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Table of contents
1.
Introduction
2.
What is Handle Compiler Design?
3.
Vital points of Handle in Compiler Design
4.
Example of Handle in Compiler Design
5.
Types of Handle in Compiler Design
6.
Advantages and Disadvantages of Handle in Compiler Design
7.
Frequently Asked Questions
7.1.
Why are handles important in compiler design?
7.2.
What is a viable prefix in compiler design?
7.3.
What is bottom-up parsing in compiler design?
8.
Conclusion
Last Updated: Mar 27, 2024

Handle in Compiler Design

Author Tashmit
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Introduction

A compiler is a software that converts high-level instructions to low-level. Compiler design is the set of steps that guide the process of compilation. Designing is done with various methods. 

Handle in compiler design

In this article, we will understand about handle in compiler design. We will also discuss vital points and examples of handle in compiler design. In the end, we will see the advantages and disadvantages of the same.

What is Handle Compiler Design?

The handle in compiler design is referred to the substring of the right-hand side in a production rule in context-free grammar. When we are constructing a parse tree, the handle is the portion of the input string that has been matched. It can be reduced to a non-terminal symbol according to a production rule.

Moving forward, let's discuss vital points of handle in compiler design.

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Vital points of Handle in Compiler Design

The concept of the handle is necessary for understanding parsing in compiler design. Some of the vital points to note are:

  1. In a context-free grammar, the handle is at the right side of the production rule.
     
  2. The part of the input string that has been recognised is represented by the handle during the process of parsing.
     
  3. In parsing algorithms such as LR parser, handles play an important role for bottom-up parsing. It uses a stack to store input and grammar rules.

Example of Handle in Compiler Design

Let us understand handle in compiler design with the help of an example:

Grammar:

S → E

E → E + T | T

T → T * F | F

F → ( E ) | id

Given input string: id + id * id

Stack Lookahead Action
[] id Shift, go to 3
[“id] + Reduce F-> id
[“F”] Shift go to 5
[“F”,”+”] id Shift go to 3
[“F”,”+”,”id”] * Shift go to 6
[“F”,”+”,”id”,”*”] id Shift go to 3
[“F”,”+”,”id”,”*”,”id”] EOF Reduce T->T*F
[“T”] + Reduce F->id
[“F”] + Shift go to 5
[“F”+“T”] EOF Reduce E-> T
[“E”] EOF Reduce S->E
["S"]    

Types of Handle in Compiler Design

There are many types of handle in compiler design:

  1. Canonical handle: It appears in a production rule in its proper position. We can say that it is the string that matches the right side of production rule from left to right. 
     
  2. Proper handle: It is a type of canonical handle which is not a suffix of an other canonical handle. It matches the right substring but there is no other substring that matches the right side of production rule.
     
  3. Unit handle: As the name suggests, it handles a single nonterminal symbol. It matches the right side that contains only one nonterminal symbol.
     
  4. Null handle: It is the handle that matches null strings. It is a substring that matches the right side that consists of no symbols.


Also check out - Phases of Compiler

Advantages and Disadvantages of Handle in Compiler Design

Here are some of the advantages and disadvantages of using handles:

Advantages: 

  1. In order to construct parse trees, it is essential to reduce the portions of input non-terminal symbols. Thus, handles help the parser to navigate through the input and avoid backtracking efficiently. 
     
  2. Handles can simplify parsing algorithms by breaking the parsing process into smaller sub-problems.

 

Disadvantages: 

  1. It is necessary to have prior knowledge of context-free grammar in order to identify handles. It is complex and can be time-consuming. 
     
  2. The parsing algorithms tend to be computationally expensive if the input is larger or the grammar is complex. Also, if the grammar is ambiguous, the usage of the handle could be reduced. 

Must Read Recursive Descent Parser and  cousins of compiler

Frequently Asked Questions

Why are handles important in compiler design?

Handles provide a systematic way of reducing portions of the input to non-terminal symbols. It is important in order to construct a parse tree according to the grammar.

What is a viable prefix in compiler design?

If a variable can be extended to a valid parse tree, it could be a viable prefix. It can then be the longes prefix which can be reduced to a non-terminal.

What is bottom-up parsing in compiler design?

The technique that starts the parsing from the input symbol all the way up to the start symbol is known as the bottom-up parsing technique. This technique involves identifying handles and performing reductions to build the parse tree.

Conclusion

In this article, we understood the handle in compiler design. We studied its vital points, advantages, and disadvantages. You can refer to errors in compiler and error recovery and handling to understand handle in compiler design better.  

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