I have found that ghc has adopted the existential data constructors. Because it was first time I have heard this term I have found some articles about it. The less complex for me was the following explanation data Worker x y = Worker {buffer :: b, input :: x, output :: y} This is wrong in Haskell98 because we can 't just hide buffer :: b. We should write instead data Worker b x y = Worker {buffer :: b, input :: x, output :: y} Let suppose that b has an indefinite type. Then with existential types we could write data Worker x y = forall b. Buffer b => Worker {buffer :: b, input :: x, output :: y} The example above is similar to the example that Paul Hudak, John Hughes, Simon Peyton Jones and Philip Wadler mention in an article "a history of Haskell". Though I can't figure out how exactly could utilize such a type. To be honest haskell is my first functional language and I 'm not so familiar with. So, an example would be very helpful