Fails to compile if optional plug dependency is missin

[aeons]

I am not sure whether this is an issue for Guardian or for Elixir/mix, but here goes.

If you create an empty project and add {:guardian, "~> 2.0"} as a dependency, compilation fails with

𝝺 mix
==> jose
Compiling 105 files (.erl)
Compiling 8 files (.ex)
warning: Poison.EncodeError.exception/1 is undefined (module Poison.EncodeError is not available or is yet to be defined)
  lib/jose/poison/lexical_encoder.ex:8: JOSE.Poison.LexicalEncodeError.exception/1

Generated jose app
==> guardian
Compiling 24 files (.ex)

== Compilation error in file lib/guardian/plug/pipeline.ex ==
** (CompileError) lib/guardian/plug/pipeline.ex:109: module Plug.Conn is not loaded and could not be found
    (stdlib 3.11.1) lists.erl:1354: :lists.mapfoldl/3
    (stdlib 3.11.1) lists.erl:1355: :lists.mapfoldl/3
    (elixir 1.10.0) expanding macro: Kernel.if/2
could not compile dependency :guardian, "mix compile" failed. You can recompile this dependency with "mix deps.compile guardian", update it with "mix deps.update guardian" or clean it with "mix deps.clean guardian"

The referenced line is here https://github.com/ueberauth/guardian/blob/62744fb61fd62afe8bbb49b75b377be14aae65e6/lib/guardian/plug/pipeline.ex#L108-L110

[doomspork]

@aeons what version of Elixir are you on?

@ueberauth/developers have any of you seen this before? I don't think I've ever used Guardian without Plug. I can try to spin up an empty project to test.

[aeons]

Elixir 1.10.

I was trying to get sans_password updated to the newest guardian, and that does not have a dependency on anything except guardian.

[Hanspagh]

This is indeed a problem

== Compilation error in file lib/guardian/plug/pipeline.ex ==
** (CompileError) lib/guardian/plug/pipeline.ex:109: module Plug.Conn is not loaded and could not be found
    (stdlib) lists.erl:1354: :lists.mapfoldl/3
    (stdlib) lists.erl:1355: :lists.mapfoldl/3
    (elixir) expanding macro: Kernel.if/2

This part is wrapped in Code.ensure_loaded, but I guess it is a problem simular to this https://github.com/elixir-lang/elixir/issues/8970

Do we want to go that direction or should we just include plug as a dependency?

[Hanspagh]

@ueberauth/developers ?

[wevnasc]

I am having the same issue, I just added the plug as a dependency and solved, but I didn't want to add the plug without to use it, is there another way to solve it?

[ramaboo]

I am seeing this as well :(

[Hanspagh]

Fixed by #663