Top Down Parsers - Recursive Descent Parsers

Summary

This video introduces top-down parsers, focusing on recursive descent parsers in compiler design. It explains how top-down parsers use leftmost derivation to generate parse trees, with emphasis on parsers without backtracking that avoid left recursion and non-determinism. The recursive descent parser is built through mutually recursive functions, each representing a non-terminal in the grammar. The video provides an example to illustrate how these parsers recognize input strings, build parse trees, and successfully parse expressions like 'I + I'. The session concludes with a preview of the upcoming discussion on LL(1) parsers.

Takeaways

• 😀 Top-down parsers are classified into two types: with backtracking and without backtracking. This session focuses on the latter.
• 😀 Top-down parsers generate parse trees from the root to the leaves using a leftmost derivation approach.
• 😀 The grammar for a top-down parser must be free of left recursion and non-determinism.
• 😀 Left recursion must be removed for top-down parsers to work correctly, and left factoring can help eliminate non-determinism.
• 😀 A recursive descent parser is a top-down parser made of mutually recursive procedures that correspond to the non-terminals in the grammar.
• 😀 The recursive descent parser mirrors the structure of the grammar by defining a procedure for each non-terminal in the grammar.
• 😀 The match function in the recursive descent parser checks whether the current input symbol matches the expected terminal and advances the input stream if they match.
• 😀 A recursive descent parser can recognize an input string by recursively calling procedures corresponding to each non-terminal in the grammar.
• 😀 The recursive descent parser constructs the parse tree as it processes the input string, using conditional branching and recursive calls.
• 😀 The session concludes with the idea that recursive descent parsers can be used to parse deterministic grammars by following the structure of the grammar and recognizing the input stream.

Q & A

What are the two categories of top-down parsers mentioned in the script?

The two categories of top-down parsers mentioned are top-down parsers with backtracking and top-down parsers without backtracking.

Which type of top-down parser is the focus of this course?

The focus of this course is on top-down parsers without backtracking.

What is a key requirement for constructing a top-down parser?

A key requirement for constructing a top-down parser is that the context-free grammar should not have left recursion or any form of non-determinism.

What does a recursive descent parser consist of?

A recursive descent parser consists of a set of mutually recursive procedures or a non-recursive equivalent, where each procedure implements one of the non-terminals of the grammar.

What does the structure of the recursive descent parser mirror?

The structure of the recursive descent parser closely mirrors the grammar it is designed to recognize.

In the example provided, what are the two main production rules of the grammar?

The two main production rules of the grammar are: 'E -> I E'' and 'E' -> + I E' | ε.

What does the 'match' procedure do in the recursive descent parser?

The 'match' procedure in the recursive descent parser checks if the current symbol in the input stream matches the expected terminal, and if it matches, it advances to the next symbol in the input stream.

What is the purpose of the 'lookahead' variable in the recursive descent parser?

The 'lookahead' variable is used to track the current symbol in the input stream, helping the parser decide which production rule to apply.

What happens when the 'lookahead' symbol matches the '$' symbol?

When the 'lookahead' symbol matches the '$' symbol, it indicates the end of the input string, and the parsing is considered successful.

How does the recursive descent parser handle ambiguity in the grammar?

The recursive descent parser handles ambiguity by using procedures that avoid non-determinism, ensuring that each non-terminal has a clear definition without overlapping choices.