On 17.05.2009, at 21:24, Mark Engelberg wrote:
> I don't want to post my actual code here, so let's run with this
> *gravity* example based off of what Adrian posted as a simple
> illustration.
Here is another solution that I consider preferable to the use of
load. It requires you to specify explicitly which vars you want to be
replaceable (which I consider an advantage), but otherwise it is very
simple to transform a piece of code into a reusable template. If you
(and/or others) consider this useful, I'd be happy to add it to
clojure-contrib.
Konrad.
; This would be in clojure-contrib
(ns deftemplate)
(use 'clojure.contrib.macro-utils)
(defmacro deftemplate [name params & forms]
(let [param-map (for [p params] (list (list 'quote p) (gensym)))
template-params (vec (map second param-map))
param-map (vec (apply concat param-map))
expansion (list 'list (list 'quote `symbol-macrolet) param-map
(list 'quote (cons 'do forms)))]
`(defmacro ~name ~template-params ~expansion)))
; This would be the slightly modified version of the original namespace
(ns gravity)
(deftemplate/deftemplate grav-template [*gravity* say-grav]
(defn halve-grav []
(/ *gravity* 2.0))
(defn mult-grav [x]
(* *gravity* x))
(defn print-grav-stats []
(say-grav *gravity*)
(say-grav (halve-grav))
(say-grav (mult-grav 2))))
(def *gravity* 1.0)
(defn say-grav [grav]
(prn "Gravity is:" grav))
; Note: it is important to give namespace-qualified symbols here,
otherwise the symbol macro
; expansion gets into an infinite loop.
(grav-template gravity/*gravity* gravity/say-grav)
; This would be the cloned-then-modified namspace
(ns gravity-variation1)
(def *gravity* 0.38)
(defn say-grav [grav]
(prn "Gravity on Mars is:" grav))
(gravity/grav-template gravity-variation1/*gravity* gravity-
variation1/say-grav)
; This would be the namespace using the two previous ones
(ns consumer)
(gravity/print-grav-stats)
(gravity-variation1/print-grav-stats)
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