I'm not in the Typed FP world, but I found the concept of pervasive use of Sum Types making invalid states (according to the project domain) of composite struct

I'm not in the Typed FP world, but I found the concept of pervasive use of Sum Types making invalid states (according to the project domain) of composite structures _unrepresentable_ to be a pretty compelling nugget to chew on. A lot of debugging is: This <object> has ended up in a weird state. Whereas the alternative could be that if the code compiles then it's impossible for that <thing> to ever be in a weird state.