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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