I'm fairly new to Clojure, so I'm still struggling to unlearn the habits of
OO-programming. While using Stuart Sierra's component library, I've found
the recommendation in the docs of using idempotent lifecycles very helpful.
The unfortunate result is that every component then has the same pattern in
its start and stop methods:
(defrecord SillyExample [...]
component/Lifecycle
(start [this]
(if (custom-started-check? this)
this
(custom-start-logic this)))
(stop [this]
(if (custom-started-check? this)
(custom-stop-logic this)
this)))
It adds some extra nesting and, potentially, duplication of the started
check's logic. In hopes of making idempotent lifecycles easier to
implement, I made the following protocol, which seems to violate the
implementation inheritance philosophy of Clojure.
(defprotocol IdempotentLifecycle
(started? [this])
(safe-start [this])
(safe-stop [this]))
(extend-protocol component/Lifecycle
my.ns.IdempotentLifecycle
(start [this]
(if (started? this)
this
(safe-start this)))
(stop [this]
(if (started? this)
(safe-stop this)
this)))
So then a use case would like more like:
(defrecord SillyExample [...]
IdempotentLifecycle
(started? [this]
(custom-started-check this))
(safe-start [this]
(custom-start-logic this))
(safe-stop [this]
(custom-stop-logic this)))
This seems like an easier end-user experience, but it feels wrong to
implement a protocol with another protocol. A more "Clojurey" feeling
option would require changes to the component library itself:
(defprotocol LifecycleStatus
(started? [this]))
(extend-protocol LifecycleStatus
java.lang.Object
(started? [_] false))
;; Lifecycle protocol stays as-is
(defn safe-start [component]
(if (started? component)
this
(start component)))
(defn safe-stop [component]
(if (started? component)
(stop component)
this))
Then component would need to use safe-start/safe-stop in place of regular
start/stop in the start-system/stop-system functions.
Maybe this is better suited to an issue/pr on his repository, but I wanted
to see if there were any comments from the community. Is there a better way
to do this?
Andrew Oberstar
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