I gave an example above of categories being useful for a very mundane purpose: https://news.ycombinator.com/item?id=10529150 https://news.ycombinator.com/item?i

I gave an example above of categories being useful for a very mundane purpose: https://news.ycombinator.com/item?id=10529150 https://news.ycombinator.com/item?id=10529150 Most categories that arise in practice aren't “just” categories - they have interesting extra structure. For instance, platonic Hask, the category of Haskell types and total functions, is bi-Cartesian-closed. The categories that arise when modeling databases are typically not closed (from a programmer's POV, this means that functions aren't first-class objects - you can't store them in tables), but they have finite coproducts (UNION ALL) and products (SELECT ... FROM multiple, tables). SQL-style aggregate functions are nothing but commutative monoid objects in these categories. etc.