Programming Bottom-Up
Last modified on August 08, 2022

http://www.paulgraham.com/progbot.html

There are two ways of programming in Lisp – top-down, and bottom-up.

Top-down = the classic paradigm of divide-and conquer, decomposing the problem hierarchically into smaller problems.

Bottom-up = defining your own DSL in Lisp, and using those primitives to make something cool.

Most programming is done top-down, but with the powers of macros you can go bottom-up, creating “little universes” in Lisp…

Personally what comes to mind is the rx macro in emacs. It’s like a nice little domain-specific language for writing regular expressions, embedded right into elisp.

Regexp matching a block comment in C:

(rx "/*"                          ; Initial /*
    (zero-or-more
     (or (not (any "*"))          ;  Either non-*,
         (seq "*"                 ;  or * followed by
              (not (any "/")))))  ;  non-/
    (one-or-more "*")             ; At least one star,
    "/")                          ; and the final /

/\*\(?:[^*]\|\*[^/]\)*\*+/

Personally, I find this to be a much more beautiful (and still somewhat succinct) way of writing regexes than the normal regex syntax.

you redefine the language itself to have the primitives you want it to have, and then you can proceed to build something (perhaps in top-down fashion now.) It’s like “meeting in the middle,” so you don’t have to build up super complex primitives, and you don’t have to decompose down to a super fine-grained level.

I wonder what the connection between this bottom-up paradigm and Bret Victor - The Future of Programming is. I feel like maybe there’s an idea that his visual programming playgrounds are like a domain-specific language (albeit not in Lisp.) They give the user a powerful set of primitives to manipulate digital objects, and the user can then build from there to make it come to life.

Also feel like emacs encourages bottom-up programming in some ways, well ofc because of Lisp, but also because of the interactivity of the environment. You write a snippet, and immediately you can evaluate that code and test out your snippet. You get a couple more snippets, try those out, and then it’s like, ok cool, what else can I build with these snippets?