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

Why Every Developer Should Learn to Build a Programming Language 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: Real-world examples of ANTLR in action: Hibernate – uses ANTLR for HQL query parsingElasticsearch – relies on ANTLR for its scripting languageApache Spark – integrates ANTLR for SQL parsing Hibernate – uses ANTLR for HQL query parsing Hibernate HQL query parsing Elasticsearch – relies on ANTLR for its scripting language Elasticsearch scripting language Apache Spark – integrates ANTLR for SQL parsing Apache Spark SQL parsing From Regex Hell to Grammar Heaven 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 (.*?) . But then someone adds attributes: (.*?) . Then nested tags appear, then comments. Then... your code becomes unmaintainable. (.*?) (.*?) ANTLR solves this elegantly: ANTLR solves this elegantly: ANTLR Grammar tableElement : ' ' tableContent* ' ' ; attribute : IDENTIFIER '=' STRING_LITERAL ; tableContent : tableRow | comment | nestedElement ; ANTLR Grammar tableElement : ' ' tableContent* ' ' ; 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 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. Python Project Structure: 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 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 Define Grammar - write .g4 files describing your language syntaxGenerate Parser - ANTLR creates Python Lexer/Parser classesBuild Interpreter - Custom visitor walks the parse tree and executes codeRun Programs - Your language comes alive! Define Grammar - write .g4 files describing your language syntax Define Grammar Generate Parser - ANTLR creates Python Lexer/Parser classes Generate Parser Build Interpreter - Custom visitor walks the parse tree and executes code Build Interpreter Run Programs - Your language comes alive! Run Programs Setup is surprisingly simple: 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:$CLASSPATH" # Generate the parser java org.antlr.v4.Tool -Dlanguage=Python3 -visitor -no-listener Grammar.g4 # Run your language! python3 main.py Shell #Install ANTLR and python runtime pip install antlr4-python3-runtime # set classpath (windows) set CLASSPATH="C:\downloads\ANTLR\antlr-4.6-complete.jar;" # Set classpath (Linux/Mac) export CLASSPATH=".:/usr/local/lib/antlr-4.6-complete.jar:$CLASSPATH" # Generate the parser java org.antlr.v4.Tool -Dlanguage=Python3 -visitor -no-listener Grammar.g4 # Run your language! python3 main.py What you get: A complete interpreter that can parse and execute a program written in your custom language syntax! What you get: 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. Grammar files are where the magic happens Step 1: Lexer Rules 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 : ' ' ; // 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 ; 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 : ' ' ; // 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 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 ; 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! The beauty of this approach 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. Now comes the exciting part Here's a complete interpreter implementation: 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 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 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 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: 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 ") 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() # 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 ") 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 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); } // 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! Run it: python main.py test.expr 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: Congratulations! ComponentWhat You BuiltReal-World UsageLexerTokenizes source code into meaningful symbolsUsed in every compiler/interpreterParserBuilds Abstract Syntax Trees from tokensPowers IDE syntax highlightingInterpreterExecutes programs by walking the ASTEnables rapid prototyping of languagesError HandlingProvides meaningful error messagesEssential for developer experience ComponentWhat You BuiltReal-World UsageLexerTokenizes source code into meaningful symbolsUsed in every compiler/interpreterParserBuilds Abstract Syntax Trees from tokensPowers IDE syntax highlightingInterpreterExecutes programs by walking the ASTEnables rapid prototyping of languagesError HandlingProvides meaningful error messagesEssential for developer experience ComponentWhat You BuiltReal-World Usage ComponentWhat You BuiltReal-World Usage Component Component Component What You Built What You Built What You Built Real-World Usage Real-World Usage Real-World Usage LexerTokenizes source code into meaningful symbolsUsed in every compiler/interpreterParserBuilds Abstract Syntax Trees from tokensPowers IDE syntax highlightingInterpreterExecutes programs by walking the ASTEnables rapid prototyping of languagesError HandlingProvides meaningful error messagesEssential for developer experience LexerTokenizes source code into meaningful symbolsUsed in every compiler/interpreter Lexer Lexer Lexer Tokenizes source code into meaningful symbols Tokenizes source code into meaningful symbols Tokenizes source code into meaningful symbols Used in every compiler/interpreter Used in every compiler/interpreter Used in every compiler/interpreter ParserBuilds Abstract Syntax Trees from tokensPowers IDE syntax highlighting Parser Parser Parser Builds Abstract Syntax Trees from tokens Builds Abstract Syntax Trees from tokens Builds Abstract Syntax Trees from tokens Powers IDE syntax highlighting Powers IDE syntax highlighting Powers IDE syntax highlighting InterpreterExecutes programs by walking the ASTEnables rapid prototyping of languages Interpreter Interpreter Interpreter Executes programs by walking the AST Executes programs by walking the AST Executes programs by walking the AST Enables rapid prototyping of languages Enables rapid prototyping of languages Enables rapid prototyping of languages Error HandlingProvides meaningful error messagesEssential for developer experience Error Handling Error Handling Error Handling Provides meaningful error messages Provides meaningful error messages Provides meaningful error messages Essential for developer experience Essential for developer experience Essential for developer experience What's next? Your language development journey has just begun: What's next? Add more features: Functions, arrays, objects, importsImprove performance: Compile to bytecode instead of tree-walkingBuild tooling: Syntax highlighting, debugger, package managerReal-world applications: DSLs for your domain, configuration languages, templating systems. Add more features: Functions, arrays, objects, imports Add more features Improve performance: Compile to bytecode instead of tree-walking Improve performance Build tooling: Syntax highlighting, debugger, package manager Build tooling Real-world applications: DSLs for your domain, configuration languages, templating systems. Real-world applications