Problem with clojure.walk and metaconstants

[ayrtonsin]

see following case

(defn simple-walk "does nothing" 
  [m]  
  (clojure.walk/postwalk 
    (fn [x] x) 
    m))

(facts "about `simple-walk`"
  (fact "with simple map,  returns the map"
        (simple-walk {:this "works"}) => {:this "works"})
  (fact "simple map with metaconstant throws java.lang.ClassCastException"
      (simple-walk {:a ..something..}) => (throws java.lang.ClassCastException)))

the exception: java.lang.ClassCastException: java.lang.Boolean cannot be cast to clojure.lang.IPersistentCollection

[philomates]

Hey @ayrtonsin, thanks for reporting this.
It looks like a duplicate of #267, which we closed on accident (it hasn't been resolved yet).

I described the underlying issue here:

the postwalk example still doesn't run because the postwalk will do a (into (empty ..a..) (map identity ..a..)) which will reduce to (conj {} ..a..) and subsequently {} due to ambiguous semantics of merging a partially-defined map (metaconstant) with a concrete map

I'm not really sure how to fix this yet, so it might be a while before it gets resolved, but I think it is worth thinking about more.