Lennart Augustsson wrote:
Yes, early ML had nested pairs. We introduced n-tuples in Lazy ML because in a lazy language n-tuples are not isomorphic to nested pairs (whereas they are in a strict language). So n-tuples are nasty because they are not inductive, but they are in many ways much more reasonable than lazy nested pairs.
although it is possible in a slightly different (and less left-right symmetric) way in Haskell: data P a b = P a !b pair: P Int (P Bool ()) triple: P Char (P Int (P Bool ()) single: P Bool () unit: ()
BTW, early ML also had binary sums. Again, they are not isomorphic to n-ary sums.
data E a b = L a | R !b data Z --since Haskell Prelude provides us with the unit type () but not the uninhabited "zero" type, I'll use GHC's EmptyDataDecls syntax for this. either: E Int (E Bool Z) 3: E Char (E Int (E Bool Z) 1: E Bool Z 0: Z Still, these don't have isomorphic *representations*: the efficiency is lacking. I wonder if it would be possible to allow {-#UNPACK#-} for those polymorphic arguments in such a way that either/both of the above were actually equivalent to n-tuple/sums. -Isaac