Return type polymorphism in Haskell
Once you&#x27;ve got return type polymorphism, you really start to miss it in other languages. The simplest example possible is &quot;mempty&quot;<p>mempty :: a<p>gets the &quot;default&quot; value of a. Which makes no sense in a language where you need an instance to have polymorphism.<p>(This, incidentally, is also why all OO serialization libraries are awful.)