Set is not a functor

set_map.scala

scala> import org.typelevel.ci._
import org.typelevel.ci._

scala> val f: String => CIString = CIString.apply
val f: String => org.typelevel.ci.CIString = $Lambda$1025/2055291664@4bbf38b8

scala> val g: CIString => String = _.toString
val g: org.typelevel.ci.CIString => String = $Lambda$1027/1152113439@62108cd3

scala> val rosses = Set("Ross", "ROSS", "ross") // three too many if you ask me
val rosses: scala.collection.immutable.Set[String] = Set(Ross, ROSS, ross)

scala> rosses.map(f).map(g)
val res0: scala.collection.immutable.Set[String] = Set(Ross)

scala> rosses.map(f andThen g)
val res1: scala.collection.immutable.Set[String] = Set(Ross, ROSS, ross)

I don't know about "tricky". CIString satisfies the contract of .equals. There's no substitution law. cats.Eq doesn't have one either. Haskell's Eq recently added it as an "expectation", but officially has no laws. We should strongly prefer that property, but it's neither illegal nor unusual to break it. A couple more examples:

• (s: Set[A] => s.toList) (ordering is undefined)
• (d: Double) => 1.0 / d (try it with 0.0 and -0.0)

A set whose equivalence relation guarantees substitution is a functor, but Scala's Set doesn't, so it's not.

Noted. Thank you.