I quite liked the types section, particularly the bit about making illegal states unrepresentable so you can enforce business rules, eg. validating email addres

I quite liked the types section, particularly the bit about making illegal states unrepresentable so you can enforce business rules, eg. validating email addresses. One of the things I'm always thinking when people talk about the merits of various type systems and the problems they solve is: "But I don't have those problems". However, there are a few good examples in there that opened my eyes a little and I'll give type-centric programming a go on my next project. Not necessarily solving problems that I have, but certainly presenting a different, hopefully clearer, way of writing some functionality.