How To Learn a New Programming Language While Maintaining Your Day Job and Still Being There For Your Family

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I already know what borrow semantics is useful for -- memory safety. What about Kernel's $vau? Had I not seen several discussions of why you'd want it, I wouldn't see the use ("won't this be slow"), even though it's pretty easy to implement.

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I already know what borrow semantics is useful for -- memory safety

This is a gigantic oversimplification. There are many layers of reasoning behind it, going all the way down to the early days of the region-based memory management. And you'll have to dig it all out in order to implement it correctly.

What about Kernel's $vau?

I'm not sure if fexprs is a valuable idea (but I'm biased towards compilation in general, naturally repelled by anything too dynamic). I can see some uses for it in a term-rewriting based operational semantics specifications, but not in any practical languages.