That's because defn binds f to the _value_ of the function, while (f) binds to the _variable_. > (defn f [] (f)) #'sandbox9672/f > (def g f) #'sandbox9

That's because defn binds f to the _value_ of the function, while (f) binds to the _variable_. > (defn f [] (f)) #'sandbox9672/f > (def g f) #'sandbox9672/g > (def f 42) #'sandbox9672/f > (g) java.lang.ClassCastException: java.lang.Long cannot be cast to clojure.lang.IFn