How to Build a Language Runtime from Scratch in Just 60 Minutes

Because we can! We are doing this for fun, not money. Today, you will build your own language runtime. A language runtime helps your code talk to the operating system. It also manages memory and handles errors. This helps programs run without problems. Many developers make custom runtimes to try new ideas. Some do it for special needs in their work. Others want to learn how languages work inside. You will see that this process is fun and feels good.

Key Takeaways

• Begin with a few main features for your language runtime. Focus on primitive types and basic syntax. This makes things easier.

• Learn why memory management matters. Use stack and heap to store data. This helps stop memory leaks.

• Add error handling with exceptions. This keeps your program safe. It also helps you fix problems when they happen.

• Test your runtime with easy programs first. Add harder parts slowly. Make sure each feature works before you add more.

• Keep learning and trying new things. Building a language runtime is fun. It helps you understand programming languages. It also helps you get better at coding.

Minimal Language Runtime

Syntax & Features

You will begin by picking a small part of C#. This helps you learn the main ideas of a language runtime. You do not need every part of C#. Choose only the most important things. This makes your work easier and more fun.

Here are the main things you need for your simple language runtime:

1. Primitive types like numbers, strings, and booleans.

2. Compound types like arrays, with easy methods such as .map and .reduce.

3. Expressions, like function calls, operators, and simple control flow such as case or switch.

4. Memory management using a basic heap and stack.

5. Simple runtime APIs for common jobs.

Tip: Begin with just a few features! You can add more later. Try to get the basics working first.

Think about which features help you learn most about runtimes. The table below shows some important features and why they matter:

When you make your own language runtime, you choose which features to use. You can try different things and see how they work.

Data Types

You need to know about data types before you build a language runtime. Every program uses data types to hold and use information. In your simple runtime, you will use basic types that are easy to handle.

To make your runtime work, you should use numbers, strings, and booleans. These types are easy to store in memory. Arrays let you put values together. You can add methods like .map and .reduce to make arrays better.

Everything in your language runtime will turn into low-level instructions. This means you must keep track of variables and manage memory well. Many runtimes use Single Static Assignment (SSA) to help with this. SSA helps you make code faster and keep track of variables.

Note: If you start with just a few data types, you can focus on learning how runtimes work. You can add more types later.

Now you have a good plan for your simple language runtime. You know what features and data types to use. Next, you will learn how to change your code into something the computer can run.

Lexer & Parser

Tokenization

You must split your code into small parts first. This is called tokenization. Each part is a token. Tokens can be numbers, words, symbols, or operators. Tokenization helps your language runtime read code.

Here are the usual steps for tokenizing source code:

1. Find tokens by looking for patterns like operators and keywords.

2. Make rules for tokenization. These rules depend on your language.

3. Write methods to handle comments and strings. This helps you avoid errors.

4. Use a parser to check the code after tokenization.

You can use tools and libraries to help with tokenization in .NET. Some popular choices are:

• Tiktoken is fast and works well with Byte Pair Encoding.

• ML.NET lets you use Tiktoken in C#.

• Blazor helps you build user interfaces in the browser.

• Telerik UI for Blazor gives you easy UI parts.

Tip: Begin with simple rules for your tokens. Add more rules as your language gets bigger.

AST Creation

After you tokenize your code, you need to build an Abstract Syntax Tree (AST). An AST shows how your code is put together. It removes extra details and keeps only what matters for analysis and running.

An Abstract Syntax Tree is a data structure that shows your program’s shape. It comes from the syntax analysis step. The AST makes code easier to check and improve. It skips details that are not needed for running your code.

An abstract syntax tree leaves out a lot of the extra information that a parse tree would have. An AST only keeps what is needed to study the source text, skipping any extra stuff used while parsing.

You can use different ways to build an AST from your tokens:

• ANTLR lets you pick which tokens become tree roots or leaves.

• You can turn AST building on or off in ANTLR.

• You can make tree nodes by hand using methods like new T(arg) or ASTFactory.create(arg).

• Short notations like #[TYPE] help you make nodes fast.

When you finish this step, your code is ready for the next part: execution.

Interpreter

Execution

Now you will make your code work. The interpreter reads your program. It runs each instruction one at a time. This is not like compiling. Compiling turns the whole program into machine code first. With an interpreter, you see what happens right away.

Here is a table that shows how interpreters and compilers are different:

Using an interpreter gives you quick feedback. You can try new ideas fast. You can fix mistakes as you go. This makes learning easier.

Your interpreter does three main things:

Here are steps to run code with your interpreter:

1. Look at a piece of code.

2. Make an instruction for your runtime.

3. Run the instruction.

Tip: You see results as soon as you write code. This helps you learn faster and fix problems quickly.

Here is a simple example in C#-like pseudocode:

// Example: Print a number
print(5 + 3);

Your interpreter reads print(5 + 3);. It figures out what 5 + 3 means. Then it prints 8. You do not need to wait for a compiler. You see the output right away.

