why?.clj

(defmacro my-defrecord [the-name [& fields] & opt+specs]
  (let [full-classname (symbol (str (.name *ns*) \. the-name))]
    `(do
       (ns-unmap *ns* '~the-name)
       (defrecord ~the-name [~@fields] ~@opt+specs)
       (ns-unmap *ns* '~the-name)
       (defn ~the-name
         ([~@fields]
            (new ~full-classname ~@fields nil nil))
         ([~@fields ~'meta ~'extmap]
            (new ~full-classname ~@fields ~'meta ~'extmap))))))

(comment

user> (defprotocol P (what [p]))
P
user> (my-defrecord Foo [] P (what [_] :old))
#'user/Foo
user> (defn call-what-on-a-Foo [] (what (Foo)))
#'user/call-what-on-a-Foo
user> (call-what-on-a-Foo)
:old
user> (my-defrecord Foo [] P (what [_] :new))
#'user/Foo
user> (call-what-on-a-Foo)
:old

)

; but this works:

(defmacro my-defrecord [the-name [& fields] & opt+specs]
  `(do
     (defrecord ~the-name [~@fields] ~@opt+specs)
     (defn ~(symbol (str "make-" the-name))
       ([~@fields]
          (new ~the-name ~@fields nil nil))
       ([~@fields ~'meta ~'extmap]
          (new ~the-name ~@fields ~'meta ~'extmap)))))

(comment

user> (defprotocol P (what [p]))
P
user> (my-defrecord Foo [] P (what [_] :old))
#'user/make-Foo
user> (defn call-what-on-a-Foo [] (what (make-Foo)))
#'user/call-what-on-a-Foo
user> (call-what-on-a-Foo)
:old
user> (my-defrecord Foo [] P (what [_] :new))
#'user/make-Foo
user> (call-what-on-a-Foo)
:new

)