Comments on: Reloading Multimethods http://twoguysarguing.wordpress.com/2010/11/05/reloading-multimethods/ colorful back and forth from two highly opinionated programmers Fri, 17 May 2013 10:05:20 +0000 hourly 1 http://wordpress.com/ By: Meikel http://twoguysarguing.wordpress.com/2010/11/05/reloading-multimethods/#comment-307 Mon, 08 Nov 2010 12:56:09 +0000 http://twoguysarguing.wordpress.com/?p=993#comment-307 To elaborate a bit more: the “problem” is that redefing the multimethod makes you loose all your methods defined with it. You change and reload the namespace containing the defmulti form. Then suddenly you have to reload also namespaces depending on it. In order to allow such reloading actions, defmulti got defonce semantics. To change the dispatch function of the multimethod you have to unmap it first: (ns-unmap ‘lambda-reductions ‘substitute).

Hope that helps.

]]> By: s http://twoguysarguing.wordpress.com/2010/11/05/reloading-multimethods/#comment-291 Sat, 06 Nov 2010 02:30:19 +0000 http://twoguysarguing.wordpress.com/?p=993#comment-291 tl;dr my guess stupid change in 1.2 with multimethods, possibly old dispatch function hanging around

]]> By: zahardzhan http://twoguysarguing.wordpress.com/2010/11/05/reloading-multimethods/#comment-290 Fri, 05 Nov 2010 22:54:26 +0000 http://twoguysarguing.wordpress.com/?p=993#comment-290 Use M-. in Emacs to see what going under the hood. Try to redifine variable binded to multimethod with def. Like (defmulti method (fn [] …)) and then reset with (def method).

(defmacro defmulti
“Creates a new multimethod with the associated dispatch function.
The docstring and attribute-map are optional.

Options are key-value pairs and may be one of:
:default the default dispatch value, defaults to :default
:hierarchy the isa? hierarchy to use for dispatching
defaults to the global hierarchy”
{:arglists ‘([name docstring? attr-map? dispatch-fn & options])
:added “1.0″}
[mm-name & options]
(let [docstring (if (string? (first options))
(first options)
nil)
options (if (string? (first options))
(next options)
options)
m (if (map? (first options))
(first options)
{})
options (if (map? (first options))
(next options)
options)
dispatch-fn (first options)
options (next options)
m (assoc m :tag 'clojure.lang.MultiFn)
m (if docstring
(assoc m :doc docstring)
m)
m (if (meta mm-name)
(conj (meta mm-name) m)
m)]
(when (= (count options) 1)
(throw (Exception. “The syntax for defmulti has changed. Example: (defmulti name dispatch-fn :default dispatch-value)”)))
(let [options (apply hash-map options)
default (get options :default :default)
hierarchy (get options :hierarchy #'global-hierarchy)]
`(let [v# (def ~mm-name)]
(when-not (and (.hasRoot v#) (instance? clojure.lang.MultiFn (deref v#))) <<<<<< PROBLEM IS HERE
(def ~(with-meta mm-name m)
(new clojure.lang.MultiFn ~(name mm-name) ~dispatch-fn ~default ~hierarchy)))))))

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