How to Build Your Own Programming Language (It’s Easier Than You Think)

Why Every Developer Should Learn to Build a Programming Language

Have you ever wondered how programming languages are actually built? Or maybe you've had a brilliant idea for a domain-specific language that could solve a particular problem in your field.

Building a programming language is way easier than most developers think. Thanks to tools like ANTLR(ANother Tool for Language Recognition), you can go from idea to working interpreter in a single weekend.

ANTLR is a parser generator that takes a grammar specification and automatically generates lexers, parsers, and tree walkers in your target language.

Real-world examples of ANTLR in action:

• Hibernate – uses ANTLR for HQL query parsing
• Elasticsearch – relies on ANTLR for its scripting language
• Apache Spark – integrates ANTLR for SQL parsing

From Regex Hell to Grammar Heaven

"Why can't I just use the regular expressions?" Every developer asks this question. Let me show why regex becomes a nightmare for anything beyond simple pattern matching.

Try parsing HTML with regex, and you'll quickly understand the problem. Want to match a simple table? You start with <table>(.*?)</table>. But then someone adds attributes: <table class="data">(.*?)</table> . Then nested tags appear, then comments. Then... your code becomes unmaintainable.

ANTLR solves this elegantly:

ANTLR Grammar

tableElement
   : '<table' attribute* '>' tableContent* '</table>'
   ;

attribute
   : IDENTIFIER '=' STRING_LITERAL
   ;

tableContent
   : tableRow
   | comment
   | nestedElement
   ;

The magic happens when ANTLR generates a complete parser that handles recursion, nested structures, and complex syntax rules automatically. No more fragile regex chains or hand-written parser nightmares!

Your First Programming Language

This section will demonstrate how to build a basic programming language using ANTLR. It implements a simple custom interpreter in Python, showing the entire pipeline from grammar definition to executing programs.

Project Structure:

   mylanguage/
     |---- Grammar.g4.  # grammar definition for my programming language
     |---- Visitor.py   # interpreter logic
     |---- interpreter.py # main interpreter entry point
     |---- main.py   # command line interface

The complete workflow

1. Define Grammar - write .g4 files describing your language syntax
2. Generate Parser - ANTLR creates Python Lexer/Parser classes
3. Build Interpreter - Custom visitor walks the parse tree and executes code
4. Run Programs - Your language comes alive!

Setup is surprisingly simple:

Shell
#Install ANTLR and python runtime
pip install antlr4-python3-runtime

# set classpath (windows)
set CLASSPATH="C:\downloads\ANTLR\antlr-4.6-complete.jar;"

# Set classpath (Linux/Mac)
export CLASSPATH=".:/usr/local/lib/antlr-4.6-complete.jar:$CLASSPATH"

# Generate the parser
java org.antlr.v4.Tool -Dlanguage=Python3 -visitor -no-listener Grammar.g4

# Run your language!
python3 main.py <code file written in new language>

What you get: A complete interpreter that can parse and execute a program written in your custom language syntax!

Writing the DNA of your Language

Grammar files are where the magic happens. They define what valid programs look like in your language. Let's build a complete example step by step.

Step 1: Lexer Rules

lexer grammar CustomLexer;
// Keywords
IF      : 'if' ;
ELSE    : 'else' ;
WHILE   : 'while' ;
FOR     : 'for' ;
FUNCTION: 'function' ;
RETURN  : 'return' ;
PRINT   : 'print' ;

// Operators
PLUS    : '+' ;
MINUS   : '-' ;
MULTIPLY: '*' ;
DIVIDE  : '/' ;
ASSIGN  : '=' ;
EQUALS  : '==' ;
LESS    : '<' ;
GREATER : '>' ;

// Delimiters
SEMICOLON: ';' ;
COMMA    : ',' ;
LPAREN   : '(' ;
RPAREN   : ')' ;
LBRACE   : '{' ;
RBRACE   : '}' ;

// Literals and Identifiers
NUMBER   : [0-9]+ ('.' [0-9]+)? ;
STRING   : '"' (~["\\\r\n] | '\\' .)* '"' ;
IDENTIFIER: [a-zA-Z_][a-zA-Z0-9_]* ;

// Whitespace (skip)
WS: [ \t\r\n]+ -> skip ;

Step 2: Parser Rules

parser grammar CustomParser;
options { tokenVocab=CustomLexer; }

// Program structure
program: statement+ EOF ;
statement
    : variableDeclaration
    | assignment
    | ifStatement
    | whileStatement
    | functionCall SEMICOLON
    | returnStatement
    | block
    ;

// Variable declarations
variableDeclaration
    : 'var' IDENTIFIER ('=' expression)? SEMICOLON
    ;

// Assignments
assignment
    : IDENTIFIER '=' expression SEMICOLON
    ;

// Control flow
ifStatement
    : 'if' '(' expression ')' statement ('else' statement)?
    ;

whileStatement
    : 'while' '(' expression ')' statement
    ;

// Expressions with precedence
expression
    : expression ('*' | '/') expression     # MultiplicativeExpr
    | expression ('+' | '-') expression     # AdditiveExpr
    | expression ('==' | '<' | '>') expression # ComparisonExpr
    | '(' expression ')'                    # ParenthesizedExpr
    | functionCall                          # FunctionCallExpr
    | IDENTIFIER                            # VariableExpr
    | NUMBER                                # NumberExpr
    | STRING                                # StringExpr
    ;

functionCall
    : IDENTIFIER '(' (expression (',' expression)*)? ')'
    ;

block
    : '{' statement* '}'
    ;

returnStatement
    : 'return' expression? SEMICOLON
    ;

The beauty of this approach is that ANTLR automatically handles operator precedence, recursion, and complex syntax relationships that would take hundreds of lines of hand-written parsing code!

