Enum.map inside quote block

[zak9...@gmail.com]

The code below is taken from "introducing Elixir" book and it works well. However, if the code in the defmacro is modified such that Enum.map is inside the quote block then the code stops working.

defmodule Functionmaker do

   # This code works

   defmacro create_functions do

       Enum.map planemo_list, fn {name, gravity} ->

           quote do

              def unquote(:"#{name_drop}")(distance) do

                 :math.sqrt(2 * unquote(gravity) * distance)

              end

           end

        end

   end

end

defmodule Drop do

   require FunctionMaker

 

   FunctionMaker.create_functions([{:mercury, 3.7}, {:venus, 8.9},

        {:earth, 9.8}, {:moon, 1.6}, {:mars, 3.7}, {:jupiter, 23.1},

        {:saturn, 9.0}, {:uranus, 8.7}, {:neptune, 11.0}])

end

The code below does not work once Enum.map is moved inside quote block:

   # This code does not work

   defmacro create_functions do

        quote do

            Enum.map unquote(planemo_list), fn {name, gravity} ->

              def unquote(:"#{name_drop}")(distance) do

                 :math.sqrt(2 * unquote(gravity) * distance)

              end

           end

        end

   end

What's wrong?

- Thanks

[Saša Jurić]

I had to hack my way a bit to get this working (e.g. I think macro should accept planemo_list as a parameter,  and you're using name_drop, when I think it should be name).

With those errors resolved, I got to what I think is your root problem. The solution would be something like:

defmodule Foo do

  defmacro create_functions(planemo_list) do

    quote bind_quoted: [planemo_list: Macro.escape(planemo_list, unquote: true)] do

      Enum.map(planemo_list, ...

      ...

I bolded the differences, the rest of the code should be the same. With this (and changes noted in my first sentence), I got the example working locally.

The problem is that in your non-working code, when you call def unquote(...), you're escaping the outermost quote, and name_drop (which should be name anyway) is resolved from the macro context where that variable doesn't exist.

I blogged about this situation in the last part of my macro series: http://www.theerlangelist.com/2014/07/understanding-elixir-macros-part-6.html

It's a pretty dense article, but hopefully illustrates the problem.

[zak9...@gmail.com]

Thanks Sasa Juric for your clarifications. Your solution works well and should be integrated into the next edition of "introducing elixir" book.

Regarding name, it should be unquote("#{name}_drop") and not unquote("#{name_drop}") 

On Saturday, March 7, 2015 at 8:19:06 PM UTC+1, zak9...@gmail.com wrote:

[Saša Jurić]

Glad it works! If you want to dive deeper into macros, I also suggest reading Metaprogramming Elixir by Chris.

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