Neat examples of GOOPS, the OO system in Guile (similar to CLOS): https://www.wedesoft.de/oop-with-goops.html (goes from rationale to demos and metaclasses!)
miniblog.
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I'm a huge fan of Swift's 'Error Handling Rationale' design document: https://github.com/swiftlang/swift/blob/9315673c003875158852579bd1f33480cdec5461/docs/ErrorHandlingRationale.md#fundamentals
It carefully defines terminology and compares with other languages, so you can understand Swift's position and preference in the design space.
Bevy is a game engine written in Rust with a superb project introduction: https://bevyengine.org/news/introducing-bevy/
Videos! Clear rationale! A ton of examples with screenshots of the results!
I still find it amazing that Google provides _free_, _unlimited_ image storage at a reasonable resolution.
It's a really nice service, but I don't understand the economic rationale.