What you are describing is structural types. It is indeed a mystery that these are so under used, especially as they are a cornerstone of type theory. Structural types are so useful that they creep into most languages in some way. Even in Java, the Kingdom of the Nouns, where the rulers refused to merge a pair class, functions essentially take tuple arguments and these tuples don't have to be named and defined. You can't return a tuple though, so there is an unfortunate asymmetry. In Haskell and OCaml, we like to describe functions in a structural way, so com.google.android.Predicate would be just "a -> Bool". You wouldn't have to convert your com.google.guava.Predicate. But even these languages lack structural records and variants and suffer for it.