Macros as text-rearrangers

gistfile1.clj

;; It often makes sense to think about macros as taking names as arguments.
;; This is in contrast to functions which take values as arguments.

;; Consider this macro (by a user in the #clojure IRC)
(defmacro getter [obj nm]
  `((memfn ~(read-string (str "get" nm))) ~obj))

;; It takes two names, obj and nm.
;; Nothing really interesting happens to obj, so let's focus on what happens with nm:
(read-string (str "get" nm))

;; It constructs a string by appending "get" to the string representing the _name_ given by nm, 
;; and then reads that string as a Clojure datum.

;; Consider this expression that uses the getter macro (taken from the same IRC user as above):
(defn mine
  [pdfdoc]
  (with-open [doc (PDDocument/load pdfdoc)]
    (let [info (.getDocumentInformation doc)]
      (getter info "Title"))))  ; the name given by nm is the string "Title"

;; This macro seems to work when the _name_ given by nm is a datum, as it is above:
(macroexpand '(getter info "Title")  ; the name given by nm is the string "Title"
;; -> ((clojure.core/memfn getTitle) info)

;; Things seem to go awry when the getter macro is handed a variable whose _value_ is the 
;; intended datum, such as in this expression (from the IRC user's question):
(defn mine2
  [pdfdoc]
  (with-open [doc (PDDocument/load pdfdoc)]
    (let [info (.getDocumentInformation doc)]
      (doseq [wd ["Title" "Subject"]]
        (do
          (println "got" wd)
          (getter info wd))))))  ; The name given by nm is wd

(macroexpand '(getter info wd)))  ; the name given by nm is wd
;; -> ((clojure.core/memfn getwd) info)

;; It seems like the intention was to hand the _value_ of wd to getter 
;; (which would have been "Title" on one iteration, then "Subject on the next"),
;; but instead getter received the _name_ of the variable, wd.