Read your current/new language’s standard library. Highly recommended for developers of all experience levels. You’ll pick up the idioms, you’ll discover something useful. You’ll laugh, you’ll cry, you’ll wonder if they’re making a sequel!
My favorites are
prelude.hs (which, sadly, does not seem to be the file that Haskell bootstraps itself from any more 🤔) and Rubinius’ implementation of
Enumerable in Ruby.
(The reference is from The Colbert Report, which is a fun-but-dated thing to know.)