README.md

Exercise: MacrosAndCodeEvaluation-2

Write a macro called times_n that takes a single numeric argument. It should define a function called times_n in the caller's module that itself takes a single argument, and that multiplies the argument by n. So, calling times_n(3) should create a function called times_3, and calling times_3(4) should return 12. Here's an example of it in use:

defmodule Test do
  require Times
  Times.times_n(3)
  Times.times_n(4)
end

IO.puts Test.times_3(4) #=> 12
IO.puts Test.times_4(5) #=> 20

Solution

See the times.exs file for the full module.

The first thing we need to do is create the function name the macro will define. We can use string interpolation; however, if we just unquote the string, we will actually get a CompileError. The macro would essentially generate the following:

def "times_3"(x) do ...

So, let's convert the string to an atom:

defmodule Times do
  defmacro times_n(n) do
    fun = String.to_atom("times_#{n}")

    # TODO
  end
end

Next, we simply write what the macro should do to define the function. The whole thing looks like:

defmodule Times do
  defmacro times_n(n) do
    fun = String.to_atom("times_#{n}")

    quote do
      def unquote(fun)(x) do
        x * unquote(n)
      end
    end
  end
end

Using the same Test module as was defined in the problem, let's test it in iex:

iex> Test.times_3(4)
12
iex> Test.times_4(5)
20