Examples - leifp/clj-isa-protocol GitHub Wiki
There will be more examples here later.
Here we extend is-a?, and therefore our multimethod dispatch, to handle maps.
The is-a? function is defined in such a way that
(is-a? specific-version-of-general-map general-map) is true. This is what
you want generally, e.g. (core/isa? specific.Float general.Number) or
(core/isa? ::specifically-mantis ::cool-insect)
(ns foo
(:use [dispatch.is-a-protocol :only [is-a? Is-A]])
(:require [dispatch.multimethods :as mm]))
(defn- map-is-a? [child parent h]
(and (map? parent)
(let [sentinel (Object.)]
(every?
(fn [[pk pv]] ;; for every key in parent
(let [cv (get child pk sentinel)]
(if (identical? cv sentinel) ;; child has that key
false
(is-a? h cv pv)))) ;; and their values are related by is-a?
parent))))
(extend-protocol Is-A
clojure.lang.IPersistentMap
(-is-a? [c p h] (map-is-a? c p h)))This now makes dispatching, say, Ring requests to more and more specific handlers easy, and easy to extend:
(mm/defmulti handler "multimethod ring handler" (fn [req] req))
(mm/defmethod handler {}
[req] {:status 404 :headers {} :body "Not found."})
(mm/defmethod handler {:request-method :get}
[req] {:status 200 :headers {} :body "Generic GET."})
(mm/defmethod handler {:request-method :get, :content-type "application/json"}
[req] {:status 200 :headers {} :body "\"GET some json.\""})
(mm/defmethod handler {:request-method :get, :content-type "application/xml"}
[req] {:status 200 :headers {} :body "<foo>GET some XML.</foo>"})
(mm/defmethod handler {:request-method :get,
:content-type "application/json"
:headers {"app/client-status" "favored-nation"}}
[req] {:status 200 :headers {} :body "\"GET some super-special json.\""})
(handler {:request-method :get})
;=> {:status 200, :headers {}, :body "Generic GET."}
(handler {:request-method :post})
;=> {:status 404, :headers {}, :body "Not found."}
(handler {:request-method :get :content-type "application/xml"})
;=> {:status 200, :headers {}, :body "<foo>GET some XML.</foo>"}
(handler {:request-method :get :content-type "application/json"})
;=> {:status 200, :headers {}, :body "\"GET some json.\""}
(handler {:request-method :get :headers {"anything" "really"} :content-type "application/json"})
;=> {:status 200, :headers {}, :body "\"GET some json.\""}
(handler {:request-method :get :headers {"anything" "really", "app/client-status" "favored-nation"} :content-type "application/json"})
;=> {:status 200, :headers {}, :body "\"GET some super-special json.\""}Notice that while you could have done something similar with clojure.core's multimethods, it's not quite as flexible:
(core/defmulti handler "multimethod ring handler" (juxt :request-method :content-type))
(core/defmethod handler [:get "application/json"] ...)
;; Oh, wait, now I want to do special things depending on the headers...
;; But I can't, unless I change the implementation of the above method.
;; Or, less likely, change the dispatch fn and value for all methods:
(core/defmulti handler "multimethod ring handler"
(juxt :request-method :content-type #(get-in % [:headers "special"])))
(core/defmethod handler [:get "application/json" nil] ...)
;; and the same for lots of methods... This is similar to the way that document DBs are more flexible than relational DBs about the data model.