast('L', $1, $3); }
;
//符号列表,用于函数调用,右递归的
symlist: NAME { $$ = newsymlist($1, NULL); }
| NAME ',' symlist { $$ = newsymlist($1, $3); }
;
//计算器的顶层规则
calclist: /* nothing */
| calclist stmt EOL {
if(debug) dumpast($2, 0);
printf("= %4.4g\n> ", eval($2));
treefree($2);
}
//识别一个函数声明 let xxx() = xxx 这样的
| calclist LET NAME '(' symlist ')' '=' list EOL {
dodef($3, $5, $8);
printf("Defined %s\n> ", $3->name); }
| calclist error EOL { yyerrok; printf("> "); }
;
%%
词法分析部分:
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$ cat fb3-2.l
/* Companion source code for "flex & bison", published by O'Reilly
* Media, ISBN 978-0-596-15597-1
* Copyright (c) 2009, Taughannock Networks. All rights reserved.
* See the README file for license conditions and contact info.
* $Header: /home/johnl/flnb/code/RCS/fb3-2.l,v 2.1 2009/11/08 02:53:18 johnl Exp $
*/
/* recognize tokens for the calculator */
%option noyywrap nodefault yylineno
%{
# include "fb3-2.h"
# include "fb3-2.tab.h"
%}
/* float exponent */ 浮点数,e开头,正负,这是指数部分
EXP ([Ee][-+]?[0-9]+)
//操作符单操作数
%%
/* single character ops */
"+" |
"-" |
"*" |
"/" |
"=" |
"|" |
"," |
";" |
"(" |
")" { return yytext[0]; }
//双操作数操作符
/* comparison ops */
">" { yylval.fn = 1; return CMP; }
"<" { yylval.fn = 2; return CMP; }
"<>" { yylval.fn = 3; return CMP; }
"==" { yylval.fn = 4; return CMP; }
">=" { yylval.fn = 5; return CMP; }
"<=" { yylval.fn = 6; return CMP; }
//关键字
/* keywords */
"if" { return IF; }
"then" { return THEN; }
"else" { return ELSE; }
"while" { return WHILE; }
"do" { return DO; }
"let" { return LET;}
//内建的一些函数
/* built in functions */
"sqrt" { yylval.fn = B_sqrt; return FUNC; }
"exp" { yylval.fn = B_exp; return FUNC; }
"log" { yylval.fn = B_log; return FUNC; }
"print" { yylval.fn = B_print; return FUNC; }
/* debug hack */
"debug"[0-9]+ { debug = atoi(&yytext[5]); printf("debug set to %d\n", debug); }
//声明的函数的函数名,字母开头
/* names */
[a-zA-Z][a-zA-Z0-9]* { yylval.s = lookup(yytext); return NAME; }
//浮点数
[0-9]+"."[0-9]*{EXP}? |
"."?[0-9]+{EXP}? { yylval.d = atof(yytext); return NUMBER; }
//其他的符号
"//".*
[ \t] /* ignore white space */
\\\n printf("c> "); /* ignore line continuation */
"\n" { return EOL; }
. { yyerror("Mystery character %c\n", *yytext); }
%%
函数实现,构造ast什么的:
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$ cat fb3-2funcs.c
/* Companion source code for "flex & bison", published by O'Reilly
* Media, ISBN 978-0-596-15597-1
* Copyright (c) 2009, Taughannock Networks. All rights reserved.
* See the README file for license conditions and contact info.
* $Header: /home/johnl/flnb/code/RCS/fb3-2funcs.c,v 2.1 2009/11/08 02:53:18 johnl Exp $
*/
/*
* helper functions for fb3-2
*/
# include <stdio.h>
# include <stdlib.h>
# include <stdarg.h>
# include <string.h>
# include <math.h>
# include "fb3-2.h"
//这些是辅助函数:构造符号表、hash算法、查找
/* symbol table */
/* hash a symbol */
static unsigned
symhash(char *sym)
{
unsigned int hash = 0;
unsigned c;
while(c = *sym++) hash = hash*9 ^ c;
return hash;
}
struct symbol *
lookup(char* sym)
{
struct symbol *sp = &symtab[symhash(sym)%NHASH];
int scount = NHASH; /* how many have we looked at */
while(--scount >= 0) {
if(sp->name && !strcmp(sp->name, sym)) { return sp; }
if(!sp->name) { /* new entry */
sp->name = strdup(sym);
sp->value = 0;
sp->func = NULL;
sp->syms = NULL;
return sp;
}
if(++sp >= symtab+NHASH) sp = symtab; /* try the next entry */
}
yyerror("symbol table overflow\n");
abort(); /* tried them all, table is full */
}
//这是构造ast的函数,根据参数,填充ast的结构。
//填充 type,节点什么的
struct ast *
newast(int nodetype, struct ast *l, struct ast *r)
{
struct ast *a = malloc(sizeof(struct ast));
if(!a) {
yyerror("out of space");
exit(0);
}
a->nodetype = nodetype;
a->l = l;
a->r = r;
return a;
}
//number的ast
struct ast *
newnum(double d)
{
struct numval *a = malloc(sizeof(struct numval));
if(!a) {
yyerror("out of space");
exit(0);
}
a->nodetype = 'K';
a->number = d;
return (struct ast *)a;
}
//比较表达式的ast
struct ast *
newcmp(int cmptype, struct ast *l, struct ast *r)
{
struct ast *a = malloc(sizeof(struct ast));
if(!a) {
