Guard test breaks the opacity of its argument

You can’t really fix it. :ets.whereis returns the opaque type tid() but your guard clause does typecheck on it. This means you’re dealing with internals you’re not supposed to deal with.

I just read about opaque types yesterday in Programming Erlang and my understanding is a little hazy.

In this case, :ets.whereis returns an opaque value, and my assumption is that this value can only be poked and prodded by other functions in the :ets module.

My nascent mental model is akin to handling radioactive materials: the material is locked away in a safe box and we can only handle it by sticking our hands into the gloves attached to the box.

Am I off base with my thinking?

In my mind an opaque type is the equivalent to the OO design guideline that all instance data should have non-public access.

In OO one motivating factor is that you don’t want anything but the instance methods mutating the instance data. In a world where everything is immutable by default that really isn’t an issue which explains to some degree why functional languages can rely on plain data structures so much.

However direct access of the client to the data to the internal organization of the data structure still couples the client to that specific organization.

An opaque type breaks that type of coupling as the client is now forced to use the functions that are supplied with the data structure to extract any information.

When done right an opaque type can give the freedom to radically change the implementation while keeping the API of client facing functions stable (example - here names could be typed as @opaque).

I understand that argument, but since :ets.whereis is part of the public API, shouldn’t the result not be opaque? My understanding is that it’s fine if I depend on the result a given public function returns me, because that’s part of the public API and not an implementation detail.

Am I missing something?

It’s complaining about is_reference - right now tab is a reference but in the future that reference may be buried deeper inside another data structure (now representing an ets table) altogether.

It actually used to be an integer not very long ago, so :ets is a very good example of this.

@peerreynders
If you change the return value of a public function, you change the public API of the module.

Yes, I defend my code should not depend on internal data structures. That’s why modules usually don’t expose their internals to the world and that’s also the why of getters and setters. But if I can’t even rely on the results of your public API, what can I rely on ?

Values returned from a public API are part of the API. Just like if someone changes :ets.whereis to receive 10 parameters instead of 1, that is a change to the public API.

Do we agree on this? I feel I need to establish some common ground before moving on.

@garazdawi

Interesting feedback, thanks for the info !

You can rely on the fact that all the :ets functions work correctly if you pass them the result of :ets.new, regardless of what the actual return value is. As long as you do that, and don’t assume anything about the actual data in the tid type, you shouldn’t experience any breaking changes if the tid type is changed.

I am sorry but I seem to be understanding something among the lines of “don’t bother with the return values of the public API and you will be fine”.

Which sounds completely … well… let’s say unreasonable. Am I the only one that sees it this way?

You can rely on the documentation. For ets, the documentation says that you will get a tid() or undefined back. The documentation never states what tid() is, so you cannot assume in your code what it is.

The same is true for any opaque datatypes, i.e. dict(), array() etc etc.