In programming language theory there are two ways to represent programs: * data, which has intensional equality (you understand it by what it is) * codata, wh

In programming language theory there are two ways to represent programs: * data, which has intensional equality (you understand it by what it is) * codata, which has extensional equality (you understand it by what it does) For data think data in a normal programming language. (Algebraic data types, if that means anything.) For codata think functions (or objects). These are duals, meaning you can map on to the other. If you have a design using data you can transform it to codata and vice versa. This opens up a lot design opportunities by exploiting this duality. There is more in the book I'm writing if this is your kind of thing: https://scalawithcats.com/ https://scalawithcats.com/