denvaar
Protocols and mix xref compile-connected cycles
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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LostKobrakai
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.
sodapopcan
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 
)
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
Anyimplementation that every file that implements the protocol will be triggered. This is expected and will happen regardless of where you have definedAny.
sodapopcan
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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