io.github.bsless/companion

"Aah, you were at my side, all along. My true mentor... My guiding moonlight..."

Why are there so many attempts to re-implement Stuart Sierra's Component library?

It's not bad, the principles are sound, the core abstraction is essentially the same across the entire ecosystem, so why is the Clojure community constantly reinventing this wheel?

Maybe the problem isn't in the core abstraction, but in the tooling and development experience surrounding it. What if instead of replacing, we'll build on top? What's really missing?

"Please don't make me define records for everything"
Manipulating and combining subsystems

Usage

Don't define more records

Every state needs to be started, stopped, have a way to access it, and maybe use it.

the component function receives the following functions:

:start takes the component as an argument and returns the state. Since the component is a record, the arguments can be assoc-ed into it.
:stop the opposite of start
:step a function of [state & args] for the common use case we would want to have for our state. for example, if our state is a connection pool, it might be jdbc/execute. If it's an Apache Kafka producer, it might be (.send ,,,).

The returned component does not just implement the Lifecycle protocol, but two other interfaces:

IDeref: want to access the state? Do you really care what key it is kept under? just deref it.
IFn: The component can also behave as a function, when you invoke it, it will call step on the state and the arguments.

And that's all you really need to make a usable component. It's less hassle to define more components like that, encouraging us to build components which do one thing.

Example:

(def c (start (component {:start (fn [_] (atom 0)) :step #(swap! % inc)})))
@c ;; => 0
(c) ;; => 1
@c ;; => 1

Participating in the great (Life)Cycle

Why not make it really easy to participate in the Lifecycle protocol?

as-component is useful for stateless components, and just takes start, stop, and init, where init is the initial value to be passed to start.

If you just need a start function, you can pass a bare function and init is taken to be {}.

Manipulating and combining subsystems

TODO

License

Distributed under the Eclipse Public License version 1.0.