Hi,

I’m trying to create some syntactic sugar through a macro. The basic function should look like the following:

f :name do
  data.something + 10
end

The macro will transform the body to an anonymous function. Yes the somewhat undefined data is intentional, it will be used to capture the argument that will be passed to the function.

I didn’t manage to create a macro that resolves the data correctly. I found now a solution that works by using the anonymous function of the capture operator. The downside is that I need to capture the argument through &1.something which does not look nice (note the real code looks more complicated since it uses a compile hook, but the following is working)

defmodule F do
  defmacro f(name, options, do: block) do
    func = quote do
      &(unquote(block))
    end
    quote do
      register(unquote(name), unquote(options), unquote(func))
    end
  end

  def register(name, options, func) do
    result = func.(%{something: 10})
    IO.puts("#{inspect name}, #{inspect options}, #{inspect func}, #{inspect result}")
  end
end

defmodule G do
  def run() do
    import F
    F.f :name, [] do
      &1.something-10
    end
  end
end

And call it with:

import G
G.run

If I convert the capturing operator to an anonymous function it should work just fine. The doc does state:

In other words, &(&1 * 2) is equivalent to fn x -> x * 2 end.

But, whenI change my macro to look like this:

defmacro f(name, options, do: block) do
    func = quote do
      fn(data) -> unquote(block) end
    end
    quote do
      register(unquote(name), unquote(options), unquote(func))
    end
  end

I get the error:

warning: variable "data" does not exist and is being expanded to "data()", please use parentheses to remove the ambiguity or change the variable name
  lib/f.ex:21: G.run/0


== Compilation error in file lib/f.ex ==
** (CompileError) lib/f.ex:21: undefined function data/0 (expected G to define such a function or for it to be imported, but none are available)

Why is this? Can someone please help me to understand this?