clj-kondo flags this: (set/difference #{:a :b} (keys {:a 1})) but it actually does work. That seq from keys can be used like a set

@kevin842 this is undeterministic behavior.

(instance? java.util.Set (keys {:a 1}))
false

user=> (set/difference #{:a :b} (keys {:a 1 :b 2 :c 3}))
Execution error (IllegalArgumentException) at user/eval7 (REPL:1).
contains? not supported on type: clojure.lang.APersistentMap$KeySeq

oh, interesting! I wonder why that is the case.. isn't it just a view over the actual collection, which it knows to support o(1) access?

I must be thinking of python, where dict.keys() does return a set-like view:

>>> "b" in {"a": 1, "b": 2}.keys() True

@kevin842 this happens because set/difference checks which input is larger and uses that to make things faster

same for union, etc

so never use those fns with something else than sets