What is an LR parser?

LR parsing is a bottom-up parsing method in compiler construction that helps analyze a programming language's syntax. It converts the source code from a high-level programming language to an abstract syntax tree in the form of a hierarchical code structure.

The LR stands for the left-to-right derivation to construct a rightmost derivation of an input. Simply, it finds the rightmost sequence of the grammar production rules that derive the input string.

Types of LR parsing

According to parsing efficiency, there are different variants of LR parsing, which are illustrated below. However, in this Answer, we will only discuss the LR parser.

We denote LR parser with LR(k), where:

• "L" means the left-to-right scanning of the input.

• "R" means the rightmost derivation.

• "k" is the unconsumed "look ahead."

Before diving into the details, let us look at the semantic diagram of the LR parser, which is given below.

As shown in the diagram, the input takes one symbol from left to right at a time, for instance, $a_2$, $a_3$,......., $a_m$. The stack is the combination of the input symbol and the state symbol. The parsing table consists of action and GoTo tables.

The working of an LR parsing

The LR parsing algorithm operates as follows:

1. Shift-reduce parsing: It is a technique used by parsers to process input step-by-step wise and construct a parse tree. The parser scans the input from left to right and performs two primary operations: shifting and reducing.

2. Shift: When the parser encounters a token in the input that matches the current state, it moves it onto a stack and transitions to a new state.

3. Reduce: If the parser identifies a specific pattern on top of the stack that matches a production rule in reverse, it combines those symbols into a non-terminal symbol. This process continues until no more reductions are possible or an error is encountered.

4. Accept: The parsing process is successful if the parser reaches the end of the input and the only symbol left on the stack is the start symbol of the grammar. This indicates that the input is syntactically correct and a parse tree has been successfully constructed.

Applications of an LR parser

Due to its efficiency and ability to handle context-free grammar, it has many applications in compiler construction and computer science. Some notable applications of LR parser are given below.

• We use an LR parser in compiler construction to implement the syntax analysis phase. Once the parser successfully constructs the abstract syntax tree (AST) for the input program, it becomes easier to write intermediate code or perform subsequent compiler phases.

• LR parsers play a significant role in processing programming languages. They are used for various tasks, including parsing expressions and statements, managing the precedence and associativity of operators, and creating syntax-directed translations for language constructs.

• LR parsers are used in natural language processing to recognize and analyze the structure of sentences. LR parser has many applications, such as in language translation, speech recognition, and sentiment analysis.

• LR parsers are employed in data validation tasks where structured input needs to be parsed and verified against specific grammar rules or data constraints.

• LR parsers are crucial in parsing various data formats and protocols, such as JSON, XML, YAML, and binary formats.

• LR parsers can evaluate complex expressions, particularly in mathematics, engineering, and scientific computing.

Conclusion

LR parsers play an essential role in compiler construction and the processing of programming languages. With the help of LR parsing techniques, compilers can ensure that the source code is correct and create meaningful syntax trees. They have natural language processing, data validation, and protocol parsing applications. Overall, LR parsers are essential components in modern language processing systems, and they are crucial for effectively managing context-free grammar, which significantly contributes to the advancement of software development and our understanding of various languages.