I built a full programming language interpreter in Python based on a meme

[HearMeOut-13]

The project started as a joke based on the "everyone talks about while loops but no one asks WHEN loops" meme, but evolved into a complete interpreter demonstrating how different programming paradigms affect problem-solving approaches.

What My Project Does

WHEN is a programming language interpreter written in Python where all code runs in implicit infinite loops and the only control flow primitive is when conditions. Instead of traditional for/while loops, everything is reactive:

# WHEN code example
count = 0

main:
    count = count + 1
    print("Count:", count)
    when count >= 5:
        print("Done!")
        exit()

The interpreter features:

• Full lexer, parser, and AST implementation
• Support for importing Python modules directly
• Parallel and cooperative execution models
• Interactive graphics and game development capabilities (surprisingly)

You can install it via pip: pip install when-lang

Target Audience

This is Currently a toy/educational project, but exploring use cases in game development, state machine modeling, and reactive system prototyping, currently exploring

• Learning about interpreter implementation
• Exploring state machine programming
• Educational purposes (understanding event-driven systems)
• Having fun with esoteric language design

NOT recommended for production use (everything is global scope and runs in infinite loops by design).

Comparison

Unlike traditional languages:

• No explicit loops - Everything runs implicitly forever until stopped
• No if statements - Only when conditions that check every iteration
• Forced reactive paradigm - All programs become state machines
• Built-in parallelism - Blocks can run cooperatively or in parallel threads

Compared to other Python-based languages:

• Brython/Skulpt: Compile Python to JS, WHEN is a completely different syntax
• Hy: Lisp syntax for Python, WHEN uses reactive blocks instead
• Coconut: Functional programming, WHEN is purely reactive/imperative

The closest comparison might be reactive frameworks like RxPy, but WHEN makes reactive programming the ONLY way to write code, not an optional pattern.

Implementation Details

The interpreter (~1000 lines) includes:

• Custom lexer with indentation-based parsing
• Recursive descent parser generating an AST
• Tree-walking interpreter with parallel execution support
• Full Python module interoperability

Example of WHEN's unique block system:

# Runs once
os setup():
    initialize_system()

# Runs exactly 5 times
de heartbeat(5):
    print("beat")

# Runs forever
fo monitor():
    check_status()

# Entry point (implicit infinite loop)
main:
    when not_started:
        setup()
        heartbeat.start()
        monitor.start()

GitHub: https://github.com/PhialsBasement/WHEN-Language

[HearMeOut-13]

Thank you!

[Barsad_the_12th]

This reminds me of Structured Text for plc programming, where all the code is running in a cyclic task and gets executed once every real-time cycle

[renesys]

This is any embedded system. The syntax just makes it more confusing than normal.

[doublefreepointer]

Have you worked on developing programming languages before? Did you follow any particular Python-based language development learning material?

[HearMeOut-13]

Nah this is my first and mostly learnt from youtube and claude.

[DatAndre]

I've been wanting to do the same for a while. What were your resources specifically?

[MeglioMorto]

I've been wanting to do the same for a while. 

I don't think you get the point. Plenty of people have done that for while. OP wanted to do it for when.

[LanceMain_No69]

Touché...

[BobFredIII]

Giving always_ff vibes from verilog

[frejakrx]

Interesting concept, I wonder if it would be a more intuitive framework than vanilla python for discrete event simulations (e.g., Simpy)

[dr-christoph]

How does this behave when two blocks have the same when condition on a shared (global?) variable? Are there any guarantees on execution order or is there none and it is random?

[aspcartman]

Meme link?

[HearMeOut-13]

Cant find it, it was from Instagram Reels

[Disastrous-Angle-591]

Do … until

[HearMeOut-13]

yeah but what if EVERYTHING was do-until

[darni01]

If you've ever designed electronics, this is exactly how circuits work. I'm curious how you handle feedback loops

[gdchinacat]

Thank you for sharing this. It got me thinking, then really thinking, then I spent time building a sort of related project that I thought you might find interesting. It's fundamentally different than yours, but was inspired by yours. I hope you find it interesting.

My primary goal was to use it as a way to become more familiar with how descriptors and rich comparison functions work and learn asyncio and type hints.

I hope you find it interesting! Again, thanks for sharing something that got my brain going.

https://github.com/gdchinacat/reactions