denvaar
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?
Where Next?
Trending in Questions
Other Trending Topics
Latest on Elixir Forum
Sponsor Spotlight
We build reliable cloud platforms for business-critical systems.
Categories:
Sub Categories:
Forums
Popular Tags
- #ecto
- #liveview
- #troubleshooting
- #learning-elixir
- #deployment
- #library
- #erlang
- #testing
- #genserver
- #mix
- #absinthe
- #remote-other
- #otp
- #plug
- #how-to-question
- #macros
- #postgres
- #elixirconf
- #channels
- #exunit
- #discussion
- #code-sync
- #podcasts
- #javascript
- #onsite
- #dialyzer
- #docker
- #authentication
- #umbrella
- #full-time-contract
- #podcasts-by-brainlid
- #ecto-query
- #blog-post
- #elixirconf-us
- #elixir-ls
- #ai
- #phoenix_html
- #iex
- #graphql
- #genstage
- #websockets
- #supervisor
- #advent-of-code
- #distillery
- #processes
- #api
- #forms
- #hex
- #security
- #metaprogramming
Our Sponsors
Build Elixir applications with speed and confidence.
Supporting innovation across the BEAM ecosystem.
Catch errors, track performance, monitor hosts and more.
Error tracking, logs, uptime, and Just Enough APM. Built for developers.
Elixir, Erlang & BEAM conferences from Code Sync - join us in 2026!
Bringing learning, work and compliance together since 1987.
Enabling companies to succeed by building software people love.
The deployment platform built for Elixir: PaaS ease, VPS control.
The team behind Membrane, Popcorn, LiveDebugger. Available for hire.
Courses that'll move you from confusion to "Aha, now I get it!"
Showing Posts 1 to 5- Show Best Posts
- Show All (oldest first)
- Show All (newest first)
sodapopcan
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 0is pretty aggressive. Some things are inherently cyclical and so long as you can keep “boundaried” off it’s all good (think db schemas withbelongs_toandhas_many). Of course I know nothing of your project so I’m just blabbing here.denvaar
https://github.com/denvaar/protocol_bug_demo
with erlang 26.1.2 and elixir 1.15.7
sodapopcan
Thanks for the repo! I was testing mostly in one file.
So TL;DR, move the impl for
Anyto the same file as thedefprotocoland 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
defprotocoland 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.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.