# Building A Simple Forth Interpreter (in python)

**Source:** https://peerdh.com/blogs/programming-insights/building-a-simple-forth-interpreter

Building A Simple Forth Interpreter
– peerdh.com


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# Building A Simple Forth Interpreter

Creating a Forth interpreter from scratch can be a rewarding experience. Forth is a stack-based programming language that emphasizes simplicity and efficiency. It is unique because it allows you to define new words (functions) easily, making it a great choice for embedded systems and low-level programming. This article will guide you through the process of building a simple Forth interpreter.

## Understanding Forth Basics

Forth operates on a stack, where data is pushed and popped. The language uses postfix notation, meaning operators follow their operands. For example, to add two numbers, you would write:

```
3 4 +
```

This pushes 3 and 4 onto the stack, then applies the `+` operator. The result, 7, remains on the stack.

### Forth Basics

## Setting Up the Environment

Before writing the interpreter, you need a programming environment. You can use any language, but Python is a good choice for its readability. Start by creating a new Python file named `forthand.py`.

### Basic Structure

Begin by defining the main structure of your interpreter. You will need a stack to hold values and a dictionary to store defined words.

```
class Forth:
    def __init__(self):
        self.stack = []
        self.words = {}

    def run(self, command):
        for token in command.split():
            self.execute(token)

    def execute(self, token):
        # Execution logic will go here
        pass
```

### Setting Up

## Implementing Stack Operations

Next, implement basic stack operations: `push`, `pop`, and `add`. These operations will be the foundation of your interpreter.

```
def push(self, value):
    self.stack.append(value)

def pop(self):
    if not self.stack:
        raise Exception("Stack underflow")
    return self.stack.pop()

def add(self):
    b = self.pop()
    a = self.pop()
    self.push(a + b)
```

### Stack Operations

## Defining Words

Forth allows you to define new words. This is done by adding them to the dictionary. Here’s how to implement a simple word definition.

```
def define_word(self, name, definition):
    self.words[name] = definition

def execute(self, token):
    if token.isdigit():
        self.push(int(token))
    elif token in self.words:
        self.words[token]()
    elif token == '+':
        self.add()
    else:
        raise Exception(f"Unknown command: {token}")
```

### Defining Words

## Handling Input

Now, you need to handle user input. You can create a simple loop that reads commands from the user.

```
def repl(self):
    while True:
        command = input("> ")
        if command == "exit":
            break
        self.run(command)
```

### Handling Input

## Putting It All Together

Now that you have the basic components, you can put everything together. Here’s the complete code for your Forth interpreter.

```
class Forth:
    def __init__(self):
        self.stack = []
        self.words = {}

    def run(self, command):
        for token in command.split():
            self.execute(token)

    def execute(self, token):
        if token.isdigit():
            self.push(int(token))
        elif token in self.words:
            self.words[token]()
        elif token == '+':
            self.add()
        else:
            raise Exception(f"Unknown command: {token}")

    def push(self, value):
        self.stack.append(value)

    def pop(self):
        if not self.stack:
            raise Exception("Stack underflow")
        return self.stack.pop()

    def add(self):
        b = self.pop()
        a = self.pop()
        self.push(a + b)

    def define_word(self, name, definition):
        self.words[name] = definition

    def repl(self):
        while True:
            command = input("> ")
            if command == "exit":
                break
            self.run(command)

if __name__ == "__main__":
    forth = Forth()
    forth.repl()
```

## Testing Your Interpreter

After implementing the interpreter, it’s time to test it. Run your script and try some commands:

```
> 3 4 +
> 5 6 +
> 10 2 +
```

You should see the results on the stack. If you want to define a new word, you can extend the interpreter further.

## Conclusion

Building a Forth interpreter is a great way to understand stack-based programming and language design. You can expand this interpreter by adding more operations, error handling, and even file input. The simplicity of Forth makes it an excellent choice for learning about interpreters and programming languages.

### Author - [LinkedIn LinkedIn](https://www.linkedin.com/in/peerdh/) - [GitHub GitHub](https://github.com/birdhouses)

## 1 comment

You could’ve implemented + as a word. I don’t think you could call this a Forth, it’s a RPN calculator. It’s missing conditionals and loops.

Martyn Buttersmere
•
May 31, 2025

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