Define function outside quote block in elixir

Question

I've a simple macro that injects a function into the calling module.

defmodule MyModule do
  defmacro __using__(_opts) do
    quote do
      def run do
        IO.puts "Hello world"
      end
    end
  end
end

This works as intended but because of the fact that the run function is nested inside the quote block, I'm unable to add documentation using ExDoc. I also want to define the run function outside as I feel that makes the code look better. Something like this:

defmodule MyModule do
  def run do
    IO.puts "Hello world"
  end

  defmacro __using__(_opts) do
    quote do
       # Some code to inject the run function
    end
  end
end

How do I go about doing this? In addition, how do I add documentation using ExDoc for a nested function?

Do you want to add the documentation to MyModule.run or do you want it to appear in the run function you inject into the module that does use MyModule? — Dogbert
– Dogbert, Commented Sep 16, 2017 at 9:57
@Dogbert I want to add the documentation to MyModule.run and in the module that does use MyModule — Mohideen Imran Khan
– Mohideen Imran Khan, Commented Sep 16, 2017 at 9:59
@Dogbert If I define the docs for the nested run function I get the documentation in the destination module but not the original module. — Mohideen Imran Khan
– Mohideen Imran Khan, Commented Sep 16, 2017 at 10:03

Dogbert · Accepted Answer · 2017-09-16 10:37:55Z

4

You can use defdelegate inside the quote and then define run in the main module.

defmodule MyModule do
  @doc """
  Prints "Hello world"
  """
  def run do
    IO.puts "Hello world"
  end

  defmacro __using__(_opts) do
    quote do
      defdelegate run, to: MyModule
    end
  end
end

defmodule A do
  use MyModule
end

A.run/0 will get an auto generated doc by default which will point the user to MyModule.run/0. This can be customized by adding a @doc "" before defdelegate, if needed.

iex(1)> MyModule.run
Hello world
:ok
iex(2)> A.run
Hello world
:ok
iex(3)> h(MyModule.run)

                                   def run()

Prints "Hello world"

iex(4)> h(A.run)

                                   def run()

See MyModule.run/0.

answered Sep 16, 2017 at 10:37

Dogbert

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3 Comments

Mohideen Imran Khan Over a year ago

Thank you very much. This works but what if the run function has arguments?

Dogbert Over a year ago

@MohideenImranKhan if you have say def add(a, b) ..., the defdelegate would be defdelegate add(a, b), to: MyModule.

Mohideen Imran Khan Over a year ago

@Dogbert Thank you.

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