Variables

Variables let you store and use data in your program. You need to keep track of variables in your interpreter. You must remember their names, values, and where they are in memory.

You may have some problems with variables. Here is a table that shows common issues:

Watch out for these problems too:

• Uninitialized Variables: If you use a variable before you set its value, you may get random results.

• Concurrency Issues: If your program runs in more than one thread, variables can cause errors like race conditions or deadlocks.

• Faulty Logic: Mistakes in your code can make your program loop forever or give wrong answers.

You can use a simple table to keep track of variables. Here is an example:

When your interpreter sees a variable, it looks up the name in the table. It gets the value and uses it in the program. If you change the value, the table updates. This helps your program remember data as it runs.

Note: Always initialize your variables before you use them. This helps you avoid strange bugs and makes your code easier to understand.

Now you know how to run code and manage variables in your language runtime. You can build programs that work well and handle data safely.

Runtime Support

Memory

You have to take care of memory in your language runtime. Memory management lets your program save and use data safely. There are two main types of memory: stack and heap. Each one does something different.

The stack is used for simple variables and function calls. The heap is for objects and arrays that need more space or last longer. Garbage collection helps remove things you do not need anymore. This means you do not have to worry as much about memory leaks.

Some basic runtime support features you should have are:

• Memory management

• Parameter passing

• Interaction with the operating system

• Stack and heap management

• Garbage collection

• Thread management

Tip: Begin with easy stack and heap management. Add garbage collection later if you want to learn more about advanced runtimes.

Errors

You need to handle errors so your program keeps working well. Errors can happen for many reasons. You must catch them and deal with them the right way.

Some common errors are:

• Division by zero

• Overflow or underflow in arithmetic operations

• Loss of precision in floating-point arithmetic

• Using arithmetic operators the wrong way

Resource errors happen if your program runs out of memory or cannot open a file. You should use exceptions instead of error codes. Exceptions help you keep error handling separate from normal code. Try using a GenericException class to make error messages clear. Using try-catch blocks makes your code easier to fix and understand.

Note: Always look for errors and use exceptions to handle them. This keeps your program safe and simple to fix.

Test Your Language Runtime

Example Code

You have built your language runtime. Now you need to see if it works. Start by writing simple programs that use the features you added. Try a basic example like printing a number or adding two values:

// Example 1: Print a message
print("Hello, world!");

// Example 2: Add two numbers
let sum = 4 + 5;
print(sum);

Run these programs through your runtime. Check if the output matches what you expect. If you see "Hello, world!" and "9" printed, your runtime handles basic instructions. Next, test more complex features. Try arrays, loops, or simple conditionals. For example:

// Example 3: Array and loop
let numbers = [1, 2, 3];
for (let i = 0; i < 3; i = i + 1) {
    print(numbers[i]);
}

Testing does not stop at running code. Use different methods to check correctness. Many developers use differential testing to compare results from your runtime with another trusted system. Some use formal methods or model checking to prove that the runtime works as expected. These steps help you find hidden bugs and make your runtime more reliable.

Debugging

When your code does not work, you need to debug. Start by using tools that help you see what happens inside your runtime. Visual Studio gives you step-by-step debugging and lets you inspect variables. You can use remote debugging to check problems on other machines. Profiling tools help you spot memory leaks or slow parts of your code.

Add assertions in your code to catch invalid states early. Unit testing helps you check small parts of your runtime, so you find issues before they grow. If you build web features, Fiddler can capture and show HTTP requests and responses.

Testing and debugging make your language runtime strong. You learn how real execution environments like the CLR work. Each test and fix brings you closer to a reliable system. 🎯

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You made a language runtime for part of C# in one hour. This project taught you about managed code, garbage collection, and just-in-time compilation. The table below shows important things you learned:

You can keep learning by looking at .NET internals or trying to make a runtime for another language. The table below shows good topics to study:

Making tools like this is fun and helps you learn. You can share ideas, join talks, and learn from other people:

• You meet other developers and show your work.

• You join creative workshops and learn new skills.

• You get smarter by talking about programming languages.

Keep trying new things and building. Every project teaches you more and makes coding more fun. 🚀

FAQ

How do you start building a language runtime?

You start by choosing a small set of features. Pick simple data types and basic syntax. Write a lexer and parser first. Build your interpreter step by step. Test each part as you go. This helps you learn and fix mistakes early.

What tools do you need for this project?

You need a code editor like Visual Studio Code. Use .NET for building and testing. Try libraries for parsing, such as ANTLR. Debugging tools help you find errors. A simple terminal or command prompt lets you run your programs.

Can you add more features later?

Yes! You can add new data types, control flow, or error handling after your basic runtime works. Start small. Add one feature at a time. Test each change. This keeps your project easy to manage and helps you learn more.

What should you do if your runtime does not work?

Check your code for mistakes. Use print statements to see what happens inside your runtime. Try debugging tools to find errors. Test with simple programs first. Fix one problem at a time. Ask for help if you get stuck.