The smallest piece of great Pharo code https://medium.com/concerning-pharo/tiny-yet-so-beautiful-1ef5149c910e (explores adding methods to built-in classes and idiomatic Smalltalk design)
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Do commas ever appear in idiomatic Clojure? It's syntactically legal but I don't think I've ever seen them in the wild.
Lisp is the only language I've seen where it's idiomatic to have some locals scoped to the *middle* of a function. It's a consequence of let blocks.
It's neat, but I rarely miss it elsewhere. I'm happy with "scoped from here to the end of the block" in e.g. C or Rust.
Arrays being truthy in JS still catches me out sometimes.
In Python or Lisp, empty lists are falsey and truthiness is safe and idiomatic.