clojure.lang.IPersistentMap
  (assocEx
    [trie opath ovalue]
    (if (get trie opath nil)
      (throw (Exception. (format "Value for key %s already exists." opath)))
      (assoc trie opath ovalue)))

1. Caused by java.lang.ClassCastException
   class clojure.lang.PersistentVector cannot be cast to class
   java.util.Map$Entry (clojure.lang.PersistentVector is in unnamed module of
   loader 'app'; java.util.Map$Entry is in module java.base of loader
   'bootstrap')

                  core.clj: 1567  clojure.core/key
            core_print.clj:  233  clojure.core/print-prefix-map/fn
            core_print.clj:   66  clojure.core/print-sequential
            core_print.clj:  229  clojure.core/print-prefix-map
            core_print.clj:  238  clojure.core/print-map

What about that stacktrace makes you think the error is coming from that line?

Looks like a bogus .toString method, maybe in the call to format when building the exception, but that is not in the stacktrace

When I comment those lines out, I don't get the error.

Oh, I should have included the line I was evaluating to cause the error. (str (trie/->TrieAgain '() 1 {})) doesn't raise. (trie/->TrieAgain '() 1 {}) does raise.

it looks like you are printing a map where keys doesn't return a seq of map entries

ser=> (keys (reify clojure.lang.IPersistentMap (seq [_] (list [:a :b]))))
Error printing return value (ClassCastException) at clojure.lang.APersistentMap$KeySeq/first (APersistentMap.java:168).
class clojure.lang.PersistentVector cannot be cast to class java.util.Map$Entry (clojure.lang.PersistentVector is in unnamed module of loader 'app'; java.util.Map$Entry is in module java.base of loader 'bootstrap')
(user=> *e
#error {
 :cause "class clojure.lang.PersistentVector cannot be cast to class java.util.Map$Entry (clojure.lang.PersistentVector is in unnamed module of loader 'app'; java.util.Map$Entry is in module java.base of loader 'bootstrap')"
 :via
 [{:type clojure.lang.ExceptionInfo
   :message nil
   :data #:clojure.error{:phase :print-eval-result}
   :at [clojure.main$repl$read_eval_print__9112 invoke "main.clj" 442]}
  {:type java.lang.ClassCastException
   :message "class clojure.lang.PersistentVector cannot be cast to class java.util.Map$Entry (clojure.lang.PersistentVector is in unnamed module of loader 'app'; java.util.Map$Entry is in module java.base of loader 'bootstrap')"
   :at [clojure.lang.APersistentMap$KeySeq first "APersistentMap.java" 168]}]
 :trace
 [[clojure.lang.APersistentMap$KeySeq first "APersistentMap.java" 168]
  [clojure.lang.RT first "RT.java" 692]
  [clojure.core$first__5402 invokeStatic "core.clj" 55]
  [clojure.core$print_sequential invokeStatic "core_print.clj" 64]
  [clojure.core$fn__7331 invokeStatic "core_print.clj" 174]

Ahhh. So before I implemented the IPersistentMap methods... I was just getting the default random deftype representation... , like "com.owoga.trie.TrieAgain@6e7c56eb". But my REPL is now trying to print something that looks like a map, because it sees it's an instance of that. Gotcha. Thanks!

maybe, not entirely sure

a tricky thing with keys is it returns a lazy seq, so exceptions may not be thrown until it is forced

in my example above the printing of the sequence returned by keys is forcing it and then throwing

(you can see the open paren dangling where it started to print the seq before it threw)

So my guess is something isn't right with the seq method, which is getting called when printing the result (not entirely sure what path through the printer is being triggered, that may be repl dependant)

Any hints on how to implement this in Clojure?

for i in range(800, 4000, 800):
    for j in range(1, 65, 16):
        print(i+j)