1. ANTLR v3 Overview (for ANTLR v2 users)
Terence Parr
University of San Francisco

2. Topics Information flow v3 grammars Error recovery Attributes
Tree construction
Tree grammars
Code generation
Internationalization
Runtime support

3. Block Info Flow Diagram

4. Grammar Syntax Trees ^(root child1 … childN) Note: No inheritance
header {…}
/** doc comment */
kind grammar name;
options {…}
tokens {…}
scopes…
action
rules…
/** doc comment */ rule[String s, int z] returns [int x, int y] throws E options {…} scopes init {…} :  |  ; exceptions
Trees
^(root child1 … childN)
Note: No inheritance

5. Grammar improvements Single element EBNF like ID*
Combined parser/lexer
Allows ‘c’ and “literal” literals
Multiple parameters, return values
Labels do not have to be unique (x=ID|x=INT) {…$x…}
For combined grammars, warns when tokens are not defined

6. Example Grammar grammar SimpleParser; program : variable* method+ ;
variable: "int" ID (‘=‘ expr)? ';’ ;
method : "method" ID '(' ')'
'{'
variable* statement+
'}' ;
statement
: ID ‘=‘ expr ';'
| "return" expr ';'
expr : ID | INT ;
ID : ('a'..'z'|'A'..'Z')+ ;
INT : '0'..'9'+ ;
WS : (' '|'\t'|'\n')+ {channel=99;}

7. Using the parser CharStream in = new ANTLRFileStream(“inputfile”);
SimpleParserLexer lexer = new SimpleParserLexer(in);
CommonTokenStream tokens = new CommonTokenStream(lexer);
SimpleParser p = new SimpleParser(tokens);
p.program(); // invoke start rule

8. Improved grammar warnings
they happen less often ;)
internationalized (templates again!)
gives (smallest) sample input sequence
better recursion warnings

9. Recursion Warnings
a : a A
| B ;
t.g:2:5: Alternative 1 discovers infinite left-recursion to a from a
// with -Im 0 (secret internal parameter)
a : b
| B ;
b : c ;
c : B b ;
t.g:2:5: Alternative 1: after matching input such as B decision
cannot predict what comes next due to recursion overflow
to c from b

10. Nondeterminisms a : (A B|A B) C ; a : (A+ B|A+ B) C ;
t.g:2:5: Decision can match input such as "A B" using multiple alternatives: 1, 2
As a result, alternative(s) 2 were disabled for that input
t.g:2:5: The following alternatives are unreachable: 2
a : (A+ B|A+ B) C ;
t.g:2:5: Decision can match input such as "A B" using multiple alternatives: 1, 2

11. Runtime Objects of Interest
Lexer passes all tokens to the parser, but parser listens to only a single “channel”; channel 99, for example, where I place WS tokens, is ignored
Tokens have start/stop index into single text input buffer
Token is an abstract class
TokenSource anything answering nextToken()
TokenStream stream pulling from TokenSource; LT(i), …
CharStream source of characters for a lexer; LT(i), …

12. Error Recovery ANTLR v3 does what Josef Grosch does in Cocktail
Does single token insertion or deletion if necessary to keep going
Computes context-sensitive FOLLOW to do insert/delete
proper context is passed to each rule invocation
knows precisely what can follow reference to r rather than what could follow any reference to r (per Wirth circa 1970)

13. Example Error Recovery
int i = 0;
method foo( {
int j = i;
i = 4 }
[program, method]: line 2:12 mismatched token:
expecting type ')'
[program, method, statement]: line 5:0 mismatched token:
expecting type ';'
One token insertion
int i = 0;
method foo() ) {
int j = i;
i = = 4; }
[program, method]: line 2:13 mismatched token:
expecting type '{'
[program, method, statement, expr]: line 4:6 mismatched token:
expecting set null
One token deletion
Note: I put in two errors each so you’ll see it continues properly

14. Attributes New label syntax and multiple return values
Unified token, rule, parameter, return value, tree reference syntax in actions
Dynamically scope attributes!
a[String s] returns [float y]
: id=ID f=field (ids+=ID)+ {$s, $y, $id, $id.text, $f.z; $ids.size();} ;
field returns [int x, int z] : … ;

15. Label properties Token label reference properties
text, type, line, pos, channel, index, tree
Rule label reference properties
start, stop; indices of token boundaries
tree
text; text matched for whole rule

16. Rule Scope Attributes
A rule may define a scope of attributes visible to any invoked rule; operates like a stacked global variable
Avoids having to pass a value down
method scope { String name; }
: "method" ID '(' ')' {$name=$ID.text;} body ;
body: '{' stat* '}’ ; …
atom init {… $method.name …} : ID | INT ;

17. Global Scope Attributes
Named scopes; rules must explicitly request access
scope Symbols { List names; }
{int level=0;}
globals
scope Symbols;
init {
level++;
$Symbols.names = new ArrayList(); }
: decl* {level--;} ;
block
scope Symbols;
init {
level++;
$Symbols.names = new ArrayList(); }
: '{' decl* stat* '}’ {level--;} ;
decl
: "int" ID ';' {$Symbols.names.add($ID);}
*What if we want to keep the symbol tables around after parsing?

