Simeon Mattes wrote:
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
I would say I don't see why this particular datatype is useful. I mean with no regard to existentials. Having output as a field in the data constructor seems strange to me.