"The Expression Problem is a new name for an old problem. The goal is to define a datatype by cases, where one can add new cases to the datatype and new functions over the datatype, without recompiling existing code, and while retaining static type safety (e.g., no casts)." - Philip Wadler Welcome to another series of C9 Lectures covering functional programming. For this series, Dr. Ralf Lämmel has generously taken the time to produce videos for Channel 9 from his office at the University of Koblenz-Landau (Germany), where he is a professor of computer science. The idea here is to take the next step from Erik Meijer’s fantastic introductory series on functional programming. Accordingly, Ralf's series will dive into more advanced areas of functional programming, again focusing on the Haskell language (the functional concepts here span beyond any one functional language, however).  To begin, Dr. Lämmel teaches us about the Expression Problem. Now put on your thinking caps, make yourself comfortable, and enjoy this installment of functional programming lectures on Channel 9. Huge thanks to Dr. Lämmel, both for doing this series for Channel 9 and for filming and producing it all by himself! Finally, thanks to Erik Meijer for suggesting this series and putting me in touch with Ralf. See Dr. Lämmel's blog post about the new lecture series here:  [Homework assignment is on slide 26 - Get the slides]