add parser notes

src/parser/parser.yy

@@ -1,9 +1,11 @@
+// Bison options
 %language "C++"
 %skeleton "lalr1.cc"
 %no-lines
 %locations
 %define api.namespace { nebula }
 %define api.parser.class { GraphParser }
+// Parameters of scanner and parser
 %lex-param { nebula::GraphScanner& scanner }
 %parse-param { nebula::GraphScanner& scanner }
 %parse-param { std::string &errmsg }
@@ -54,6 +56,7 @@ static constexpr size_t kCommentLengthLimit = 256;
                       const nebula::GraphParser::location_type& loc);
 }
 
+// Define types of semantic values
 %union {
     bool                                    boolval;
     int64_t                                 intval;
@@ -402,6 +405,18 @@ static constexpr size_t kCommentLengthLimit = 256;
 %type <boolval> opt_with_properties
 %type <boolval> opt_ignore_existed_index
 
+/* Define precedence and associativity of tokens.
+ * Associativity:
+ * The associativity of an operator op determines how repeated uses of the operator nest:
+ * whether ‘x op y op z’ is parsed by grouping x with y first or by grouping y with z first.
+ * %left specifies left-associativity (grouping x with y first) and %right specifies right-associativity (grouping y with z first).
+ * %nonassoc specifies no associativity, which means that ‘x op y op z’ is considered a syntax error.
+ *
+ * Precedence:
+ * The precedence of an operator determines how it nests with other operators.
+ * All the tokens declared in a single precedence declaration have equal precedence and nest together according to their associativity.
+ * When two tokens declared in different precedence declarations associate, the one declared later has the higher precedence and is grouped first.
+ */
 %left QM COLON
 %left KW_OR KW_XOR
 %left KW_AND

src/parser/scanner.lex

@@ -1,3 +1,5 @@
+/* Flex definitions section */
+/* Flex options */
 %option c++
 %option yyclass="GraphScanner"
 %option nodefault noyywrap
@@ -11,6 +13,7 @@
 #include "GraphParser.hpp"
 #include "graph/service/GraphFlags.h"
 
+/* YY_USER_ACTION is called to advance location after each time a pattern is matched. */
 #define YY_USER_ACTION                  \
     yylloc->step();                     \
     yylloc->columns(yyleng);
@@ -19,6 +22,13 @@ static constexpr size_t MAX_STRING = 4096;
 
 %}
 
+/* Define some exclusive states.
+ * Each state is referenced within a `<>` in the rules section
+ * <DQ_STR> double quoted string literal
+ * <SQ_STR> single quoted string literal
+ * <LB_STR> accent quoted label, eg. `v2`
+ * <COMMENT> comment
+ */
 %x DQ_STR
 %x SQ_STR
 %x LB_STR
@@ -60,6 +70,15 @@ LABEL_FULL_WIDTH            {CN_EN_FULL_WIDTH}{CN_EN_NUM_FULL_WIDTH}*
 
 %%
 
+ /* Flex rules section */
+ /* How Does the input is matched?
+  * When the generated scanner is run, it analyzes its input looking for strings which match any of its patterns.
+  * 1. If it finds more than one match, it takes the one matching the most text.
+  * 2. If it finds two or more matches of the same length, the rule listed first in the flex input file is chosen.
+  * Once the match is determined, the text corresponding to the match is made available in the global character pointer yytext, and its length in the global integer yyleng.
+  * The action corresponding to the matched pattern is then executed, and then the remaining input is scanned for another match.
+  * The last rule `.` could match any single character except `\n`.
+  */
  /* Reserved keyword */
 "GO"                        { return TokenType::KW_GO; }
 "AS"                        { return TokenType::KW_AS; }
@@ -538,3 +557,4 @@ LABEL_FULL_WIDTH            {CN_EN_FULL_WIDTH}{CN_EN_NUM_FULL_WIDTH}*
                             }
 
 %%
+/* Flex user code section */