You want some way of specifying an interface - that doesn't necessarily imply classes. Also, class-based inheritance tends to be a poor fit for interfaces; not

You want some way of specifying an interface - that doesn't necessarily imply classes. Also, class-based inheritance tends to be a poor fit for interfaces; note for example that C#/Java have their own special-purpose interface mechanics, and even C++ which allows multiple inheritance tends to use implicit template-based interface implementation instead. Discriminated unions on the other hand are - the name says it - intended for cases where you want to discriminate between multiple possible values. They're by design not intended to hide differences behind an interface; they don't address the same problem at all. All in all, I think that neither classes nor discriminated unions solve the interface problem (nor were they ever really designed to do so) - that's a different problem.