You might already know of this since you mentioned pattern matching, but there is the `sealed` class modifier which allows for exhaustive matching on class subt

You might already know of this since you mentioned pattern matching, but there is the `sealed` class modifier which allows for exhaustive matching on class subtypes. It's very limited: https://dart.dev/language/class-modifiers#sealed https://dart.dev/language/class-modifiers#sealed I recall there being a reason unions aren't generally implemented, though I'm not sure where I read it. It was to do with union types being unnecessarily complex for the compiler and the code they tend to produce, at no real benefit to the programmer or the resulting program. Go's lack of unions is essentially based around the same reasoning. It looks like it's an open discussion for Dart, but I can't see any explicit plans for an implementation.