In a really wonderful paper of some few years back, Swierstra introduced the idea of modular datatypes using ideas from universal algebra.  Modular datatypes allow one to assemble a bigger datatype from component datatypes, and combine functions written on the component datatypes in a modular way.   In this episode I introduce the paper and the problem (dubbed the expression problem by Phil Wadler) it is trying to solve.  Modular datatypes are a different form of modularity that I would like to consider in the context of the discussion of module systems we have been engaged in now for a while in Chapter 13 of the podcast.