I am also really annoyed by how Set and Map's map methods work in Scala (and have argued about it here: https://contributors.scala-lang.org/t/set-map-deduping-a

I am also really annoyed by how Set and Map's map methods work in Scala (and have argued about it here: https://contributors.scala-lang.org/t/set-map-deduping-and-possibly-unsafe-behavior/3436 https://contributors.scala-lang.org/t/set-map-deduping-and-p...), I actually don't think that for all the shortcomings of the Set map method, it actually violates any of the Functor laws: 1) For all sets s, s.map(identity) == s ==> true 2) For all sets s, functions f and g, s.map(x => f(g(x)) == s.map(g).map(f) ==> true On the other hand, Map's map method is much more horrible, and does violate the 2nd functor law where f == g == _.swap : Map(1 -> 2, 2 -> 2).map(_.swap).map(_.swap) ==> Map(2 -> 2) Map(1 -> 2, 2 -> 2).map(_.swap.swap) ==> Map(1 -> 2, 2 -> 2) Further, it behaves differently depending on if it is known to be a Map at compile-time, or if it is only known to be an Iterable[(K, V)], due to overloading: Map(1 -> 2, 2 -> 2).map(_.swap) ==> Map(2 -> 2) (Map(1 -> 2, 2 -> 2): Iterable[(Int, Int)]).map(_.swap) ==> List(2 -> 1, 2 -> 2)