Data.IntMap could be cleaned up some if single-field, single strict constructor GADTs/existentials could be unpacked even when wrapping a sum type. We could then have data Status = E | NE data IntMap' (s :: Status) a where Bin :: ... -> ... -> !(IntMap' NE a) -> !(IntMap' NE a) -> IntMap' NE a Tip :: ... -> a -> IntMap' NE a Nil :: IntMap' E a data IntMap a = forall s . IM {-# UNPACK #-} !(IntMap' s a) The representation would be the same as that of a newtype, but the pattern matching semantics would be strict. In the GADT case, this would essentially allow any fixed concrete datatype to serve directly as a witness for an arbitrary set of type equalities demanded on construction. Is there any hope something like this could happen?