Develop a recursive descent parser and scanner for the EBNF grammar like those described in chapter 8:
EBNF = { Production } EOF .
Production = nonterminal "=" Expression "." .
Expression = Term { "|" Term } .
Term = [ Factor { Factor } ] .
Factor = nonterminal
| terminal
| "[" Expression "]"
| "(" Expression ")"
| "{" Expression "}" .
For your convenience a skeleton of the code appears below
using Library;
using System;
using System.Text;
class Token {
public int kind;
public string val;
public Token(int kind, string val) {
this.kind = kind;
this.val = val;
} // constructor
} // Token
class EBNFParser {
// +++++++++++++++++++++++++ File Handling and Error handlers ++++++++++++++++++++
static InFile input;
static OutFile output;
static string NewFileName(string oldFileName, string ext) {
// as in textbook - no need to expand this
static void ReportError(string errorMessage) {
// as in textbook - no need to expand this
static void Abort(string errorMessage) {
// Abandons parsing after issuing error message
ReportError(errorMessage);
output.Close();
System.Environment.Exit(1);
}
// +++++++++++++++++++++++ token kinds enumeration +++++++++++++++++++++++++
const int
noSym = 0, // add to this
EOFSym = 1;
// +++++++++++++++++++++++++++++ Character Handler ++++++++++++++++++++++++++
const char EOF = '\0';
static bool atEndOfFile = false;
// Declaring ch as a global variable is done for expediency - global variables
// are not always a good thing
static char ch; // look ahead character for scanner
static void GetChar() {
// Obtains next character ch from input, or CHR(0) if EOF reached.
// Reflect ch to output
if (atEndOfFile) ch = EOF;
else {
ch = input.ReadChar();
atEndOfFile = ch == EOF;
if (!atEndOfFile) output.Write(ch);
}
} // GetChar
// +++++++++++++++++++++++++++++++ Scanner ++++++++++++++++++++++++++++++++++
// Declaring sym as a global variable is done for expediency - global variables
// are not always a good thing
static Token sym;
static void GetSym() {
// Scans for next sym from input
while (ch > EOF && ch <= ' ') GetChar();
// Rest of scanner comes here
sym = new Token(symKind, symLex.ToString());
}
// +++++++++++++++++++++++++++++++ Parser +++++++++++++++++++++++++++++++++++
static void Accept(int wantedSym, string errorMessage) {
// Checks that lookahead token is wantedSym
if (sym.kind == wantedSym) GetSym(); else Abort(errorMessage);
} // Accept
static void Accept(IntSet allowedSet, string errorMessage) {
// Checks that lookahead token is in allowedSet
if (allowedSet.Contains(sym.kind)) GetSym(); else Abort(errorMessage);
} // Accept
// Parser Routines come here
// +++++++++++++++++++++ Main driver function +++++++++++++++++++++++++++++++
public static void Main(string[] args) {
// Open input and output files from command line arguments
if (args.Length == 0) {
Console.WriteLine("Usage: EBNFParser FileName");
System.Environment.Exit(1);
}
input = new InFile(args[0]);
output = new OutFile(NewFileName(args[0], ".out"));
GetChar(); // Lookahead character
GetSym(); // Lookahead symbol
EBNF(); // Start to parse from the goal symbol
// if we get back here everything must have been satisfactory
Console.WriteLine("Parsed correctly");
output.Close();
} // Main
} // class EBNFParser
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