LALR Parser in Compiler Design

Types of LALR Parser in Compiler Design

LALR are of two types:

1. Simple LALR parser: It is also known as SLR parser. Simple LALR uses a simple parse table construction algorithm. It only handles a limited class of grammar therefore, it is less powerful.
    
2. Canonical LALR parser: It is also known as CLR parser. Canonical LALR uses a complex parse table construction algorithm. It can handle a large range of class grammar more efficiently.

Diagrams and Tables 

Let us understand state tables with the help of an example.

Consider we have a grammar 

S -> L = R | R

L -> * R | id

R -> L

Step 1: Compute the LR(0) sets of the grammar: To computer the LR(0) set of the grammar we will use closure and go to operations.
 

Step 2: Compute the lookahead sets for the LR(0) items: To compute the lookahead sets for LR(0) we will use the FOLLOW set of the grammar.
 

Step 3: Compute the LR(1) sets of the grammar: To compute the LR(1) set we will use LR(0) items and their lookahead sets.
 

Step 4: Construct the LALR(1) parsing table: To construct LALR(1) parsing table we can use LR(1) sets.

Applications of LALR Parser in Compiler Design

There are many applications of LALR parsers in compiler design. A few of them are mentioned below.

1. Parsing programming language source code: LALR parsers are commonly used for parsing the source code. It can handle a huge variety of grammar efficiently in less time.
    
2. Generating error messages: LALR parsers can be used to create error messages. If the source code contains syntax errors, it analyses the state and input symbols. Hence, find the position and nature of error in the code.
    
3. Code optimisation: It is used to optimise the performance of code. LALR can analyse the structure of the input source code and generate code based on that analysis.
    
4. Flexibility: It is flexible and can handle a specific grammar rule. Therefore, it is suitable for use in a wide range of compiler designs.

Also check out - Phases of Compiler , cousins of compiler and Cross Compiler

Advantages and Disadvantages of LALR Parser in Compiler Design

Some of the advantages and disadvantages of LALR parser are:

Advantages:

1. Efficient: LALR parsers are efficient with respect to time and space complexity. 
    
2. Can handle large grammars: They can handle huge classes of grammars than any other parser.
    
3. Construction of abstract syntax tree: It is used to create an abstract syntax tree (AST) of the input source code. 
    

Disadvantages:

1. Complexity: It requires a significant amount of effort and expertise to generate an LALR parser. Since it involves the generation of a parse table which can be large and complex.
    
2. Overhead: The parsing process can introduce overhead in the compiler, which can impact performance.
    
3. Limited lookahead: The amount of lookahead used by an LALR parser is limited, which can result in parsing errors in some cases.

Must Read Recursive Descent Parser.

Frequently Asked Questions

What is the significance of an LALR handle?

An LALR handle is used to reduce a sequence of input symbols to the non-terminal on the left-hand side of the production. It is an important concept in LALR parsing as it helps to identify the portion of the input string that can be reduced.

How is an LALR handle identified?

An LALR handle is identified by examining the viable prefix of the input string and the right-hand side of the grammar production. The handle is a substring of the right-hand side that matches the right end of the viable prefix.

What is the role of an LALR handle in the parsing process?

In the parsing process, an LALR handle is used to reduce a sequence of input symbols to the non-terminal on the left-hand side of the production. This reduces the size of the stack and simplifies the parsing process.

Conclusion

LALR is a right substring of production rule. It is supposed to match with the right end of prefix. In this article we understood its uses, features, types, advantages and disadvantages. To dig deeper you can refer to LR parser and bottom-up parsing techniques.

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