How to get the AST that would be generated by the given macro?

Macros are just functions that take AST as parameters and are expected to return AST. Generally AST is created using quote blocks. But you can quote any code whether in a macro of not. So you to get the AST of the code you show, it would be a matter of:

defmodule TestModule do
  defmacro test_macro(param1, param2, param3) do
    quote do
      param1 + param2 + unqoute(param3)
    end
  end
end

Note that:

1. In this case param1 and param2 in the quote block have nothing to do with the parameters passed to the macro. They are variable references that would need to be referenceable in the calling site.
2. unquote will dereference the parameter param3. If your intent was to get the AST for unquote(param3) then change the quote do block to be quote unquote: false do

Do note that due to macro hygene, simply having param1 as a bound variable at the call site will actually not work without opting out of hygene with var!

@bartblast you can see the AST by putting an IO.inspect after your quote do block

defmodule TestModule do
  defmacro test_macro(param1, param2, param3) do
    param1 + param2 + unqoute(param3)
  end
  |> IO.inspect
end

@kip, @benwilson512
Sorry, I forgot about wrapping the code inside the macro with quote, but that was my intent (I edited the module code).

I need to get the AST programmatically without touching the current code.
So, I’d like to implement the following function that can be called anywhere:

defmodule Helpers do
  def get_macro_generated_ast(module, function, args) do
    …
  end
end

And calling this function like this:
Helpers.get_macro_generated_ast(TestModule, :test_macro, [1, 2, 3])
Would return the AST which i specified in my first post.

What you are looking for is Macro.expand_once/2 or Macro.expand/2 if you want to expand it fully. Just beware, that it is no safer than Code.eval_quoted/3, so do not use it on untrusted input.

@hauleth
That’s what I did initially, but there are 2 problems with this:

Before running:

Macro.expand((quote do: TestModule.test_macro(1, 2, 3)), __ENV__)

I need to explicitely require the TestModule.

the result AST contains an “unquote” expression instead of a value:

{:+, [context: TestModule, import: Kernel],
 [
   {:+, [context: TestModule, import: Kernel],
    [
      {:param1, [counter: -576460752303422365], TestModule},
      {:param2, [counter: -576460752303422365], TestModule}
    ]},
   {:unqoute, [], [{:param3, [counter: -576460752303422365], TestModule}]}
 ]}

Yes, because require mean “ensure that given module is compiled”. Module must be compiled to be able to use macros from it.

Ouch… it looks like it’s not possible to do with the standard lib, then. I’ll have to develop some custom solution for this. Thank you all for help!

You can always use my library:
https://github.com/olafura/beam_to_ex
https://github.com/olafura/beam_to_ex_ast

test.ex

defmodule TestModule do
  defmacro test_macro(param1, param2, param3) do
    p1_and_p2 = param1 + param2
    quote do
      unquote(p1_and_p2) + unquote(param3)
    end
  end

  def testing do
    test_macro(1, 2, 3)
  end
end

elixirc text.ex
mix run run.exs --beam Elixir.TestModule.beam

defmodule TestModule do
  def(MACRO-test_macro(&("CALLER"), _param1, _param2, _param3)) do
    _p1_and_p2 = _param1 + _param2
    {:+, [context: TestModule, import: Kernel], [_p1_and_p2, _param3]}
  end
  def testing() do
    3 + 3
  end
end

What exactly you want?

Do you mean what I need this code for?