yyerror("out of space");
exit(0);
}
a->nodetype = '0' + cmptype;
a->l = l;
a->r = r;
return a;
}
//函数的ast
struct ast *
newfunc(int functype, struct ast *l)
{
struct fncall *a = malloc(sizeof(struct fncall));
if(!a) {
yyerror("out of space");
exit(0);
}
a->nodetype = 'F';
a->l = l;
a->functype = functype;
return (struct ast *)a;
}
//调用的ast
// call funcname();这种
struct ast *
newcall(struct symbol *s, struct ast *l)
{
struct ufncall *a = malloc(sizeof(struct ufncall));
if(!a) {
yyerror("out of space");
exit(0);
}
a->nodetype = 'C';
a->l = l;
a->s = s;
return (struct ast *)a;
}
//引用的ast
struct ast *
newref(struct symbol *s)
{
struct symref *a = malloc(sizeof(struct symref));
if(!a) {
yyerror("out of space");
exit(0);
}
a->nodetype = 'N';
a->s = s;
return (struct ast *)a;
}
//赋值表达式 ast
struct ast *
newasgn(struct symbol *s, struct ast *v)
{
struct symasgn *a = malloc(sizeof(struct symasgn));
if(!a) {
yyerror("out of space");
exit(0);
}
a->nodetype = '=';
a->s = s;
a->v = v;
return (struct ast *)a;
}
//条件表达式的ast
struct ast *
newflow(int nodetype, struct ast *cond, struct ast *tl, struct ast *el)
{
struct flow *a = malloc(sizeof(struct flow));
if(!a) {
yyerror("out of space");
exit(0);
}
a->nodetype = nodetype;
a->cond = cond;
a->tl = tl;
a->el = el;
return (struct ast *)a;
}
//符号list
struct symlist *
newsymlist(struct symbol *sym, struct symlist *next)
{
struct symlist *sl = malloc(sizeof(struct symlist));
if(!sl) {
yyerror("out of space");
exit(0);
}
sl->sym = sym;
sl->next = next;
return sl;
}
//释放符号list
void
symlistfree(struct symlist *sl)
{
struct symlist *nsl;
while(sl) {
nsl = sl->next;
free(sl);
sl = nsl;
}
}
//定义一个函数
//func(parma1,parma2…);
/* define a function */
void
dodef(struct symbol *name, struct symlist *syms, struct ast *func)
{
if(name->syms) symlistfree(name->syms);
if(name->func) treefree(name->func);
name->syms = syms;
name->func = func;
}
static double callbuiltin(struct fncall *);
static double calluser(struct ufncall *);
//ast求值
double
eval(struct ast *a)
{
double v;
if(!a) {
yyerror("internal error, null eval");
return 0.0;
}
switch(a->nodetype) {
/* constant */
case 'K': v = ((struct numval *)a)->number; break;
/* name reference */
case 'N': v = ((struct symref *)a)->s->value; break;
/* assignment */
case '=': v = ((struct symasgn *)a)->s->value =
eval(((struct symasgn *)a)->v); break;
/* expressions */
case '+': v = eval(a->l) + eval(a->r); break;
case '-': v = eval(a->l) - eval(a->r); break;
case '*': v = eval(a->l) * eval(a->r); break;
case '/': v = eval(a->l) / eval(a->r); break;
case '|': v = fabs(eval(a->l)); break;
case 'M': v = -eval(a->l); break;
/* comparisons */
case '1': v = (eval(a->l) > eval(a->r))? 1 : 0; break;
case '2': v = (eval(a->l) < eval(a->r))? 1 : 0; break;
case '3': v = (eval(a->l) != eval(a->r))? 1 : 0; break;
case '4': v = (eval(a->l) == eval(a->r))? 1 : 0; break;
case '5': v = (eval(a->l) >= eval(a->r))? 1 : 0; break;
case '6': v = (eval(a->l) <= eval(a->r))? 1 : 0; break;
//这部分是对条件表达式的ast的求值
//比如if else 这些
/* control flow */
/* null if/else/do expressions allowed in the grammar, so check for them */
case 'I’:
//条件成立,走then或者do的分支
if( eval( ((struct flow *)a)->cond) != 0) {
if( ((struct flow *)a)->tl) {
v = eval( ((struct flow *)a)->tl);
} else
v = 0.0; /* a default value */
//不成立,走else分支
} else {
if( ((struct flow *)a)->el) {
v = eval(((struct flow *)a)->el);
} else
v = 0.0; /* a default value */
}
break;
// while语句
case 'W':
v = 0.0; /* a default value */
//条件成立,走do的逻辑
if( ((struct flow *)a)->tl) {
while( eval(((struct flow *)a)->cond) != 0)
v = eval(((struct flow *)a)->tl);
}
//不成立,凉凉,啥都不做
break; /* last value is value */
//语句列表
//
case 'L': eval(a->l); v = eval(a->r); break;
//函数
//func(param…);
case 'F': v = callbuiltin((struct fncall *)a); break;
//用户调用部分
//比如 call xxx();
case 'C': v = calluser((struct ufncall *)a); break;
default: printf("internal error: bad node %c\n", a->nodetype);
}
return v;
}
//一些内建函数的实现,cc支持的
static double
callbuiltin(struct fncall *f)
{
enum bifs functype = f->functype;
double v = eval(f->l);
switch(functype) {
