On 12.12.2012 02:16, Mark Engelberg wrote:
Hi Mark,
Here's my take on the problem. I hope you can adapt it to your situation.
(ns example.canvas
"A canvas is an abstract interface to a graphics API. The canvas
namespace does not build new functionality on top of the APIs, that's a
job for another namespace.")
(defprotocol Canvas
(draw-triangle [canvas point-a point-b point-c])
(draw-circle [canvas position size]))
(defmulti make-canvas-of-type (fn [type] type))
(defn make-canvas
([] (make-canvas-of-type :svg))
([type] (make-canvas-of-type type)))
;---
(ns example.svg-canvas
"A canvas that produces SVG images."
(:import example.canvas.Canvas)
(:require [example.canvas :refer [make-canvas-of-type]]))
(defmethod make-canvas-of-type :svg [_]
(reify Canvas
(comment ...)))
;---
(ns example.png-canvas
"A canvas that produces PNG images."
(:import example.canvas.Canvas)
(:require [example.canvas :refer [make-canvas-of-type]]))
(defmethod make-canvas-of-type :png [_]
(reify Canvas
(comment ...)))
;---
(ns example.artist
"The artist uses a canvas to provide advanced rendering operations. It
can work with any canvas you give it, so it doesn't care what kind of
different canvases there are."
(:require [example.canvas :refer [draw-triangle draw-square]]))
(defn create-masterpiece [canvas triangle-budget circle-budget]
"Bigger budgets = better art!"
(comment
(draw-triangle ...)
(draw-circle ...)))
;---
(ns example.patron
"The patron doesn't have an artistic vision (that's the artist's job),
nor does it know how to talk to a graphics API (that's what the canvases
are for). What it *does* know is how many triangles and squares we can
afford, and where we want them drawn."
(:require [example.artist :refer [create-masterpiece]
example.canvas :refer [make-canvas]]))
(defn commision-a-masterpiece []
(create-masterpiece (make-canvas) 500 1000))
--
Timo
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