Conditional dependencies in mix.exs

Questions / Help
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#1

I have a program which can provide different services, depending on its run-time configuration (through a configuration file or command-line options). Some of the services, but not all, depend on external libraries, such as HTTPoison, so I add in my mix.exs:

      {:httpoison, "~> 1.8"}

But I would like to avoid this dependency (HTTPoison brings a lot of other libraries) if not necessary. How to make it conditional, and allow the program to know if it has been included or not, so it can produce a proper error message if the user tries to activate the service, without having the library?

I’ve read this StackOverflow post, which seems to be about a different problem, and this Elixir Forum discussion which was inconclusive.

#2

There’s an optional option, though it’s meant more for dependencies within libraries, so the app using the library can choose to include it or not:

{:httpoison, "~> 1.8", optional: true}

But maybe something like this:

def dep_enabled?(env_key), do: not is_nil(System.get_env(env_key))
def deps() do 
[{:httpoison, "~> 1.8", optional: dep_enabled?("POISON_IS_GOOD_FOR_YOU")} 
...
#3

Actually, re-reading the docs it looks like in your case (if this is a top level app) it would always be included, so maybe a similar approach but using only to specify [:dev, :test] if the dep is disabled, and [:dev, :test, :prod] if it should be enabled?

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#4

I think target is what you want.
In your mix.exs you would use it like:

{:httpoison, "~> 1.8", targets: :your_target_name}

and during build just set MIX_TARGET=your_target_name to build your application with httpoison.

I think during runtime you can check if httpoison was compiled via:

:application.info[:loaded]

It should appear in the list of loaded applications. Not sure if there is a more elegant way.

#5

Unfortunately, the code cannot compile (“HTTPoison.Response.struct/0 is undefined”) so I also need a compile-time condition.

#6

Yes, of course. I forgot to mention that.

You can do something like this:

defmodule SomeModule do
  if Mix.target() == :your_target_no do
    def some_function do
      HTTPoison.get("elixirforum.com")
    end
  else
    def some_function do
      # Do something without HTTPoison
    end
  end
end

Not sure if that suits your needs and how complex the httpoison integration is in your app.

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