A Road to Lisp: Why Lisp

I get this on both FF and Chrome.

My first thought was that the article was redacted lol

metoo

> The former have been table stakes for interpreted languages

I used to think so too back in the day when I was getting into Clojure. It was much later when I realized that when Lisp people talk about the REPL they’re usually talking not so much about the interactive CLI where you can evaluate commands easily but more so the ability to connect your program to a live session where you can quickly evaluate forms within your text editor and in the context of your running application, which enables much more interactive development than in other interpreted languages.

Largely agreed.

I do think it is worth highlighting how many advanced parts of hot-reloading have already been covered in Common Lisp. Same with highlighting how the REPL is largely not used to directly type into, but is instead a very powerful interface for tools to interact with a running image.

But, again agreed that simply these existing are not that notable today.

But with Clojure and immutable by default, hot reload is a real thing, it sounds like not just on JVM but even among lisps.

What has Lisp ever done for us?

If it helps, https://taeric.github.io/CodeAsData.html was my attempt at exploring "code as data" and what makes lisp different here. My specific focus was more to point out that "eval" in lisp doesn't just take in a string. But I think the same general points remain.

If you haven't read it, I'd suggest taking a look at Paul Graham's book On Lisp [0]. He says better, and with more examples than I'd provide in a comment block, what I'd write on the subject. Jump to chapter 8 for his discussion on the topic, referring back to chapter 7 if you find the macro definitions difficult to read.

[0] https://www.paulgraham.com/onlisptext.html

Sure, macros are functions that take functions as input, and produce new functions as output. But they take the function's symbols as input and produce a new set of symbols. So a macro can extend the syntax of the language without having to modify the core language system. Anyway, what's unique about Lisp macros vs say, Rust macros, or C style preprocessors, is "homoiconicity". The data structure that a Lisp macro takes as an input, the lisp code, is the same data structure that the language uses normally (S-expressions, lists...), so writing a macro requires few new language skills compared to writing normal lisp (again, compare writing Rust macros, a dark art in comparison).

You could use them to:

1. Come up with an algorithm to define an algorithm.

2. Code expansion. Instead of typing out 1000 classes that are best represented as a template of a single class, you can define a macro then use it.

3. C++ at least uses them to provide generics.

4. They let you peel back the layers of abstraction to use the language itself as an API. Useful if you want to write static analysis to do analysis on code quality, security, linting, etc.

5. Anything you can imagine, it's metaprogramming.

There's a bit of a mental model flip to make maybe.

> they're code that's generated at compile time

They're code that generates code at compile time. Macros can actively walk the AST of the parameters they process and rewrite them completely into new shapes. That transformed AST is what then actually gets compiled.

one way to see it is that it's a function that runs at compile time. for instance instead of dumping magic numbers/tables in a codebase you could put the code and substitute to their value at compile time.

but also it can change your code, so you get to do all the java annotation magic stuff.