Bringing Your Language to Life: The Interpreter Implementation

Now comes the exciting part - making your language actually DO something! The CustomVisitor class is where your grammar rules become executable code.

Here's a complete interpreter implementation:

from antlr4 import *
from CustomParser import CustomParser
from CustomParserVisitor import CustomParserVisitor

class MyInterpreter(CustomParserVisitor):
    def __init__(self):
        self.variables = {}  # Variable storage
        self.functions = {}  # Function definitions

    def visitProgram(self, ctx):
        """Entry point - execute all statements"""
        result = None
        for statement in ctx.statement():
            result = self.visit(statement)
            if isinstance(result, ReturnValue):
                break
        return result

    def visitVariableDeclaration(self, ctx):
        """Handle: var x = 10;"""
        name = ctx.IDENTIFIER().getText()
        if ctx.expression():
            value = self.visit(ctx.expression())
            self.variables[name] = value
        else:
            self.variables[name] = None
        return None

    def visitAssignment(self, ctx):
        """Handle: x = 20;"""
        name = ctx.IDENTIFIER().getText()
        value = self.visit(ctx.expression())
        self.variables[name] = value
        return value
 
    def visitIfStatement(self, ctx):
        """Handle: if (condition) statement else statement"""
        condition = self.visit(ctx.expression())
        if self.is_truthy(condition):
            return self.visit(ctx.statement(0))
        elif len(ctx.statement()) > 1:  # else clause
            return self.visit(ctx.statement(1))
        return None

    def visitWhileStatement(self, ctx):
        """Handle: while (condition) statement"""
        while True:
            condition = self.visit(ctx.expression())
            if not self.is_truthy(condition):
                break
            result = self.visit(ctx.statement())
            if isinstance(result, ReturnValue):
                return result
        return None

    def visitAdditiveExpr(self, ctx):
        """Handle: expression + expression"""
        left = self.visit(ctx.expression(0))
        right = self.visit(ctx.expression(1))
        operator = ctx.getChild(1).getText()
        if operator == '+':
            return left + right
        elif operator == '-':
            return left - right

    def visitMultiplicativeExpr(self, ctx):
        """Handle: expression * expression"""
        left = self.visit(ctx.expression(0))
        right = self.visit(ctx.expression(1))
        operator = ctx.getChild(1).getText()
        if operator == '*':
            return left * right
        elif operator == '/':
            if right == 0:
                raise RuntimeError("Division by zero")
            return left / right

    def visitComparisonExpr(self, ctx):
        """Handle: expression == expression"""
        left = self.visit(ctx.expression(0))
        right = self.visit(ctx.expression(1))
        operator = ctx.getChild(1).getText()     
        if operator == '==':
            return left == right
        elif operator == '<':
            return left < right
        elif operator == '>':
            return left > right

   
    def visitVariableExpr(self, ctx):
        """Handle: identifier references"""
        name = ctx.IDENTIFIER().getText()
        if name in self.variables:
            return self.variables[name]
        raise RuntimeError(f"Undefined variable: {name}")

    def visitNumberExpr(self, ctx):
        """Handle: number literals"""
        text = ctx.NUMBER().getText()
        if '.' in text:
            return float(text)
        return int(text)

    def visitStringExpr(self, ctx):
        """Handle: string literals"""
        text = ctx.STRING().getText()
        # Remove quotes and handle escape sequences
        return text[1:-1].replace('\\"', '"')

    
    def visitFunctionCall(self, ctx):
        """Handle: print("Hello World")"""
        name = ctx.IDENTIFIER().getText()
        args = []
        if ctx.expression():
            args = [self.visit(expr) for expr in ctx.expression()]    
        # Built-in functions
        if name == 'print':
            output = ' '.join(str(arg) for arg in args)
            print(output)
            return None    
        raise RuntimeError(f"Unknown function: {name}")

   
    def is_truthy(self, value):
        """Determine if a value is truthy"""
        if value is None:
            return False
        if isinstance(value, bool):
            return value
        if isinstance(value, (int, float)):
            return value != 0
        if isinstance(value, str):
            return len(value) > 0
        return True

class ReturnValue:
    """Wrapper for return values"""
    def __init__(self, value):
        self.value = value

The main execution script:

# main.py
import sys
from antlr4 import *
from CustomLexer import CustomLexer
from CustomParser import CustomParser
from CustomInterpreter import CustomInterpreter

def main():
    if len(sys.argv) != 2:
        print("Usage: python main.py <source_file>")
        return
    # Read source code
    with open(sys.argv[1], 'r') as file:
        source_code = file.read()


    # Create lexer and parser
    input_stream = InputStream(source_code)
    lexer = CustomLexer(input_stream)
    token_stream = CommonTokenStream(lexer)
    parser = CustomParser(token_stream)
    
    # Parse the code
    tree = parser.program()
    # Execute with our interpreter
    interpreter = CustomInterpreter()
    try:
        interpreter.visit(tree)
    except Exception as e:
        print(f"Runtime error: {e}")

if __name__ == '__main__':
    main()

Example program

// test.expr
{
  num1 = 25;
  num2 = 10;
  while(num1 <> num2) {
    if (num1 > num2) {
      num1 = num1 - num2;
    } else {
      num2 = num2 - num1;
    }
  }
  print("The greatest common divider is: ");
  print(num1);
}

Run it: python main.py test.expr and watch your language come alive!

Your Language Development Journey Starts Now

Congratulations! You've just learned how to build a complete programming language from scratch. Here's what you've accomplished:

What's next? Your language development journey has just begun:

• Add more features: Functions, arrays, objects, imports
• Improve performance: Compile to bytecode instead of tree-walking
• Build tooling: Syntax highlighting, debugger, package manager
• Real-world applications: DSLs for your domain, configuration languages, templating systems.