[Haskell-cafe] dependent types

in modeling real application we have often the case that the type of some object depends on a value. e.g. small_boat is a vessel with size less than a constant. big_boat is a vessel with a larger size. for example, many legal classifications are expressed in this form (e.g. car vs. truck) depending on the type, operations are applicable or not (e.g. road with restriction 'no trucks'). how could one model such situations in haskell? i assume there are several possibilities...