[fredrikholm]

They make it near impossible to extend from the outside.

I can pass a Reader interface to your function, but I cannot (easily) add a

> | UnusualProtocol(Socket)

as a third party consumer.

Other than that, sum types are the better abstraction. With exhaustive first class pattern matching (eg. with proper matching on destructuring), nothing comes close in terms of ergonomics.

[hermanradtke]

That is a feature. Compose instead of extending.

[Yoric]

OCaml has extensible sum types. They work very nicely for some ranges of problems.