Adding more to __using__?

Is there a way to put more functions into __using__ block? It gets very long and hard to read. I cannot figure out how to split up the quote do into multiple functions. Is there a trick?

@vrod I’ll start by saying: If this is a problem you’re running into, you are 100% putting too much inside of a __using__ block. Code inside of a using block should generally look like this:

defmodule SomeThing do
  defmacro __using__(opts) do
    quote do
      def foo(arg) do
        SomeThing.foo(__MODULE__, arg)
      end
    end
  end

  def foo(module, arg) do
    # complex multi-line implementation here
  end
end

NOT

defmodule SomeThing do
  defmacro __using__(opts) do
    quote do
      def foo(arg) do
        # complex multi-line implementation here
      end
    end
  end
end

Doing it the first way will improve compile times, debugging ability, and avoid the problem you are running into.

BUT, to actually answer your question, you can do this;

defmacro __using__(opts) do
  [
    first_part_stuff(opts),
    second_part_stuff(opts),
  ]
end

defp first_part_stuff(opts) do
  quote do
    # things here
  end
end

defp second_part_stuff(opts) do
  quote do
    # things here
  end
end

Basically, return a list of quote do blocks.

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I see, thank you. In your example, you end up having more public functions available, yes? I think sometimes this might make a problem?

You can @doc false those functions if you want to make it clear that they aren’t supposed to be used.

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I don’t know if I quiet understand your question, but you can put as many functions as you please inside the quote do block in using.

For instance:

defmodule Using do
  defmacro __using__(_opts) do
    quote do
      def a(), do: :a

      def b(), do: :b
    end
  end
end

defmodule Foo do
  use Using
end

And then

 $iex -S mix
iex(1)> Foo.a
:a
iex(2)> Foo.b
:b

If the only purpose of the __using__ macro is to define functions that have no dependency on macro arguments then I think just import MyModule is far clearer in intent and easier to maintain. I can’t tell if your use case fits this description though.

Otherwise, as @benwilson512 says, the quoted code should do as little as possible and delegate to normal functions early for the same reasons as clear intent and maintainability.

2 Likes

I have arguments passing in use, so import does not work (maybe there is a trick?). My purpose is to simplify many functions: when I use with arguments, I can make many functions in a module that are much more simpler. For instance instead of something1(shared1, shared2, x, y), something2(shared1, shared2, x, y), …
I can do use Thing, shared1: "a", shared2: "b" and then have much simpler functions like something1(x, y) because the macro put the shared values where they are needed.

The quote gets long with @doc especially, so even if there is no complex logic in the functions and they are simple calls to some other functions like @benwilson512 says, it is helpful to organize things in smaller pieces sometimes. I hope that makes sense

I would recommend against that. Be explicit and avoid magic.
Later you will loose understanding of what’ s going on with your code.

1 Like

Yea I agree with @eksperimental here, this use of macros generally causes more trouble than it’s worth.

If you find yourself with functions that take too many arguments, consider creating a struct and then passing on that struct. For example:

defmodule SomeModule do
  defstruct [shared1: "a", shared2: "b", foo: nil, bar: nil]
end

Then you construct a %SomeModule{} struct and it will take on the relevant defaults, and also provide a place to store other values you want to set. Then you can pass this to your various functions, and add extra arguments as appropriate.

3 Likes