Mousebender

Map operation on sets isn’t closed, both in Python:

>>> type( map(lambda x: x**2, set([1,2,3,4])) )
<type 'list'>


and Ruby:
>> Set.new([1,2,3,4]).map { |x| x**2 }.class
=> Array


At least Haskell does the right thing:
Main> typeOf( Set.map (\x -> x^2) (Set.fromList([1,2,3,4::Integer])) )
Set Integer


An obvious workaround is to operate on lists and convert to sets once you need them. You have to ask yourself whether it will cause performance problems in your application.