You can do similar existential types in Haskell: {-# LANGUAGE ExistentialQuantification #-} data Foo = forall a. Show a => Foo a [Foo 10, Foo "bla

You can do similar existential types in Haskell: {-# LANGUAGE ExistentialQuantification #-} data Foo = forall a. Show a => Foo a [Foo 10, Foo "blarg", Foo (Foo (Foo "str")) However, I think having the extra constructor there is very important. If you were willing to overlook extra syntax and the behavior of the type, then even a normal tagged union starts to look like sub-typing! In fact, in practice, that's exactly what you use where you would use sub-typing in a different language. There are some significant differences, but I'm sure you can see a parallel. There is also an intuitive parallel between existential types and normal tagged unions. The existential type is somewhat like a way to create a tagged union for an unbounded number of types. This naturally means it can't actually be tagged--the tag loses its meaning and you now can't recover the type of the contents--but in practical terms they're similar. As I said, thinking of it like sub-typing is a pretty good intuitive guide (although it might lead you astray sometimes). I was just being pedantic; there's something about formal semantics and type theory that just raises the pedant in me :P.