Smalltalk isn't about editing code, but executing a very long lived mutable program!
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Excellent deep dive on parsing, executing and optimising in an interpreter written in Rust:
A cute touch in Emacs 25.1: executing the bytecode of a macro teaches Emacs to highlight it!
Awesome Emacs package of the day: eval-in-repl: https://github.com/kaz-yos/eval-in-repl -- it's a great way of interactively executing code to play with it.