1. LL(1) Parser Generators
SLLGEN and JavaCC
11/27/2018
PPL LL(1) Parser Generators

2. PPL LL(1) Parser Generators
class dictionaries (11 kinds)
inductive
nonleft-recursive 9 10 11 8 7 6 1 2
LL(1) 3
1: ideal case
2: JavaCC does not complain (Oct ). Grammar contains useless symbols. Ok if we don’t want to parse but forces cyclic objects. 4
nonambiguous 5
Venn Diagram
11/27/2018
PPL LL(1) Parser Generators

3. 11 kinds of class dictionaries
Why 11 and not 16?
Four properties: nonambiguous, LL(1), inductive, non-left recursive: 16 sets if independent
But: implication relationships
LL(1) implies nonambiguous: 12 left
LL(1) and inductive imply nonleft-recursive: 11 left
11/27/2018
PPL LL(1) Parser Generators

4. PPL LL(1) Parser Generators
SLLGEN Error messages
What are the error messages you get from the SLLGEN Parser Generator if you violate any of the four properties?
11/27/2018
PPL LL(1) Parser Generators

5. PPL LL(1) Parser Generators
JavaCC Error messages
What are the error messages you get from the JavaCC Parser Generator if you violate any of the four properties?
Answers follow.
11/27/2018
PPL LL(1) Parser Generators

6. Inductive class dictionaries
inductiveness already defined for class graphs
contains only good recursions: recursions that terminate
Car = Motor.
Motor = <belongsTo> Car.
bad recursion, objects must be cyclic,
cannot use for parsing: useless nonterminals
11/27/2018
PPL LL(1) Parser Generators

7. Inductive class dictionaries
A node v in a class graph is inductive if there is at least one finite tree object of class v.
A class graph is inductive if all its nodes are inductive.
Car = Motor Transmission.
Motor = <belongsTo> Car.
Transmission = .
Which nodes are inductive?
11/27/2018
PPL LL(1) Parser Generators

8. Inductiveness style rule to follow
Maximize the number of classes which are inductive.
Reasons: cyclic objects
cannot be parsed directly from sentences.
require visitors to break infinite loops.
it is harder to reason about cyclic objects.
No message from the Java Compiler Compiler!
11/27/2018
PPL LL(1) Parser Generators

9. Left-recursive class dictionaries
Bring us back to the same class without consuming input.
Java Compiler Compiler:
left recursion detected:
A -> C -> A
A : B | C.
B = “b”.
C = A.
11/27/2018
PPL LL(1) Parser Generators

10. Ambiguous class dictionaries
cannot distinguish between objects. Print is not injective (one-to-one).
Fruit : Apple | Orange.
Apple = “a”.
Orange = “a”.
But: undecidable
11/27/2018
PPL LL(1) Parser Generators

11. Java Compiler Compiler: error message
Warning: Choice conflict …
A common prefix is “a”.
Consider using a lookahead of 2 ...
11/27/2018
PPL LL(1) Parser Generators

12. LL(1) class dictionaries
A special kind of nonambiguous class dictionaries. Membership can be checked efficiently.
11/27/2018
PPL LL(1) Parser Generators

13. Java Compiler Compiler LL(1) error messages: Rule 2
A = [B]. B = .
Error message: expansion can be mapped by empty string, line x, column y in Parser.jj.
A = [B] [“b” C]. B = “b”. C = .
Warning only: Choice conflict … line x column y. Expansion nested within construct and expansion following construct have common prefixes one of which is b.
11/27/2018
PPL LL(1) Parser Generators

14. PPL LL(1) Parser Generators
Style rule
Ideally, make your class dictionaries LL(1), nonleft-recursive and inductive.
11/27/2018
PPL LL(1) Parser Generators