Compilation warnings (clause cannot match) in mix-and-otp tutorial

Questions / Help
1

I’m going through the Mix and OTP tutorial and when I compile the project I see many compilation warnings for what appear to be redundant pattern-match clauses.

$ mix new kv --module KV
$ cd kv
$ mix compile
Compiling 1 file (.ex)
Generated kv app
warning: this clause cannot match because a previous clause at line 1 always matches
  lib/list/chars.ex:1

warning: this clause cannot match because a previous clause at line 1 always matches
  lib/list/chars.ex:1

...
warning: this clause cannot match because a previous clause at line 6 always matches
  lib/inspect.ex:6

77 warning in total, though many are duplicates. Because I’m new to Elixir, I’d like to understand this before continuing. Specifically, I’m wondering:

  • Where (if anywhere) can I find the files it’s complaining about (e.g. lib/list/chars.ex)?
  • What (if anything) should I do to address these warnings?
  • Is this something I should report an issue for?

The guide lists the following version requirements:

  • Elixir 1.9.0 onwards
  • Erlang/OTP 20 onwards

I’m using Elixir 1.9.1 and Erlang/OTP 22

Erlang/OTP 22 [erts-10.4.3] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [hipe]

Elixir 1.9.1 (compiled with Erlang/OTP 22)

I’m running on NixOS, in case that’s relevant.

2

I’ve never seen those warnings in that context…

But the filenames mentioned in the warnings appear to be from the standard library.

But perhaps @DianaOlympos or @hauleth can help you, both of them use Nix as well as far as I can remember.

3

Yes it is. I am encountering the same issue. I haven’t dug what is the problem, but so far I just get used to ignoring it (everything works as expected). It is somehow related to the protocols, but I didn’t found the reason for that warnings yet.

4

It seems to be related to the ERL_LIBS environment variable.

I tried installing elixir systemwide in my nixos config rather than using nix-shell -p elixir and the compiler warnings disappeared. Comparing env inside and outside the nix-shell I noticed that in the nix-shell the environment had:

ERL_LIBS=/nix/store/h7zp9vg05m4jimbvp4d9majqnq4qa8jb-elixir-1.9.1/lib/elixir/lib:/nix/store/h7zp9vg05m4jimbvp4d9majqnq4qa8jb-elixir-1.9.1/lib/elixir/lib

My first thought was that because the above contains the same path twice, something was loading twice, leading to the warnings. This was not the case, however – removing the duplicate entry had no effect.

Unsetting ERL_LIBS eliminated the warnings. I don’t know if it will break something else further down the line, but so far I’ve been able to use mix new, mix compile, and mix test without issue.

2 Likes
5

Is it maybe possible that you are using an elixir version that is compiled on one version of OTP, with another version of OTP? I believe you can see this info when running elixir --version.

6

Not so far as I can tell. It looks like it was compiled with and is using OTP 22

$ elixir --version
Erlang/OTP 22 [erts-10.4.3] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [hipe]

Elixir 1.9.1 (compiled with Erlang/OTP 22)
1 Like
7

It is not possible in Nix.

8

So you can keep ERL_LIBS but we should probably not get it in double. I think this come from https://github.com/NixOS/nixpkgs/blob/076860e0340a5e4a909b9a710e186508b14d1c90/pkgs/development/beam-modules/build-mix.nix#L41 and https://github.com/NixOS/nixpkgs/blob/076860e0340a5e4a909b9a710e186508b14d1c90/pkgs/development/interpreters/elixir/setup-hook.sh

I will check later and validate it is what is happening then push a patch.

9

I’m not sure this is NixOS related. I’m getting similar behaviour in Fedora 30 with Elixir packages built by asdf-vm. Unsetting ERL_LIBS does make the warnings go away. Seems to affect about any project (third-party or my own) EXCEPT for an umbrella app. Very strange.

10

I created Nix user repository mostly forked from beam modules from nixpkgs.

I unset ERL_LIBS from setup-hook and it seems that warnings have disappeared.

Try my repo https://github.com/jechol/nix-beam