Scalaz used to have a scalaz.Functor for scala.collection.Set but it was eventually removed because it relied on Any’s == method. You can read more about why Functor[Set] is a bad idea at Fake Theorems for Free. If Set had been truly parametric, we wouldn’t have been able to define a Functor in the first place. Luckily, a truly parametric Set has recently been added to Scalaz as scalaz.ISet, with preliminary benchmarks also showing some nice performance improvements. I highly recommend us...
