Covariance and contravariance in Scala

This post explains concepts of covariance and contravariance. It focuses on real world metaphors and on code examples that show how those concepts can be used in Scala. Metaphors are borrowed from Erik Meijer's talk Contravariance is the Dual of Covariance Implies Iterable is the Dual of Observable (Making Money Using Math) [Meijer2014].

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