One of the core tenets of object orientation is encapsulation,
where in theory, only the public functions and data are available outside the object.
While this may be the ideal, there is usually some leakiness to the abstraction,
where internal data or private functions are accessible from the outside world.
elixir, by creating sharply-defined process boundaries, enforces the separation
even to the point of ensuring that you can’t crash another object process.
It implements this via something called the GenServer mix-in protocol
(NOTE: strikethru text indicates my shift from ruby-specific terms to more
elixir-specific terms).
The man page for GenServer
shows the mechanics of how one creates a GenServer with start_link() and
handle messages with handle_call() and handle_cast() but not much on how to
structure the code that uses a GenServer. It requires the caller to spend
some mental energy on
the how and the why of using start_link() instead of focusing on
the task you’re trying to perform.
Hiding the required functions within the API leads, IMO, to a much more readable and object-oriented interface for the users of the object. For example, here is my class-ization of the canonical Stack example:
It extends the example with some additional public functions and the
private functions to support them. In actual use I’d expect a find_or_create()
function would be more common than just a create() function, but it
all depends on how you want to use your object. Also, if this wasn’t
slimmed down to make it more reasable I’d use
doctests
to verify correctness.
Saying that GenServer is elixir’s way of building objects is doing it a bit of a dis-service. An object, in ruby or python or java, is often an ephemeral thing, living and breathing only briefly in memory after being fetched from the DB and before being persisted back to the database.
A GenServer however lives and breathes in memory, where it can immediately respond without querying the database. The object’s public interface has a facade that runs locally in the calling process, but the private interface and data live in an entirely separate process. And the separate process can live on the same computer as the caller or can trivially be run on a separate computer.
A friend once pointed out that I only write blog posts when looking for a new position. Hmmm… fairly true. I’m currently looking for a position using elixir, or a company interested in making the transition from ruby to elixir. If you know of such a position, in the Bay Area or remote, please message me at either john.baylor@gmail.com or on twitter @JohnB.