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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.
@rossabaker believe it or not, I am kind of relieved, because when I saw your case-insensitive string example, part of me asked, what sort of function is this? Is it pure? Does it rely on some side effects or some state to produce its value? I didn't pursue the question sufficiently though. And when I briefly tried to come up with a function g that could complement the f function that turns "a1", "a2", etc into "a", when I noticed that I wasn't coming up with a candidate, I also asked myself if such a function would have to resort to some kind of 'trick' to behave in the desired way.
Well, thank you very much for letting me know.
The quest for compelling examples of how a Set is not a functor continues!