This pattern appears in clojure.zip, also. (Any point in updating that?)
On Tue, Nov 6, 2018 at 10:51 AM Alex Miller <a...@puredanger.com> wrote:
>
> On Tuesday, November 6, 2018 at 9:25:31 AM UTC-6, John Schmidt wrote:
>>
>> Nice to see continued progress on Clojure 1.10!
>>
>> It is not clear to me what metadata extension provides that is not
>> already possible with direct definitions or external extensions. Some
>> additional background or a small motivating example would be much
>> appreciated in clearing up the confusion!
>>
>
> Metadata extension allows you to implement protocols on a per-value basis
> (the others are type/class-oriented). This opens up a whole new range of
> potential uses for protocols. Rich was already using this approach for the
> new datafy/nav functionality - this made it generic for all protocols.
>
> Any collection instance can carry metadata, so you can take any collection
> instance and dynamically extend a protocol to it by adding metadata. So if
> you think about something like Component, which has a Lifecycle protocol
> for start/stop, you can now make lightweight components without needing to
> make a type or reify:
>
> $ clj -Sdeps '{:deps {org.clojure/clojure {:mvn/version "1.10.0-beta5"},
> com.stuartsierra/component {:mvn/version "0.3.2"}}}'
> Clojure 1.10.0-beta5
> user=> (require '[com.stuartsierra.component :as component])
> nil
> user=> (def c (with-meta {:state :init}
> {`component/start (fn [c] (assoc c :state :started))
> `component/stop (fn [c] (assoc c :state :stopped))}))
> #'user/c
> user=> (component/start c)
> {:state :started}
> user=> (component/stop c)
> {:state :stopped}
>
>
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