Intentions are a tool for runtime polymorphism in Clojure and ClojureScript. They behave much same as multimethods, with one key exception: where multimethods override inherited behavior, intentions combine it.
Add the following dependency to your project:
[intentions "0.2.1"]An intention is created with defintent:
(defintent valid?
:dispatch :type
:combine #(and %1 %2))An intention needs both a dispatch function, and a combine function. The combine function combines two return values into one. In this case, we perform a logical AND on the values.
Once an intention is stated, behavior can be added using conducts. These are analogous to methods:
(derive ::square ::quad)
(defconduct valid? ::quad [shape]
(= (count (:sides shape)) 4))
(defconduct valid? ::square [shape]
(apply = (:sides shape))Unlike methods, conducts with derived dispatch values combine the
functionality of their parents. Because ::square derives from
::quad, both conducts are applied, then combined using and:
(valid? {:type ::square, :sides [2 2 2 2]})
-> true
(valid? {:type ::square, :sides [2 2 2]})
-> falseConducts are combined in a fixed order down the inheritance tree, with
parents evaluated before children. So in the above case, the conduct
for ::quad is evaluated before ::square. This ordering is
particularly useful when using a combining function like merge.
When the inheritance order is ambiguous (such as in the case of a
diamond dependency), dependencies are converted to strings and
ordered alphanumerically, so as to always provide a consistent
ordering. This can be overridden by using the prefer-conduct
function.
For example:
;; Diamond dependency graph
(derive ::b ::a)
(derive ::c ::a)
(derive ::d ::b)
(derive ::d ::a)
(defintent order-example
:dispatch identity
:combine concat)
(defconduct ::b [_] '(b))
(defconduct ::c [_] '(c))If we were to dispatch on ::d, the order in which to apply ::b and
::c is ambiguous, so we default to alphanumeric ordering:
(order-example ::d)
-> (b c)To explicit specify an ordering, use prefer-conduct:
(prefer-conduct order-example ::c ::b)
(order-example ::d)
-> (c b)Copyright © 2016 James Reeves
Distributed under the Eclipse Public License either version 1.0 or (at your option) any later version.