Multiple methods in multimethod match dispatch value

code.clj

;; Happens sometimes when clojure.pprint/simple-dispatch or clojure.pprint/print-method
;; or whatnot don't have a stated preference for an object type that matches multiple
;; dispatch values. Here's a way around that.

;; Define a type that will match multiple dispatch values (IDeref & IPersistentMap)
user> (defrecord Foo [bar]
  clojure.lang.IDeref
  (deref [this]
    (if bar
      this
      (assoc this :bar 1))))
user.Foo
      
;; Now, try to pprint an instance...
user> (clojure.pprint/pprint (map->Foo {}))
IllegalArgumentException Multiple methods in multimethod 'simple-dispatch' match dispatch value: class user.Foo -> interface clojure.lang.IDeref and interface clojure.lang.IPersistentMap, and neither is preferred  clojure.lang.MultiFn.findAndCacheBestMethod (MultiFn.java:178)

;; That's the error we expected, so let's force it to print as a map
user> (defmethod clojure.pprint/simple-dispatch Foo [o] 
  ((get-method clojure.pprint/simple-dispatch clojure.lang.IPersistentMap) o))
#object[clojure.lang.MultiFn 0x60f87fd7 "clojure.lang.MultiFn@60f87fd7"]

;; And try to print again
user> (pprint (map->Foo {}))
{:bar nil}

;; And print the deref'd version
user> (pprint @(map->Foo {}))
{:bar 1}

For anyone googling this, you can also simply do

(prefer-method print-method Foo Bar)
(prefer-method print-method Foo Baz)

... and so on for all of the cases where printing Foo conflicts with something else.