View Source Creating Plugins

Basics

A plugin is basically just a module that provides an init/1 callback, taking a Credo.Execution struct as its only parameter and returning a Credo.Execution. That's basically it.

defmodule CredoDemoPlugin do
  def init(exec) do
    # but what do we do here??
    exec
  end
end

The Credo.Plugin module provides a number of functions for extending Credo's core features.

defmodule CredoDemoPlugin do
  @config_file File.read!(".credo.exs")

  import Credo.Plugin

  def init(exec) do
    exec
    |> register_default_config(@config_file)
    |> register_command("demo", CredoDemoPlugin.DemoCommand)
    |> register_cli_switch(:castle, :string, :X)
    |> append_task(:convert_cli_options_to_config, CredoDemoPlugin.ConvertCliSwitchesToPluginParams)
    |> prepend_task(:set_default_command, CredoDemoPlugin.SetDemoAsDefaultCommand)
  end
end

You can find more information on Credo.Plugin and the functions imported:

Development

Plugins are generally developed by putting them in a Hex package, referencing that package in mix.exs and then configuring the plugin in .credo.exs.

However, for local development it can be beneficial to develop a plugin inside a project. But referencing modules from the current Mix project in .credo.exs does not work out of the box, because the project is not loaded for every mix task.

To be able to use a module from the current project in .credo.exs, run mix app.config first:

mix do app.config + credo

This way, local plugins can be referenced in .credo.exs.

Another, even more pragmatic way to do it is to run app.config from .credo.exs directly (since it is just a script file):

Mix.Task.run("app.config", [])

%{
  configs: [
    %{
      name: "default",
      plugins: [
        {MyProject.CredoPlugin, []}
      ]
    }
  ]
}

This should naturally taken with a grain of salt, e.g. taking steps that this is only active during development.

View Source Creating Plugins

Basics

Development

Further reading