If you've ever fancied implementing a type checker, there's a fun example in readable JS here: https://languagengine.co/blog/so-you-want-to-write-a-type-checker/
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I'm having fun writing a simple type checker, but I'm learning firsthand why syntax-directed checking doesn't work. It prevents inference.
My checker catches real bugs, but it can't handle cases like this:
[1, 2].map(fun(x) { x + 1; })
I think I need bidirectional checking.
Really cute approach to reporting type errors: when there's a type error, show an example of a runtime error that the type check has prevented!
Data-Driven Techniques for Type Error Diagnosis https://escholarship.org/uc/item/59s4h4pv
Deno has the cutest PL mascot I think I've ever seen. Are there any other contenders?