Add ANTLR grammar for greetings and farewells with corresponding parser and listener implementations

main.py

@@ -1,27 +1,38 @@
 import sys
 
 from antlr4 import *
+
 from HelloLexer import HelloLexer
 from HelloParser import HelloParser
 from HelloListener import HelloListener
 
-from antlr4.tree.Trees import Trees
-
+from TreesUser import TreesUser # contains Marina's PrettyPrint()
 
 def main(argv):
     if len(argv) == 1:
-        InputStream = StdinStream()
+        inpstream = StdinStream()
     else:
         inpname = argv[1]
-        InputStream = FileStream(inpname)
-    lexer = HelloLexer(InputStream)
+        inpstream = FileStream(inpname)
+    # The lexer analyses the characters on inpstream and produces tokens
+    lexer = HelloLexer(inpstream)
+    # on the token stream (belonging to a standard ANTLR class)
     tokstream = CommonTokenStream(lexer)
+    # The parser produces an abstract syntax tree
     parser = HelloParser(tokstream)
+    # When we execute the parse, we need to give it the starting rule of our grammar,
+    listener = HelloListener()
 
-    tree = parser.start()
+    # which in our case is 'greeting' (as defined in hello.g4)
+    tree = parser.greeting()
+    tree = parser.bye()
 
-    print(Trees.toStringTree(tree, None, parser))
+    # traditional Lisp-style parse tree
+    print(TreesUser.toStringTree(tree, None, parser))
+    # pretty-print the parse tree
+    print(TreesUser.PrettyPrint(tree, None, parser))
 if __name__ == '__main__':
     main(sys.argv)
 
 
+