I just got burned by the clojure.set/difference function (v 1.5.1) wherein
I had in error passed a vector for the second argument. This causes
inconsistent results depending on the relative lengths of the arguments.
That is, as soon as the second argument becomes longer than the first we
get a different branch in the code and the behaviour changes:
user=> (clojure.set/difference #{4 5} [4 5])
#{}
user=> (clojure.set/difference #{4 5} [4 5 6])
#{4 5}
user=> (clojure.set/difference #{:a :b} {:a 1 :b 2})
#{:a :b}
user=> (clojure.set/difference #{:a :b} {:a 1 :b 2 :c 3})
#{}
What I would have liked to happen in this case is for an exception to be
thrown, or for the second argument to be implicitly converted to a set. An
exception is thrown if the first element is not a set, so perhaps we should
also have that behaviour in the case of the second.
There is inconsistency for at least the union function in the face of non
set arguments as well:
user=> (clojure.set/union #{1} [1])
#{1}
user=> (clojure.set/union #{1} [1 2])
[1 2 1]
I can't think of any case where the current behaviour would be desirable,
the most relevant post I can find on the issue quoted Rich Hickey as saying
that the behaviour of the functions on non sets should not be relied
upon<https://groups.google.com/d/msg/clojure/lQIP6-qJqWQ/8dFmHeXjI_gJ>.
Another post<https://groups.google.com/d/msg/clojure/7QVFwtXWAi8/bhtkyXUWqxYJ>I
read suggested (weakly) that someone had run across this behaviour and
not found it as perverse as me.
Can anyone else think of a reason why we should not add type hints to the
functions, or why coercing the arguments to sets is better (or something
else I haven't thought of)?
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