tools/gn: add "gn help grammar" with formal grammar

tools/gn/parser.cc

Index: tools/gn/parser.cc
diff --git a/tools/gn/parser.cc b/tools/gn/parser.cc
index 2a037fb24fa3a8275d3a05ae8327da902fbabe3c..1fa432e3a8061704d7cac1ea900948327140f582 100644
--- a/tools/gn/parser.cc
+++ b/tools/gn/parser.cc
@@ -9,14 +9,105 @@
 #include "tools/gn/operators.h"
 #include "tools/gn/token.h"
 
-// grammar:
-//
-// file       := (statement)*
-// statement  := block | if | assignment
-// block      := '{' statement* '}'
-// if         := 'if' '(' expr ')' statement [ else ]
-// else       := 'else' (if | statement)*
-// assignment := ident {'=' | '+=' | '-='} expr
+const char kGrammar_Help[] =
+    "GN build language grammar\n"
+    "\n"
+    "Tokens\n"
+    "\n"
+    "  GN build files are read as sequences of tokens.  While splitting the\n"
+    "  file into tokens, the next token is the longest sequence of characters\n"
+    "  that form a valid token.\n"
+    "\n"
+    "White space and comments\n"
+    "\n"
+    "  White space is comprised of spaces (U+0020), horizontal tabs (U+0009),\n"
+    "  carriage returns (U+000D), and newlines (U+000A).\n"
+    "\n"
+    "  Comments start at the character \"#\" and stop at the next newline.\n"
+    "\n"
+    "  White space and comments are ignored except that they may separate\n"
+    "  tokens that would otherwise combine into a single token.\n"
+    "\n"
+    "Identifiers\n"
+    "\n"
+    "  Identifiers name variables and functions.\n"
+    "\n"
+    "      identifier = letter { letter | digit } .\n"
+    "      letter     = \"A\" ... \"Z\" | \"a\" ... \"z\" | \"_\" .\n"
+    "      digit      = \"0\" ... \"9\" .\n"
+    "\n"
+    "Keywords\n"
+    "\n"
+    "  The following keywords are reserved and may not be used as\n"
+    "  identifiers:\n"
+    "\n"
+    "          else    false   if      true\n"
+    "\n"
+    "Integer literals\n"
+    "\n"
+    "  An integer literal represents a decimal integer value.\n"
+    "\n"
+    "      integer = [ \"-\" ] digit { digit } .\n"
+    "\n"
+    "String literals\n"
+    "\n"
+    "  A string literal represents a string value consisting of the quoted\n"
+    "  characters with possible escape sequences and variable expansions.\n"
+    "\n"
+    "      string    = `\"` { char | escape | expansion } `\"` .\n"
+    "      escape    = `\\` ( \"$\" | `\"` | char ) .\n"
+    "      expansion = \"$\" ( identifier | \"{\" identifier \"}\" ) .\n"
+    "      char      = /* any character except \"$\", `\"`, or newline */ .\n"
+    "\n"
+    "  After a backslash, certain sequences represent special characters:\n"
+    "\n"
+    "          \\\"    U+0022    quotation mark\n"
+    "          \\$    U+0024    dollar sign\n"
+    "          \\\\    U+005C    backslash\n"
+    "\n"
+    "  All other backslashes represent themselves.\n"
+    "\n"
+    "Punctuation\n"
+    "\n"
+    "  The following character sequences represent punctuation:\n"
+    "\n"
+    "          +       +=      ==      !=      (       )\n"
+    "          -       -=      <       <=      [       ]\n"
+    "          !       =       >       >=      {       }\n"
+    "                          &&      ||      .       ,\n"
+    "\n"
+    "Grammar\n"
+    "\n"
+    "  The input tokens form a syntax tree following a context-free grammar:\n"
+    "\n"
+    "      File = StatementList .\n"
+    "\n"
+    "      Statement     = Assignment | Call | Condition .\n"
+    "      Assignment    = identifier AssignOp Expr .\n"
+    "      Call          = identifier \"(\" [ ExprList ] \")\" [ Block ] .\n"
+    "      Condition     = \"if\" \"(\" Expr \")\" Block\n"
+    "                      [ \"else\" ( Condition | Block ) ] .\n"
+    "      Block         = \"{\" StatementList \"}\" .\n"
+    "      StatementList = { Statement } .\n"
+    "\n"
+    "      Expr        = UnaryExpr | Expr BinaryOp Expr .\n"
+    "      UnaryExpr   = PrimaryExpr | UnaryOp UnaryExpr .\n"
+    "      PrimaryExpr = identifier | integer | string | Call\n"
+    "                  | identifier \"[\" Expr \"]\"\n"
+    "                  | identifier \".\" identifier\n"
+    "                  | \"(\" Expr \")\"\n"
+    "                  | \"[\" [ ExprList [ \",\" ] ] \"]\" .\n"
+    "      ExprList    = Expr { \",\" Expr } .\n"
+    "\n"
+    "      AssignOp = \"=\" | \"+=\" | \"-=\" .\n"
+    "      UnaryOp  = \"!\" .\n"
+    "      BinaryOp = \"+\" | \"-\"                  // highest priority\n"
+    "               | \"<\" | \"<=\" | \">\" | \">=\"\n"
+    "               | \"==\" | \"!=\"\n"
+    "               | \"&&\"\n"
+    "               | \"||\" .                     // lowest priority\n"
+    "\n"
+    "  All binary operators are left-associative.\n";
 
 enum Precedence {
   PRECEDENCE_ASSIGNMENT = 1,  // Lowest precedence.