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Practically anything in Haskell

  • Monads. Yes - the big scary word that makes increadibly easy parsers, IO, operations on Lists and other things so easy (once you notice common pattern)
  • Arrows. The same for advanced users ;)
  • Standard stuff like lambdas etc.
  • Currying functions
  • Algebraic data types
  • Pattern matching

And many more.

PS. Yes. I am Haskell fanboy if anyone asked.