spy.clj

(def spy-plugin
  {::pc/wrap-resolve (fn [resolve]
                       (fn [env input]
                         (let [resolver-sym (-> env ::pc/resolver-data ::pc/sym)
                               _ (prn {:sym    resolver-sym
                                       :input  input
                                       :entity (p/entity env)})
                               output (resolve env input)]
                           (prn {:sym    resolver-sym
                                 :output output
                                 :entity (p/entity env)})
                           output)))})
(let [parser (p/parser {::p/plugins [(pc/connect-plugin)
                                     spy-plugin]})
      indexes (pc/register {}
                           [(pc/constantly-resolver :a 42)
                            (pc/single-attr-resolver :a :c inc)])
      env {::p/reader               [p/map-reader
                                     pc/reader2
                                     pc/open-ident-reader
                                     p/env-placeholder-reader]
           ::pc/indexes             indexes
           ::p/placeholder-prefixes #{">"}}]
  (parser env [:c]))