Specificity in ClojureScript

This commit was pushed up to Clojurescript core recently, adding a new macro specify. This means we can now have instance-level implementations of protocols on specific values:

    (ns example1)

    (defprotocol Listable
      (render [items]))

    (def elements
     (specify ["one" "two" "three"]
       (render [items]
           (apply str (map #(str "<li>" % "</li>") items))

    (println (satisfies? Listable elements))
    ; true

    (println (satisfies? Listable ["one" "two" "three"])) ; false
    ; false

    (println (render elements))
    ; <ul><li>one</li><li>two</li><li>three</li></ul>

    (println elements)
    ; [one two three]

    (println (= elements ["one" "two" "three"]))
    ; true
    (println (identical? elements ["one" "two" "three"]))
    ; false

This is pretty important feature for Clojurescript. extend-type is still powerful, but there is the odd time where you want to make a specific value conform to an interface. Because we’re on Javascript, we can get this level of modularity without a huge performance cost.

Note that this relies on version 0.0-2138 of Clojurescript.

This change was authored by the indomitable David Nolen whose latest open source offering in a Clojurescript interface over Facebook’s React.


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