Bug with MapEntry ?

[DjebbZ]

Hello, long time no issue :)

I think I found a bug yesterday while using specter. When navigating into map entries, I noticed that you can call Clojure's key on it while in a select, but not when in a transform/multi-transform/set-val. My understanding (and the error I got) is that a MapEntry is kept as is in a select, but is transformed into a PersistentVector while in a transform.

Below a complete reproduction of the bug, and also a show case of what I was trying to do (navigating to map entries where the key isn't some specific key):

> clj -Sdeps "{:deps {org.clojure/clojure {:mvn/version \"1.9.0\"} com.rpl/specter {:mvn/version \"1.1.1\"}}}"
Clojure 1.9.0
user=> (require '[com.rpl.specter :as sp])
nil
user=> (def data {:a 1 :b 2 :c 3})
#'user/data
user=> (sp/select [sp/ALL #(not= (key %) :a)] data) ;; works as expected
[[:b 2] [:c 3]]
user=> (sp/setval [sp/ALL #(not= (key %) :a)] sp/NONE data) ;; `key` fails
ClassCastException clojure.lang.PersistentVector cannot be cast to java.util.Map$Entry  clojure.core/key (core.clj:1559)
user=> (sp/setval [sp/ALL #(not= (first %) :a)] sp/NONE data) ;; `first` OK
{:a 1}
user=> (sp/transform [sp/ALL #(not= (key %) :a)] sp/NONE data) `key` fails
ClassCastException clojure.lang.PersistentVector cannot be cast to java.util.Map$Entry  clojure.core/key (core.clj:1559)
user=> (sp/transform [sp/ALL #(not= (first %) :a)] sp/NONE data) `first` OK
{:a 1}
user=> (sp/multi-transform [sp/ALL #(not= (key %) :a) (sp/terminal sp/NONE)] data) `key` fails
ClassCastException clojure.lang.PersistentVector cannot be cast to java.util.Map$Entry  clojure.core/key (core.clj:1559)
user=> (sp/multi-transform [sp/ALL #(not= (first %) :a) (sp/terminal sp/NONE)] data) `first` fails
{:a 1}
user=> (sp/multi-transform [sp/ALL #(not= (first %) :a) (sp/terminal-val sp/NONE)] data) trying `terminal-val` instead of `terminal` out of curiosity
{:a 1}
user=> (sp/multi-transform [sp/ALL #(not= (key %) :a) (sp/terminal-val sp/NONE)] data) `key` fails
ClassCastException   [trace missing]
user=> 

Bug? To be clear, for me the bug is that MapEntry are transformed to Vector whereas I expected them to be MapEntry.

End of report!

[jeff303]

This is happening because small maps (PersistentArrayMap) store the keys and values in a single array, and the all-transform is operating on entries as two-item vectors (here).

I suspect there may be some way to reassign key to first when evaluating this anonymous function (only when the structure is a PersistentArrayMap, i.e. using the protocol mechanism).

For now, though, it seems that using MAP-KEYS with a predicate does what you want.

(sp/setval [sp/MAP-KEYS (sp/pred #(not(= :a %)))] sp/NONE data)
{:a 1}