Maybe, well, the usual polymorphic dispatch?
Take the tipical "foreach" loop in ruby/python/java/c#
for var in list
dostuff
all rely (through coroutines, lambdas, side effects) on the list to know how to be iterable. If you change from a cons list to an array/file/socket/prime generator it will still work.
Of course, perfectly doable with, say, haskell type classes, but I don't know how to it in arc. Any hints?
