Introduction: You Don't Need C++ to Build a Programming Language

When you search for "how to build a programming language," most tutorials point you toward low-level languages like C, C++, or Rust. While these are great for high-performance, system-level compilers, they have a steep learning curve. If your goal is educational, novelty, or web-focused, Python and JavaScript are actually the perfect tools for the job.

In this guide, we will break down how you can design and implement your own custom programming language that blends Python's clean logic with HTML/CSS/JS frontend capabilities.

Understanding the 3 Pillars of Language Design

Before writing code, you need to understand how a computer reads a programming language. The process generally follows three steps:

  • The Lexer (Lexical Analyzer): Takes your raw code (a string of text) and breaks it down into individual units called "tokens" (like keywords, numbers, operators, or strings).
  • The Parser: Takes those tokens and organizes them into a tree structure called an Abstract Syntax Tree (AST). This defines the grammar and relationship between elements.
  • The Interpreter / Compiler: The interpreter reads the AST and executes it on the fly. A compiler (or transpiler) converts the AST into another language (like Python or JavaScript) that can run directly on a browser or machine.

The Easiest Route: Building a Transpiler to JavaScript

Since you are comfortable with Python and want to integrate web elements (HTML/CSS/JS), the most rewarding approach is to build a transpiler in Python that converts your custom language into standard HTML, CSS, and JavaScript. This gives your language instant access to the web browser environment!

Step 1: Define Your Syntax

Let's design a simple, custom language called WebPy. We want to write simple code that automatically generates HTML and JavaScript interactive elements. For example:

button "Click Me" alert("Hello World!")

Step 2: Write a Simple Lexer in Python

We can use Python's built-in regular expressions (re) to tokenize our code. Here is a basic lexer:

import re

TOKEN_TYPES = [
    ('BUTTON', r'button'),
    ('STRING', r'"[^"]*"'),
    ('ACTION', r'alert\([^\)]*\)'),
    ('SKIP', r'[ \t\n]+'),
]

def tokenize(code):
    tokens = []
    position = 0
    while position < len(code):
        match = None
        for token_type, regex in TOKEN_TYPES:
            pattern = re.compile(regex)
            match = pattern.match(code, position)
            if match:
                text = match.group(0)
                if token_type != 'SKIP':
                    tokens.append((token_type, text))
                position = match.end(0)
                break
        if not match:
            raise SyntaxError(f'Illegal character at position {position}')
    return tokens

# Example usage:
code = 'button "Click Me" alert("Hello World!")'
tokens = tokenize(code)
print(tokens)

Step 3: Create the Parser and Code Generator

Now, let's parse these tokens and generate actual HTML and JavaScript. We will read the tokens sequentially and output a clean HTML file.

def compile_to_html(tokens):
    html_output = "<!DOCTYPE html>\n<html>\n<head><title>My WebPy App</title></head>\n<body>\n"
    js_output = "<script>\n"
    
    i = 0
    button_count = 0
    while i < len(tokens):
        if tokens[i][0] == 'BUTTON':
            # Expect a string next for the button text
            if i + 1 < len(tokens) and tokens[i+1][0] == 'STRING':
                btn_text = tokens[i+1][1].replace('"', '')
                btn_id = f"btn_{button_count}"
                html_output += f'  <button id="{btn_id}">{btn_text}</button>\n'
                
                # Expect an action next
                if i + 2 < len(tokens) and tokens[i+2][0] == 'ACTION':
                    action_code = tokens[i+2][1]
                    js_output += f'  document.getElementById("{btn_id}").addEventListener("click", () => {{ {action_code}; }});\n'
                    i += 3
                else:
                    i += 2
                button_count += 1
            else:
                raise SyntaxError("Expected string after 'button'")
        else:
            i += 1
            
    html_output += "\n" + js_output + "</script>\n</body>\n</html>"
    return html_output

# Compile our code
compiled_html = compile_to_html(tokens)
with open("index.html", "w") as f:
    f.write(compiled_html)
print("Compilation successful! index.html generated.")

Taking it to the Next Level: Powerful Libraries

If you want to build a more complex language without writing a parser entirely from scratch, you should look into standard parsing libraries:

  • Lark (Python): A modern, user-friendly parsing library for Python. It can parse any context-free grammar and build ASTs automatically.
  • PLY (Python Lex-Yacc): A classic implementation of lex and yacc parsing tools for Python.
  • Chevrotain or Nearley (JavaScript): If you decide to write the entire compiler in JavaScript so it runs natively inside the browser, these are excellent parsing toolkits.

Conclusion

You don't need to learn C++ to build a programming language. By leveraging your Python skills to parse text and your HTML/JS skills to render the output, you can create highly interactive, visual, and custom domain-specific languages (DSLs) with relative ease. Start small with a basic transpiler, and gradually add more complex structures like variables, loops, and custom CSS styling rules!