How do I use \*print-dup\* to print records in clojure? A simple case

Question

I'm trying to use *print-dup* to allow writing clojure data to a file and then read it back, however, I'm getting problems even with this simple case. Is there something I am doing wrong? What do I need to do to get this to work?

Clojure 1.3.0-alpha3-SNAPSHOT 
user=> (defrecord TreeNode [val left right]) ;;create the record 
user.TreeNode 
user=> (TreeNode. 5 nil nil) 
#:user.TreeNode{:val 5, :left nil, :right nil} ;; it works just fine 
user=> (binding [*print-dup* true] (prn (TreeNode. 5 nil nil))) ;; use *print-dup* to support reading in and preserving type 
#=(user.TreeNode/create {:val #=(java.lang.Long. "5"), :left nil, :right nil}) ;; this is the form we need to copy paste 
nil 
user=> #=(user.TreeNode/create {:val #=(java.lang.Long. "5"), :left nil, :right nil}) ;;trying to copy and paste 
IllegalArgumentException No matching method found: create 
clojure.lang.Reflector.invokeMatchingMethod (Reflector.java:50) ;;we have an error 
user=>

Answer 1

Records don't print in an eval-able form. We added our own defrecord2 that includes support for creating constructor functions and print/pprint support to print them using the constructor function (which can be eval'ed back into the original record). This is doc'ed more here and here and the code is here.

I asked Rich Hickey about this issue at the Clojure Conj conference in Oct. 2010 and he said constructor functions and reader support for records are planned for the future.

Answer 2

As an update, as of alpha8, in the simple case, *print-dup* with records now works.

 user=> (defrecord TreeNode [val left right])
 user.TreeNode
 user=> (TreeNode. 5 nil nil)
 #user.TreeNode{:val 5, :left nil, :right nil}
 user=> (binding [*print-dup* true] (prn (TreeNode. 5 nil nil)))
 #user.TreeNode[5, nil, nil]
 nil
 user=> #user.TreeNode[5, nil, nil]
 #user.TreeNode{:val 5, :left nil, :right nil}
 user=> 

Answer 3

Work around in 1.2:

(defn- extend-print-dup [record-class]
  (defmethod print-dup record-class [o w]
    (.write w "#=(")
    (.write w (.getName ^Class (class o)))
    (.write w ". ")
    (dorun (map (fn [a]  (print-dup a w) (.write w " ")) (vals o)))
    (.write w ")")))

(defrecord Hl7Field [protospec segname fname pos hl7type values])
(extend-print-dup Hl7Field)

=> (def a (Hl7Field. "p" "PV1" "toto" 0 "ST"  ["c" "d"]))
=> (binding [*print-dup* true] (prn a))
#=(higiebus.bus.protocol.hl7.Hl7Field. "p" "PV1" "toto" 0 "ST" ["c" "d"] )

=> #=(higiebus.bus.protocol.hl7.Hl7Field. "p" "PV1" "toto" 0 "ST" ["c" "d"] )
{:protospec "p", :segname "PV1", :fname "toto", :pos 0, :hl7type "ST", :values ["c" "d"]}

You could wrap the call to extend-print-dup in a custom defrecord macro.