Nim's term-rewriting macros are wild: https://nim-lang.org/docs/manual.html#term-rewriting-macros (you can leverage Nim's side-effect analysis or enforce canonicalisation!)
miniblog.
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Neither Spotify nor Skype use peer to peer functionality any more.
Are there any consumer apps that still leverage P2P designs? Has bandwidth just become so cheap that it's unnecessary?
I've seen markdown projects leverage link syntax for custom extensions, e.g. [click me](button://).
Markdown also supports embedded HTML. Perhaps you could add custom tags for extensions?
I really like the one-module-per-file model of JavaScript or Python.
If you're storing code in files, you might as well leverage file boundaries. If modules are a separate abstraction (e.g Rust, OCaml), it's harder to learn and choose how to organise code.