On Thu, Feb 19, 2009 at 4:51 PM, mikel <mev...@mac.com> wrote:
>
> If I can implement a custom printing function that gets called on
> generic functions, that presupposes that I have some way to tell that
> an object is a generic function. So, yes, if I solve the problem, then
> the problem is solved. :-)
Hm, perhaps I was unclear. I've been playing with a little patch to
allow you to attach a print fn to anything that can hold metadata:
(defmacro do-writes [w & exprs]
(let [wsym (gensym)]
`(let [~wsym ~(with-meta w {:tag 'java.io.Writer})]
~@(for [e exprs]
`(.write ~wsym (str ~e))))))
user=> #^{:custom-print #(do-writes %2 "#<foo: " (:a %) ">")} {:a 1 :b 2}
#<foo: 1>
To extend further my already over-extended example from earlier:
(import '(clojure.lang IFn IDeref IMeta))
(defn make-thing [a b]
(let [state (atom {})]
(proxy [AFn IDeref IMeta]
[{:custom-print
#(do-writes %2 "#<thing: a " a ", b " b ": " @@% ">")}]
(deref [] state)
(invoke
([] "no arg")
([x] "one arg")))))
(def thing (make-thing 1 2))
(swap! @thing assoc :z 1)
user=> thing
#<thing: a 1, b 2: {:z 1}>
All that and no need for an AOT compilation step first.
I've attached a patch to support the :custom-print metadata, in case
anyone wants to play with it.
> I think the only amenity that leaves unaccounted-for is the minor
> matter of some sort of fastidiousness about having GenericFunction
> as a peer of MultiFn, but truthfully, I'm not sure there's really
> any reason to want that.
Both extend AFn, so I think they're siblings even if one of them has a
goofy name.
--Chouser
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-~----------~----~----~----~------~----~------~--~---
commit cfe7088f295886337c127a78d289676612133121
Author: Chouser <chou...@n01se.net>
Date: Fri Feb 20 12:03:15 2009 -0500
Support :custom-print
diff --git a/trunk/src/clj/clojure/core.clj b/trunk/src/clj/clojure/core.clj
index ecd417d..f83f585 100644
--- a/trunk/src/clj/clojure/core.clj
+++ b/trunk/src/clj/clojure/core.clj
@@ -1979,7 +1979,12 @@
(def #^{:private true} print-initialized false)
-(defmulti print-method (fn [x writer] (class x)))
+(defmulti print-method
+ (fn [x writer]
+ (if (:custom-print ^x)
+ :custom-print
+ (class x))))
+
(defmulti print-dup (fn [x writer] (class x)))
(defn pr-on
diff --git a/trunk/src/clj/clojure/core_print.clj b/trunk/src/clj/clojure/core_print.clj
index 9ec08df..5469b1b 100644
--- a/trunk/src/clj/clojure/core_print.clj
+++ b/trunk/src/clj/clojure/core_print.clj
@@ -68,6 +68,9 @@
(pr-on m w))
(.write w " "))))
+(defmethod print-method :custom-print [o, #^Writer w]
+ ((:custom-print ^o) o w))
+
(defmethod print-method nil [o, #^Writer w]
(.write w "nil"))
