I'm implementing basic I/O in Trifle lisp today! It's remarkably hard to have code test coverage for simple things like opening a file.
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It is remarkably hard to escape command line arguments safely on Windows, and the standard libraries of multiple languages have needed patching: https://flatt.tech/research/posts/batbadbut-you-cant-securely-execute-commands-on-windows/
Blogging about my programming language project: choosing the basic syntax!
https://www.garden-lang.org/blog/syntax.html
I've been learning some Common Lisp by writing some simple JSON munging programs. I'm sure it's not best practice — I should probably deserialise to CLOS automatically — but it's a nice way to get comfortable with the basics.
It feels weird deliberately ignoring helpers though.