This came up on Slack. A naïve implementation of "lispify" to turn vectors into lists used this code:

But when called like this:

It produces this error: java.lang.ClassCastException: clojure.lang.Keyword cannot be cast to java.util.Map$Entry

My initial reaction was to change the condition to (and (vector? e) (not (map-entry? e))) but that still failed, because while walking the hash map, the MapEntry [:a "b"] was turned into a PersistentVector.

At this point, we can switch to using prewalk and it works as expected:

Now we get the expected result:

This seems unintuitive at best and feels like a bug: postwalk should preserve the MapEntry type rather than converting it to a PersistentVector.

Cause: The problem seems to be this line https://github.com/clojure/clojure/blob/master/src/clj/clojure/walk.clj#L45:

Proposed: Change to:

This would preserve the type of the subelement.

Patch: clj-2031-w-test-v2.diff
Screened by: Alex Miller