What did category theory ever do for us (functional programmers)?

What did category theory ever do for us (functional programmers)? - An extreme pragmatic and un-academic approach. Examples are in Scala.

Talk given at Scale by the Bay 2019.

Slides: https://github.com/winitzki/sofp/blob/master/sofp-src/what-did-category-theory-do-for-us.pdf

Summary:

- Main features that functional programmers use to write better code; no category theory required
- Examples of typeclasses with laws, and why laws are important in practice
- Formulating typeclasses and laws via a generalized "lifting" type signature with "twisted" function types
- Definition of Category and Functor as generalizations that cover the laws of functors, contrafunctors, filterable functors, monads, applicative functors, comonads, etc.
- Examples of categories that are used to describe functors, monads, applicatives, and filterable functors
- Filterable functors in more detail, with backdrop of category theory
- "Type constructor libraries": free functor / free monad/ etc., and Church encoding as examples
- What the new book ("The Science of Functional Programming") will do
- Conclusion 1: programmers need to learn functional programming and not category theory
- Conclusion 2: basic definitions of category theory (category, functor) are useful as condensed formulations of general laws
- Conclusion 3: category theory will not help us derive or prove laws for specific typeclasses, because category theory only talks about laws that apply generally to a large number of very different typeclasses
- Q & A session. I was corrected by Oscar Boykin who described practical uses of the function "liftOpt" (which I claimed to be useless in practice).