Is there a guide for relying on optional dependencies in a library?

axelson · August 28, 2021, 9:08pm

This is the best “guide” I’ve been able to find on this, which is to use Code.ensure_loaded?/1 with a module that you expect will be inside the optional dependency:

And one example of that approach being used is in Dataloader:

github.com

absinthe-graphql/dataloader/blob/91db9e4f3cb8e3b375c997d83e4b1f57f97e3bf7/mix.exs#L64


      
          # See: https://github.com/elixir-lang/elixir/pull/8263
          defp test_apps(:test) do
            [:ecto_sql, :postgrex]
          end
          
          defp test_apps(_), do: []
          
          defp deps do
            [
              {:telemetry, "~> 1.0 or ~> 0.4"},
              {:ecto, ">= 3.4.3 and < 4.0.0", optional: true},
              {:ecto_sql, "~> 3.0", optional: true, only: :test},
              {:postgrex, "~> 0.14", only: :test, runtime: false},
              {:dialyxir, "~> 1.0.0", only: [:dev, :test], runtime: false},
              {:ex_doc, "~> 0.24", only: :dev, runtime: false}
            ]
          end
          
          def docs do
            [
              extras: [

github.com

absinthe-graphql/dataloader/blob/91db9e4f3cb8e3b375c997d83e4b1f57f97e3bf7/lib/dataloader/ecto.ex#L1


      
          if Code.ensure_loaded?(Ecto) do
            defmodule Dataloader.Ecto do
              @moduledoc """
              Ecto source for Dataloader
          
              This defines a schema and an implementation of the `Dataloader.Source` protocol
              for handling Ecto related batching.
          
              A simple Ecto source only needs to know about your application's Repo.
          
              ## Basic Usage

Although there’s no way to know for sure what a module will be in any given dependency, but generally there is a module that corresponds to the application name so this isn’t a large problem in practice.