case B_sqrt:
return sqrt(v);
case B_exp:
return exp(v);
case B_log:
return log(v);
case B_print:
printf("= %4.4g\n", v);
return v;
default:
yyerror("Unknown built-in function %d", functype);
return 0.0;
}
}
//函数调用的实现,比较重要的部分。
static double
calluser(struct ufncall *f)
{
//获取函数的信息,函数名,参数等
struct symbol *fn = f->s; /* function name */
//形参
struct symlist *sl; /* dummy arguments */
//实参
struct ast *args = f->l; /* actual arguments */
//保存的参数
double *oldval, *newval; /* saved arg values */
double v;
int nargs;
int i;
if(!fn->func) {
yyerror("call to undefined function", fn->name);
return 0;
}
//获取参数数量,从形参列表遍历获得
/* count the arguments */
sl = fn->syms;
for(nargs = 0; sl; sl = sl->next)
nargs++;
//为保存参数分配空间
/* prepare to save them */
oldval = (double *)malloc(nargs * sizeof(double));
newval = (double *)malloc(nargs * sizeof(double));
if(!oldval || !newval) {
yyerror("Out of space in %s", fn->name); return 0.0;
}
//参数可能是表达式,所以需要对其求值。
//比如 max(1+2,5) 就需要对第一个参数先求值,然后再进行计算。
/* evaluate the arguments */
for(i = 0; i < nargs; i++) {
if(!args) {
yyerror("too few args in call to %s", fn->name);
free(oldval); free(newval);
return 0;
}
if(args->nodetype == 'L') { /* if this is a list node */
newval[i] = eval(args->l);
args = args->r;
} else { /* if it's the end of the list */
newval[i] = eval(args);
args = NULL;
}
}
//保存形参的旧值,然后更新新值
//比如 max(1+2,4-1) 更新成 max(3,3)
/* save old values of dummies, assign new ones */
sl = fn->syms;
for(i = 0; i < nargs; i++) {
struct symbol *s = sl->sym;
oldval[i] = s->value;
s->value = newval[i];
sl = sl->next;
}
free(newval);
/* evaluate the function */ //参数都更新完了之后,就可以直接去计算函数值了
v = eval(fn->func);
/* put the dummies back */ //因为之前更改了形参的list,现在要恢复
sl = fn->syms;
for(i = 0; i < nargs; i++) {
struct symbol *s = sl->sym;
s->value = oldval[i];
sl = sl->next;
}
free(oldval);
return v;
}
//释放节点,分情况,因为不同的操作符,子树数目不同,所以要分情况。
void
treefree(struct ast *a)
{
switch(a->nodetype) {
/* two subtrees */
case '+':
case '-':
case '*':
case '/':
case '1': case '2': case '3': case '4': case '5': case '6':
case 'L':
treefree(a->r);
/* one subtree */
case '|':
case 'M': case 'C': case 'F':
treefree(a->l);
/* no subtree */
case 'K': case 'N':
break;
case '=':
free( ((struct symasgn *)a)->v);
break;
case 'I': case 'W':
free( ((struct flow *)a)->cond);
if( ((struct flow *)a)->tl) free( ((struct flow *)a)->tl);
if( ((struct flow *)a)->el) free( ((struct flow *)a)->el);
break;
default: printf("internal error: free bad node %c\n", a->nodetype);
}
free(a); /* always free the node itself */
}
void
yyerror(char *s, ...)
{
va_list ap;
va_start(ap, s);
fprintf(stderr, "%d: error: ", yylineno);
vfprintf(stderr, s, ap);
fprintf(stderr, "\n");
}
int
main()
{
printf("> ");
return yyparse();
}
//把ast dump出来做显示,方便调试
/* debugging: dump out an AST */
int debug = 0;
void
dumpast(struct ast *a, int level)
{
printf("%*s", 2*level, ""); /* indent to this level */
level++;
if(!a) {
printf("NULL\n");
return;
}
switch(a->nodetype) {
/* constant */
case 'K': printf("number %4.4g\n", ((struct numval *)a)->number); break;
/* name reference */
case 'N': printf("ref %s\n", ((struct symref *)a)->s->name); break;
/* assignment */
case '=': printf("= %s\n", ((struct symref *)a)->s->name);
dumpast( ((struct symasgn *)a)->v, level); return;
/* expressions */
case '+': case '-': case '*': case '/': case 'L':
case '1': case '2': case '3':
case '4': case '5': case '6':
printf("binop %c\n", a->nodetype);
dumpast(a->l, level);
dumpast(a->r, level);
return;
case '|': case 'M':
printf("unop %c\n", a->nodetype);
dumpast(a->l, level);
return;
case 'I': case 'W':
printf("flow %c\n", a->nodetype);
dumpast( ((struct flow *)a)->cond, level);
if( ((struct flow *)a)->tl)
dumpast( ((struct flow *)a)->tl, level);
if( ((struct flow *)a)->el)
dumpast( ((struct flow *)a)->el, level);
return;
case 'F':
printf("builtin %d\n", ((struct fncall *)a)->functype);
dumpast(a->l, level);
return;
case 'C': printf("call %s\n", ((struct ufncall *)a)->s->name);
dumpast(a->l, level);
return;
default: printf("bad %c\n", a->nodetype);
return;
}
}

