For "Week 5" I did TCL, and it kind of set my brain on fire with "wait… you can just DO that?".

In TCL, strings are anonymous functions and vice versa. In my old Emily language, "if" and "while" blocks were implemented by passing in lambdas; but in TCL you do the same thing by *passing in a string containing {the code to execute}*. I started trying to imagine what "hygienic" TCL would be; if TCL is a C macro, I want an ML or Rust macro. I tried to imagine something closer to LISP than TCL.

This is what I came up with; I think of it as "0-lisp". In LISP a "2-LISP" is a LISP where functions and variables live in different namespaces, and in a "1-LISP" they live in one namespace. Projecting backward, in my 0-LISP, the distinction itself disappears totally; functions are lists and *any list can be executed*. I wanted to know what this changed. Here's what I found:

It helps very little, and makes certain important things a huge pain. Oops! Still, it was very educational to learn this.

Anyway, now I have a LISP interpreter written in Rust.

https://github.com/mcclure/lisp0-experiment/tree/2025-04-07

It's a very minimal "MVP"; the language currently lacks:

- Variable scopes
- Loops
- Conditionals
- Floating point numbers
- Errors lack backtraces or line numbers (and cannot be recovered from)

It has

- One global scope
- Ints, strings, arrays and hashtables
- Tail recursion
- File I/O

But math and recursion means it's Turing complete. Which means it's sufficient to use it for a AOC problem!

Now, since *I'm* the one writing this LISP, I do get to make some changes. The main thing I wanted to change, as it's the #1 thing that infuriates me writing LISP, is parentheses. This "LISP" has a syntax where each line (as separated by newlines or commas) implicitly has () around it. Meanwhile, {} and [] are shorthand, for

{x,y,z} => '((x)(y)(z))

[x,y,z] => (make-array [(x),(y),(z)])

In other words {} is functions (quoted lists) and [] is array literals.

Is this still a LISP? I don't care!

Before I can start, I need to make some utilities. By "utilities" I mean "if" and "while" statements, which again, this language currently lacks. MVP, remember. Implementing these in userland is possible! It's *ugly*, though. For `if`, I simulate branching by putting the two possible outcomes (functions) into an array, casting the condition to a bool and then the bool to an int, and using that as an index to the array.

"while" is even more nightmarish. I don't… understand the use of make-quote.

Like, I wrote the code, but I don't understand it. I initially wrote it with all the `make-quote`s being `return`s, and that didn't work, so I experimentally replaced them with make-quotes and it did work. I need to reread the interpreter code to really understand why that was required.

If even the designer can't understand the model here, the model is probably too complicated for anyone ELSE to write self-modifying code either! That's a sign of a problem, or better abstractions needed.

Anyway, I like testing things before I use them, so after implementing my "if" and "while" I decide to write a simple Fizzbuzz program. This causes me to immediately realize—

I FORGOT TO IMPLEMENT > AND <

I just forgot!! Fortunately I *did* include bit arithmetic ops, so I implement <, >, <=, >= in userland as well, by testing bit (1<<63) as a proxy for 2's compliment negative. This is actually a little worrisome. I'm not sure if this code truly "works" or if I'm just leveraging UB in Rust.