Several tickets have been created suggesting that several functions in the clojure.set namespace might throw an exception if given non-sets as arguments, because in some cases they return unexpected values. This list is a sample, not intended to be complete: CLJ-810, CLJ-1682, CLJ-1953, CLJ-1954
Now that clojure.spec exists, documenting the expected argument types of these functions can be done more precisely. The specs could be used to dynamically detect incorrect argument types passed to these functions during testing, and via any other ways built on top of clojure.spec in the future.
Alex Miller suggested in a Slack discussion that perhaps it would be helpful if contributors would help implement such specs for functions within Clojure.
The approach taken with the proposed patch is simply to spec that the :args of clojure.set/subset?, superset?, union, intersection, and difference must all be sets, and spec the :ret types of the first two as boolean, and the last three as set. This seems to match all known comments given when declining previous tickets proposing changes to the behavior of these functions.
An alternate approach not taken with the patch above would be to also allow nil as arguments and return values, but it is not clear whether that is part of the intended contract of these functions, or an incidental aspect of their current implementations.
I (Andy Fingerhut) looked through all of Clojure and contrib projects for occurrences of clojure.set/union, difference, intersection, superset?, and subset?, and there is only one such call that I can quickly determine does not pass sets to them. It is the one in clojure.data/diff for which there is a patch that can correct this (assuming it is considered a bug) on this ticket: CLJ-1087
Many such calls I could quickly determine always passed sets (e.g. because the arguments were wrapped in (set ...) calls). On a small fraction of them, I gave up trying to figure it out after a minute, since determining the answer fully from code inspection would require going back up the call tree a ways.
Please suggest any other ways that these changes could be evaluated in order to increase your confidence that they are correct.