Maybe share the code you've tried + your project.clj in a thread?

I haven't used Leiningen for years, but I'm happy to try and help with this @U01EB0V3H39

FWIW, you'll probably need this in your project.clj:

:prep-tasks ["compile"]
  :aot [my.CustomException]

why do you want a custom exception class?

Then you can do:

$ lein repl
Compiling my.CustomException
...
max.core=> (import '(my CustomException))
my.CustomException
max.core=> (CustomException. "Ex" (ex-info "t" {}) {:a 1})
#error {
 :cause "t"
 :data {}
 :via
 [{:type my.CustomException
   :message "Ex"
   :data {:a 1}
   :at [max.core$eval1619 invokeStatic "form-init4063394007754078810.clj" 1]}
  {:type clojure.lang.ExceptionInfo
   :message "t"
   :data {}
   :at [max.core$eval1619 invokeStatic "form-init4063394007754078810.clj" 1]}]
...

@U050ECB92 he said it was "for a weird experiment thing."

(he posted a whole bunch of caveats he wanted to head off 🙂 )

Thanks Sean! Here’s the file I’m trying to gen-class: scratch/CustomException.clj

(ns scratch.CustomException
  (:gen-class
   :extends java.lang.Error
   :constructors {[String Throwable clojure.lang.IPersistentMap] [String Throwable]} ; mapping of my-constructor -> superclass constuctor
   :init init
   :state state ; name for the var that holds your internal state
   :main false
   :prefix "my-ex-"))

(defn my-ex-init [msg t context]
  ;; first element of vector contains parameters for the superclass constructor
  ;; Second element will be your internal state 
  [[msg t] context])

(defn my-ex-getData [this]
  (.state this)) ; accessing the internal state

(defproject scratch "0.1.0-SNAPSHOT"
  :dependencies [[org.clojure/clojure "1.10.3"]]
  :main ^:skip-aot scratch.core
  :target-path "target/%s")

(ns scratch.core
  (:import scratch CustomException))

Looking into this a little more, it seems like in the REPL I might need to compile the class? I ran (compile 'scratch.CustomException) but I still get the class not found exception

I’ll try the leiningen think you suggested as well and see if that works. it’d still be nice to have a way to iterate on it in the repl

Ok, trying the project.clj changes you suggested, I get this error (abbreviated) when starting the repl:

[{:type clojure.lang.Compiler$CompilerException
   :message Syntax error compiling at (scratch/core.clj:1:1).
   :data #:clojure.error{:phase :compile-syntax-check, :line 1, :column 1, :source scratch/core.clj}
   :at [clojure.lang.Compiler load Compiler.java 7652]}
  {:type java.lang.ClassNotFoundException
   :message scratch
   :at [java.net.URLClassLoader findClass URLClassLoader.java 471]}]

Maybe my ns’s/files aren’t matching up? What should a gen-class’d clj file be named?

It should be (:import (scratch CustomException))

or (:import scratch.CustomException)

In the first one, you're importing CustomException from the package scratch -- and you can import multiple classes if needed. In the second one, you're importing a specific class.

In your code, you're telling it to import scratch and to import CustomException, neither of which exist as top-level classes.

(sorry I didn't answer sooner but it seemed you'd already gone to bed so I wasn't watching Slack after a while)

No worries, I was bouncing around between a few things last night. I really appreciate the help!

Ok, I think I’ve got everything hooked up now. Only one small thing isn’t working: when I make changes to the gen-classed ns, I can’t seem to see those changes in other ns’s without restarting the repl. I’ve been running (compile scratch.CustomException) , but I think perhaps my (:import ) isn’t refreshing even when I re-eval the ns?

Yeah, I'm not sure that you can hot reload gen-class'd classes. I try to avoid gen-class as much as possible because it's a bit of a mess all around.

What’s the alternative? Can you hot-reload plain old .java files?

java files are not classes, in java when you recompile a class the program generally needs to be restarted before you can use the new version

java doesn't have "loading files" the way clojure does, as the compiler is not built into the vm

(IDE's provide versions of this, I'm not specific on the details, I assume the cursive IDE is most likely to make this usable)

Are you using chrome console for that? It adds $ as a synonym of document.querySelector but it is available only in the context of console.