But it actually does not do so. The IO data type has a single value constructor: IO. Other monadic types do use ADTs - for example Either has both a Right and Left constructor - but ADTs are used extensively outside of any monadic type as well because the basic idea of composite types is actually very useful. So I cannot imagine how it is a bad thing that Scala has this useful feature, just because it does not also require an IO Monad? Does that mean ML is mistaken in its use of ADTs as well, even though it precedes Haskell?