Passing options to module

Questions / Help
,
1

I’m trying to pass some options to a module when I use it, but anything beyond a simple list seems to get quoted (?) and arrives in abstract tree syntax (AST) format. Forgive the OO terminology, but here’s an example of the issue:

defmodule MyChild do
   use MyParent, default_sort: %{updated_at: -1}
end

Then, in MyParent:

defmodule MyParent do
   defmacro __using__(opts) do
     IO.inspect(opts)
    # ...
   end
end

The output of the inspection is something like this:

[
  default_sort: {:%{}, [line: 20], [updated_at: {:-, [line: 20], [1]}]}
]

What’s going on there?

Specifically, I would like to create a module attribute in the __using__ block so that all of the functions provided in the __using__ block and any functions in the calling MyChild module can reference it. But so far, the only solution I’ve found is to declare the module attribute before the use statement, e.g.

defmodule MyChild do
   @default_sort %{updated_at: -1}
   use MyParent
end

That works, but it feels slightly weird to have the module attribute before the use.

Thanks for any insights.

2

Macros always receive AST and are always required to return AST. So what you are seeing is normal. As the guide says:

… However, as we learned in the previous chapter, the macro will receive quoted expressions, inject them into the quote, and finally return another quoted expression.

I typically find the following construct useful for dealing with this when I want to use the macro arguments at compile time (and with @before_compile delegate the production of the final AST to a regular function).

bind_quoted is recommended in most cases and takes care compile-time unquoting. If you are passing something like a map you will have to Macro.escape/1 it first. The example below is from ex_cldr so its just an example. But you can see it is setting a module attribute.

  defmacro __using__(opts \\ []) do
    quote bind_quoted: [opts: opts] do
      @cldr_opts opts
      @before_compile Cldr.Backend.Compiler
    end
  end
5 Likes