clojure.walk/postwalk fails when given a metaconstant

[rafaeldff]

This works:

 (fact (map identity [...a... ...b...]) => [...a... ...b...])

While this fails:

user=> (require '[clojure.walk :as w])
nil

user=> (fact (w/postwalk identity [...a... ...b...]) => [...a... ...b...])

FAIL at (NO_SOURCE_FILE:1)
    Expected: [...a... ...b...]
      Actual: java.lang.ClassCastException: java.lang.Boolean cannot be cast to clojure.lang.IPersistentCollection
false

[marick]

This is a side effect of https://github.com/marick/Midje/wiki/Data-Prerequisites, a feature I've decided is not useful.

Because of that feature, metaconstants look like collections to Clojure. In its processing, postwalk tries to create an empty metaconstant. That (for some reason that I haven't looked into the Clojure source to find) throws the error. You get the same error from this:

(fact (prn (empty ..a..)))

… which throws the same exception.

I'm not going to fix this, because there are probably innumerable cases where metaconstants don't look quite like collections. At some point, I'll take out this (mis)feature, but not until I'm prepared to make breaking changes. (A 2.0 version)