An introduction to algebraic data types, which are used in most (all?) statically-typed functional programming languages, such as Haskell, O'Caml, F#. (For C++ people, this is actually the same as boost::variant.) It's a good introduction if you know more classical programming languages like C, since it describes how it maps to structs & unions, but it also describes some of the theory behind it (i.e. why it's called an _algebraic_ data type). - andrep