hmm I guess there is something in clojure I have never run into before and don't actually understand what is happening

(ns test.thing)

(defrecord Thing []
  clojure.lang.IFn
  (invoke [this]
    "thing"))

(def thing
  (Thing.))

(def bar (let [] (thing)))

(def foo (thing))

what is different with (def foo (thing)) that it ends up failing with

Execution error (AbstractMethodError) at test.thing.Thing/applyTo (server.clj:-1).
Method test/thing/Thing.applyTo(Lclojure/lang/ISeq;)Ljava/lang/Object; is abstract

why is it trying to apply?

as soon as its used in a top-level def it just fails. (def {:foo (thing)}) or any other collection also fails. wrapping it in a let magically makes it work?

I guess I must implement applyTo from IFn but I wonder why?

(going from defrecord to deftype changes the error message but still complains about applyTo)

For "simple" code (like invoking a function bound to a var) unless aot compiling, the compiler doesn't bother generating bytecode, it runs a minimal interpreter, which invokes funtions using apply

Which is why the let version works fine, that doesn't count as simple, so it generates bytecode that calls the invoke method for function invocation

TIL. thanks