Programming Language Syntax 7

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Presentation transcript:

Programming Language Syntax 7 http://flic.kr/p/zCyMp

Recall: Recursive Descent Parsers … …

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

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

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

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 …

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

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 …

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

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

Activity: Run FIRST/EPS algorithm on grammar

Solution

Algorithm for computing FOLLOW sets

Activity: Run FOLLOW algorithm on grammar

Solution

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

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)

Solution

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

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

What’s next? Homework 3 assigned next class