1. Programming Language Syntax 7

2. Recall: Recursive Descent Parsers… …

3. Recall: Recursive Descent Parsers…
What types of grammars can recursive descent parsers handle?
Answer: LL grammars …

4. Recall: Recursive Descent Parsers…
What procedures does a recursive descent parser contain?
Answer: 1 for each production
+ match() …

5. Recall: Recursive Descent Parsers…
Each procedure contains what type of statement?
Answer: A branching statement (case or if/else) …

6. Recall: Recursive Descent Parsers…
What does each branch do?
Answer: Predict right-hand side and execute corresponding procedure(s) or match, or identify parse error …

7. Recall: Recursive Descent Parsers…
Does this algorithm recognize sentences in a language?
Create a parse tree?
Answer: Yes
Answer: No …

8. Recall: Recursive Descent Parsers…
How could it be modified to create a parse tree?
Answer:
When each procedure is called, hang a node from node associated w/ caller …

9. Recall: Recursive Descent Parsers…
How does the parser generator determine the predict sets?
Answer:
Computes two mappings: FIRST, FOLLOW (+EPS) …

10. Recall Mappings (aka functions in the mathematical sense)
FIRST maps
each left-hand side of a production to
a set of terminals that can start it
Follow maps
a set of terminals that can follow it
EPS maps
true or false, depending whether the right-hand side can be empty or not

11. Activity: Run FIRST/EPS algorithm on grammar

12. Solution

13. Algorithm for computing FOLLOW sets

14. Activity: Run FOLLOW algorithm on grammar

15. Solution

16. Given the FIRST, FOLLOW, and EPS sets/values, can you deduce the PREDICT sets?

17. Algorithm for computing PREDICT sets
Based on computed PREDICT sets, how can you tell if a grammar is LL(1)?
If two rules with the same left-hand side have the non-disjoint (overlapping) PREDICT sets, then the grammar is not LL(1)

18. Solution

19. ANTLR 4 generates recursive descent parsers
ANTLR-Generated Parser (Java) …
ANTLR Grammar … …
EBNF Grammar

20. ANTLR 4 is Adaptive LL(*) aka ALL(*)
Can process all non-left-recursive grammars
Can process grammars with direct left recursion
ANTLR 3 was just LL(*), so it couldn’t do this

21. What’s next?
Homework 3 assigned next class