warning-handlers: clojure.lang.PersistentList cannot be cast to clojure.lang.IFn

[pesterhazy]

The warning-handlers key doesn't seem to work as explained in the docs.

Check this demo repo, the simple result of doing lein new figwheel asdf and adding the warning-handlers key.

When you run

lein cljsbuild once

in the repo you get the following exception

clojure.lang.ExceptionInfo: clojure.lang.PersistentList cannot be cast to clojure.lang.IFn at line 3 src/asdf/core.cljs {:file "src/asdf/core.cljs", :line 3, :column 1, :tag :cljs/analysis-error}

It looks as though the (fn ...) form is interpreted literally, as a list, rather than evaluated to a fn. But as I understand the docs this is how to use the feature.

A similar exception is caused when using a symbol:

Wrong number of args (3) passed to: Symbol

Not sure if this is cljsbuild issue or a clojurescript compiler issue.

[lread]

@pesterhazy, I made the very same mistake you did.
The :warning-handlers does not go under :compiler, it is specified at the same level as :compiler.
The README clearly shows this, but somehow I messed it up too.

(defproject lein-cljsbuild-example "1.2.3"
  :plugins [[lein-cljsbuild "1.0.4"]]
  :cljsbuild {
    :builds {:id           "example"
             :compiler     {}
             :warning-handlers [my.ns/custom-warning-handler ;; Fully-qualified symbol
                                ;; Custom function (to be evaluated at project build-time)
                                (fn [warning-type env extra]
                                  (when-let [s (cljs.analyzer/error-message warning-type extra)]
                                    (binding [*out* *err*]
                                      (cljs.analyzer/message env s))))]}})

[danielcompton]

I have also made this class of mistake before at #437 😄.