Blog: ANTLR Calculator

• The project demonstrates the following concepts of ANTLR4 considering the "Calculator" example using the Python language:
  • Grammar
  • Lexer
  • Parser
  • Difference between Visitor and Listener

Project Setup

• Requirements
  • Python 3
  • Java

ANTLR Setup

1. Download the antlr jar into /usr/local/lib directory

cd /usr/local/lib
curl -O http://www.antlr.org/download/antlr-4.13.1-complete.jar

2. Add the path in the CLASSPATH

export CLASSPATH="/usr/local/lib/antlr-4.13.1-complete.jar:$CLASSPATH"

3. Create alias to use ANTLR tools with a shorter command

alias antlr4=' java -jar /usr/local/lib/antlr-4.13.1-complete.jar'

Python environment setup

1. Create a Python virtual environment

   virtualenv path/to/venv/antlr_env
   

2. Activate the virtual environment

   source path/to/venv/antlr_env/bin/activate
   

3. Install packages

   pip install -r requirements.txt
   

Generate ANTLR Lexer, Parser, Listener, Visitor

• To generate lexer, parser, listener, and visitor files from the grammar execute the command:

antlr4 -Dlanguage=Python3 -visitor antlr/Calculator.g4

Visualize parsed input as tree

• To visualize the parsed input as tree execute the command (here "3+5*(2-8)" is considered as an example expression)
• Press Ctrl+D additionally after typing the expression (if you can not see the tree visualization directly)

antlr4-parse antlr/Calculator.g4 expression -gui
3+5*(2-8)

Evaluate expression using Visitor/Listener

Visitor:

• To evaluate an expression using visitor execute the command:

python3 calculate_with_visitor.py 3+5*(2-8)

• You should get the output

The result of '3+5*(2-8)' is: -27

Listener:

• To evaluate an expression using listener execute the command:

python3 calculate_with_listener.py 3+5*(2-8)

• You should get the output

The result of '3+5*(2-8)' is: -27