Chapter 3 - add vars and lexical scope. (#7)

* Chapter 3 - add vars and lexical scope.

* Fix typo

* Overhaul graph viz

Makes the scopes pointing into the graph being build more visually appealing.
Constants are a printed with just the constant value.

* Fix control edge color

* Fix missing grouping operator parse

Add a parens test.
Shorten names 'Identifier' to 'Id'
_lexer.match -> match
require() passes a Node thru, so can be flow-coded.
Uniform syntax error messages.
Handy Lexer.toString for debugging.
Drop Optional for null returns; less total code; less objects; less confusion.
camelCase some fcns.

* All parser helpers return Node, not void.

Drop _ret.  parseBlock and parseStatement now the same.

* CLosely align chap2 and chap3 parsers

Mostly whitespace aligned; dragged a few functions around to make more sense.
I think the diff from Ch2 and Ch3 is down a few differences about what is a statement, plus all the symbol stuff for scopes and variables.

* Add examples illustrating the symbol table creation.

* Update docs and comments

---------

Co-authored-by: Cliff Click <cliffc@acm.org>

1 files changed, 72 insertions, 0 deletions

diff --git a/chapter03/docs/03-grammar.md b/chapter03/docs/03-grammar.md
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+# Grammar for Chapter 1
+
+```antlrv4
+grammar SimpleLanguage;
+
+program
+    : statement+ EOF
+    ;
+
+statement
+    : returnStatement
+    | declStatement
+    | blockStatment 
+    | expressionStatement
+    | metaStatement
+    ;
+
+metaStatement
+    : '#showGraph' ';'
+    ;
+
+expressionStatement
+    : IDENTIFIER '=' expression ';'
+    ;
+
+blockStatement
+    : '{' statement+ '}'
+    ;
+
+declStatement
+    : 'int' IDENTIFIER '=' expression ';'
+    ;
+
+returnStatement
+    : 'return' expression ';'
+    ;
+
+expression
+    : additiveExpression
+    ;
+    
+additiveExpression
+    : multiplicativeExpression (('+' | '-') multiplicativeExpression)*    
+    ;
+    
+multiplicativeExpression
+    : unaryExpression (('*' | '/') unaryExpression)*
+    ;
+
+unaryExpression
+    : ('-') unaryExpression
+    | primaryExpression
+    ;
+
+primaryExpression
+    : IDENTIFIER
+    | INTEGER_LITERAL
+    | '(' expression ')'
+    ;
+
+INTEGER_LITERAL
+    : [1-9][0-9]*
+    | [0]
+    ;
+    
+IDENTIFIER
+    : NON_DIGIT (NON_DIGIT | DIGIT)*
+    ;
+    
+NON_DIGIT: [a-zA-Z_];
+DEC_DIGIT: [0-9];
+```
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