A lovely example of extensible type systems evolving independently from the underlying language! https://twitter.com/soprano/status/898664668079693824
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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
Playing with optional type signatures in Python, I realise that the return type is the most important to me.
I'd much rather have a function with only a return type instead of a function with only parameter types. It's often quick to add too.
On thinking of books like software, treating them as evolving artifacts, and URLs versus paper book publishers: https://cs.brown.edu/~sk/Memos/Books-as-Software/