Protocols and mix xref compile-connected cycles

Questions / Help
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1

I’m not sure if this is the way things are intended to be, but I’ve noticed that the following modules (in a new mix project using erlang 26.1.2 & elixir 1.15.7) produce a cycle error:

defprotocol MyProto do
  @fallback_to_any true

  def foo(x)
end

defimpl MyProto, for: Any do
  def foo(_x), do: :any
end

defimpl MyProto, for: Specific do
  def foo(_x), do: :specific
end

mix xref graph --format cycles --label compile-connected --fail-above 0

1 cycles found. Showing them in decreasing size:

Cycle of length 3:

    lib/any.ex
    lib/my_proto.ex
    lib/any.ex

** (Mix) Too many cycles (found: 1, permitted: 0)

So, is this intended behavior? If so, what’s the best way to ignore it? Otherwise, can someone explain why it’s expected?

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2

Using your exact example, and a few variations of it, I am unable to reproduce this. Does this happen for you with this exact code or is there more to it?

More generally, I think --fail-above 0 is pretty aggressive. Some things are inherently cyclical and so long as you can keep “boundaried” off it’s all good (think db schemas with belongs_to and has_many). Of course I know nothing of your project so I’m just blabbing here.

3

with erlang 26.1.2 and elixir 1.15.7

4

Thanks for the repo! I was testing mostly in one file.

So TL;DR, move the impl for Any to the same file as the defprotocol and you’re good.

In terms of why this is, hopefully someone else can explain as I’m interested myself. I have an inkling but I don’t want to make stuff up (at least not this time :upside_down_face:)

Of note:

  • 1.18.2 only produces a cycle length of 2. I’ve helped nuke compiled time deps in a few projects over the past few years and each new version of Elixir makes good strides in eliminating unnecessary deps.

  • Practically speaking, in both 1.15 and 1.18, it’s only when you change the Any implementation that every file that implements the protocol will be triggered. This is expected and will happen regardless of where you have defined Any.

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5

I looked into it a little bit and I’m assuming it’s this line. This is going to create a compile time dep on the module defined by defprotocol and then of course that module needs to know about the protocol module. I also didn’t realize until I tried it out that in general so long as two modules are defined in the same file then no compile time dep is created between them, so that would explain that.

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6

The elixir compiler works on a per file basis. So there’s simply no need to track and dependencies between things in a single file.

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