Quote, defmacro and modules

Questions / Help
, ,
1

I’m trying to understand how macros and module definitions interact with each others and I’m having few cases which I don’t understand. The below example has some pseudo doctest to show my doubts.

What I need is to support case 5. Thanks for the support in advance

defmodule Foo do
  @doctest """
  Case 1 OK: Trivial (and useless) case.. (Foo.Case1)

      iex> Foo.Case1.foo
      "local stuff"

  Case 2 OK: Injects the Case2 in WrapMod and uses its args
  WARNING: comment Case4 for this to work (head scratching)

      iex> WrapMod.Case2.foo
      [foo: "bar"]

  Case 3 FAILED!!!! Injects used args into the Foo.Case3.foo function

  Error:

    example.ex:24: undefined function args/0 (expected Foo.Case3 to define
    such a function or for it to be imported, but none are available)

  Desiderata:

    iex> Foo.Case3.foo
    [foo: "bar"]

  Case 4 KINDA NOT OK: Super clunky combo of case 2 and 1

  It works, but it HIDES!!!!! Case 2: Call to `WrapMod.Case2.foo` is not available
  anymore

      iex> WrapMod.Case4.foo
      "local stuff"
      iex> WrapMod.Case2.foo
      UndefinedFunctionError

  Case 5 FAILED: Hyper clunky combo of case 2 with case 3 <- which is not working :(

  This is where I'd want to get to: I would like to be able to invoke
  and use the evaluated Foo.Case3.foo from within the quoted  Case5 quoted module code

  """

  defmacro __using__(args) do

    #Case 1
    defmodule Case1 do
      def foo() do
        "local stuff"
      end
    end

    #Case 2
    quote do
      defmodule Case2 do
        def foo() do
          unquote(args)
        end
      end
    end

    #Case 3
    #defmodule Case3 do
    #  def foo() do
    #    quote do
    #      unquote(args)
    #    end
    #  end
    #end

    # Case 4: comment me to get back WrapMod.Case2.foo which is otherwise no more available!!!
    quote do
      defmodule Case4 do
        def foo() do
          Case1.foo()
        end
      end
    end

    #quote do
    #  defmodule Case5 do
    #    def foo() do
    #      Case3.foo()
    #    end
    #  end
    #end

  end
end

defmodule WrapMod do
  use Foo, foo: "bar"
end

For the records, case 3 works if I change it like this:

    #Case 3
    defmodule Case3 do
      def foo() do
        quote do
          var!(args)
        end
      end
    end

but this is giving me

iex(1)> Foo.Case3.foo
{:var!, [context: Foo.Case3, import: Kernel], [{:args, [], Foo.Case3}]}

which is clearly not what I want

2

I figured how to sort out the reason why Case4 obliterates Case2. Injection through quote expands the AST returned by the __using__ macro. In order to expand both cases, they need to stay together a the end of the __using__ macro and wrapped into a list:

[
   quote do
      defmodule Case2 do
        def foo() do
          unquote(args)
        end
      end
    end,
   quote do
      defmodule Case4 do
        def foo() do
          Case1.foo()
        end
      end
    end
]
3

As in regular functions, macro returns the last expression.
It is not necessary to wrap two quote expressions into a list. You can simply wrap both module definitions into 1 quote block:

quote do
  defmodule Case2 do
    def foo() do
      unquote(args)
    end
  end

  defmodule Case4 do
    def foo() do
      Case1.foo()
    end
  end
end
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