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Tony Hoare's InfoQ talk in which he describes introducing null references in ALGOL W in 1965 and calls it his billion-dollar mistake. He discusses language designer responsibility and run-time checks.

Rich Hickey's Clojure/Conj 2018 keynote argues that Maybe/Option types and rigid type signatures are a poor way to model data that changes over time. He builds the case around optionality, schema growth and compatibility, and presents the ideas behind Clojure spec as an alternative.

Sandi Metz argues that hidden assumptions in code are missing concepts, and shows how to name and expose them so code is easier to understand and change. It covers null objects and messages in Ruby.

Stuart Marks explains the intended purpose and proper use of Java 8's Optional class and why it caused so much controversy. He gives guidelines for using its small API correctly.