0x03: 使用

测试环境 ubuntu 16.04 x64
这部分没啥意思,随便测试下就好了,主要还是看上面的代码。

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# muhe @ ubuntu in ~/flexbison [20:49:52] $ ./fb3-2 > 1+123 = 124 > 1 = 1 > 2 = 2 > 1.1+2 = 3.1 > 3.33333/1.2344 = 2.7
> let max(x,y) = if x >= y then x;else y;; Defined max > max(0.1,-0.2) = 0.1 > max(99999999999999999999999999999999999999999,999999999999999999999999999999999999999999999999999999999999999999999999999) = 1e+75 >

设置了debug之后可以看到ast,方便调试:

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2
3
> let max(x,y) = if x >= y then x;else y;; Defined max > max(1,2) call max binop L number 1 number 2 = 2 >
>

0x04: sql分析

书中第四章是一个sql的分析器,含词法分析、语法分析,代码量还好不算特别大。

目录结构:

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2
3
4
5
6
7
8
9
$ tree .
.
├── Makefile
├── glrmysql.l #mysql子集词法分析器
├── glrmysql.y #mysql子集语法分析器
├── lpmysql.l
├── lpmysql.y
├── pmysql.l
└── pmysql.y

*.l是词法分析部分,*.y是语法分析部分。

这里有三份代码,glrxxxxx是第四章的代码,lpmxxxx是第八章的代码,pmysqlxx是第九章的代码,这里只看第四章的代码,这个例子比较简单,简化了很多东西。

0x05: 引用

在编程概念中,表达式和语句分别是什么概念?
左递归

文章目录
  1. 1. 0x00: 起因
  2. 2. 0x01: 一些概念
    1. 2.1. 语句和表达式
    2. 2.2. 左递归
  3. 3. 0x02: 高级计算器的实现
    1. 3.1. 先看语法分析的部分:
    2. 3.2. 词法分析部分:
    3. 3.3. 函数实现,构造ast什么的:
  4. 4. 0x03: 使用
  5. 5. 0x04: sql分析
  6. 6. 0x05: 引用
,