18. Tree Support
TreeAdaptor; How to create and navigate trees (like ASTFactory from v2); ANTLR assumes tree nodes are Object type
Tree; used by support code
BaseTree; List of children, w/o payload (no more child-sibling trees)
CommonTree; node wrapping Token as payload
ParseTree; used by interpreter to build trees

19. Tree Construction
Automatic mechanism is same as v2 except ^ is now ^^ expr : atom ( '+'^^ atom )* ;
^ implies root of tree for enclosing subrule a : ( ID^ INT )* ; builds (a 1) (b 2) …
Token labels are $label not #label and rule invocation tree results are $ruleLabel.tree
Turn on options {output=AST;} (one can imagine output=text for templates)
Option: ASTLabelType=CommonTree;

20. Tree Rewrite Rules
Maps an input grammar fragment to an output tree grammar fragment
variable
: type declarator ';' -> ^(VAR_DEF type declarator) ;
functionHeader
: type ID '(' ( formalParameter ( ',' formalParameter )* )? ')'
-> ^(FUNC_HDR type ID formalParameter+)
atom
: …
| '(' expr ')' -> expr

21. Mixed Rewrite/Auto Trees
Alternatives w/o -> rewrite use automatic mechanism
b : ID INT -> INT ID
| INT // implies -> INT ;

22. Rewrites and labels
Disambiguates element references or used to construct imaginary nodes
Concatenation += labels useful too:
forStat
: "for" '(' start=assignStat ';' expr ';' next=assignStat ')' block
-> ^("for" $start expr $next block) ;
block
: lc='{' variable* stat* '}’ -> ^(BLOCK[$lc] variable* stat*)
/** match string representation of tree and build tree in memory */
tree : ‘^’ ‘(‘ root=atom (children+=tree)+ ‘)’ -> ^($root $children)
| atom ;

23. Loops in Rewrites
Repeated element ID ID -> ^(VARS ID+) yields ^(VARS a b)
Repeated tree ID ID -> ^(VARS ID)+ yields ^(VARS a) ^(VARS b)
Multiple elements in loop need same size ID INT ID INT -> ^( R ID ^( S INT) )+ yields (R a (S 1)) (R b (S 2))
Checks cardinality + and * loops

24. Preventing cyclic structures
Repeated elements get duplicated a : INT -> INT INT ; // dups INT! a : INT INT -> INT+ INT+ ; // 4 INTs!
Repeated rule references get duplicated a : atom -> ^(atom atom) ; // no cycle!
Duplicates whole tree for all but first ref to an element; here 2nd ref to atom results in a duplicated atom tree
*Useful example “int x,y” -> “^(int x) ^(int y)” decl : type ID (‘,’ ID)* -> ^(type ID)+ ;
*Just noticed a bug in this one ;)

25. Predicated rewrites
Use semantic predicate to indicate which rewrite to choose from
a : ID INT -> {p1}? ID
-> {p2}? INT
-> ;

26. Misc Rewrite Elements
Arbitrary actions a : atom -> ^({adaptor.createToken(INT,"9")} atom) ;
rewrite always sets the rule’s AST not subrule’s
Reference to previous value (useful?)
b : "int"
( ID -> ^(TYPE "int" ID)
| ID '=' INT -> ^(TYPE "int" ID INT) ) ;
a : (atom -> atom) (op='+' r=atom -> ^($op $a $r) )* ;

27. Tree Grammars
Syntax same as parser grammars, add ^(root children…) tree element
Uses LL(*) also; even derives from same superclass! Tree is serialized to include DOWN, UP imaginary tokens to encode 2D structure for serial parser
variable
: ^(VAR_DEF type ID)
| ^(VAR_DEF type ID ^(INIT expr)) ;

28. Code Generation
Uses StringTemplate to specify how each abstract ANTLR concept maps to code; wildly successful!
Separates code gen logic from output; not a single character of output in the Java code
Java.stg: 140 templates, 1300 lines

29. Sample code gen templates
/** Dump the elements one per line and stick in debugging
* location() trigger in front. */
element() ::= <<
<if(debug)>
dbg.location(<it.line>,<it.pos>);<\n>
<endif>
<it.el><\n>
>>
/** match a token optionally with a label in front */
tokenRef(token,label,elementIndex) ::= <<
<if(label)>
<label>=input.LT(1);<\n>
match(input,<token>,FOLLOW_<token>_in_<ruleName><elementIndex>);

30. Internationalization
ANTLR v3 uses StringTemplate to display all errors
Senses locale to load messages; en.stg: 76 templates
ErrorManager error number constants map to a template name; e.g.,
RULE_REDEFINITION(file,line,col,arg) ::=
"<loc()>rule <arg> redefinition”
/* This factors out file location formatting; file,line,col inherited from
* enclosing template; don't manually pass stuff in. */
loc() ::= "<file>:<line>:<col>: "

31. Runtime Support
Better organized, separated: org.antlr.runtime org.antlr.runtime.tree org.antlr.runtime.debug
Clean; Parser has input ptr only (except error recovery FOLLOW stack); Lexer also only has input ptr
4500 lines of Java code minus BSD header

32. Summary v3 kicks ass it sort of works! http://www.antlr.org/download/…
ANTLRWorks progressing in parallel