Recursive ascent parser

Recursive ascent parser is a type of parser used in computer science for syntax analysis in compilers. It is a variant of the LR parser and is designed to parse the context-free grammars that are common in programming languages. Unlike the more widely known recursive descent parser, which is a top-down parser, the recursive ascent parser operates in a bottom-up fashion. This article provides an overview of recursive ascent parsers, including their structure, operation, and comparison with other parsing techniques.

Overview

A recursive ascent parser is constructed as a series of mutually recursive functions, where each function represents a state in the LR parsing table. This approach is essentially the inverse of a recursive descent parser, which constructs its recursive functions based on the non-terminals of a grammar. In contrast, recursive ascent parsers are built around the states of an LR parsing table, making them more suited for grammars that are difficult to parse using top-down methods.

Operation

The operation of a recursive ascent parser is based on the LR parsing algorithm. It involves shifting input tokens onto a stack until a rule from the grammar can be applied to reduce a series of tokens on the stack to a single non-terminal symbol. This process is repeated until the entire input is consumed and reduced to a single start symbol, indicating successful parsing.

Each function in a recursive ascent parser corresponds to a state in the LR parsing table and includes logic for handling the shift and reduce operations based on the input token and the state's transitions. The parser moves between states (functions) by calling the functions corresponding to the next state, passing along the parsing stack and input tokens.

Advantages and Disadvantages

Recursive ascent parsers combine the advantages of LR parsing with the implementation simplicity of recursive functions. They are capable of parsing a wide range of grammars, including those that are not suitable for recursive descent parsers due to left recursion or ambiguity.

However, the main disadvantage of recursive ascent parsers is their complexity in terms of implementation. Constructing the parser requires a detailed understanding of the LR parsing table and the ability to translate this table into a series of recursive functions. This can be more challenging and less intuitive than the direct correspondence between grammar rules and functions in a recursive descent parser.

Comparison with Other Parsing Techniques

Recursive ascent parsers are most directly compared to recursive descent parsers, as both employ recursive functions for parsing. However, while recursive descent parsers are more straightforward to implement for simple grammars, recursive ascent parsers offer the robustness of LR parsing, making them suitable for more complex grammars.

Compared to table-driven LR parsers, recursive ascent parsers offer a more readable and maintainable codebase at the expense of some performance and flexibility. The explicit coding of states as functions can make the parser easier to debug and modify, but it may not match the efficiency of a compact table-driven approach.

Conclusion

Recursive ascent parsers represent a powerful, though less commonly used, approach to syntax analysis in compilers. They offer a balance between the implementation simplicity of recursive functions and the comprehensive grammar coverage of LR parsing. Despite their complexity, recursive ascent parsers provide a valuable tool for dealing with complex grammars in compiler design.