opposite.ex

Creation of an opposite macro

We want opposite 1 + 1
to return to 0 or essentially run 1 - 1

opposite 1 + 1
Transforms 1 + 1 into an AST
opposite({:+, [], [1,1]}

Our macro would then transform that AST (Abstract syntax tree)

The macro accepts an AST as an agruement

defmacro opposite({:+, context, [a, b]}) do
     {:- , context, [a, b]}
end

We can use the quote operator to generate the final AST:

defmodule Math.Opposite do
     defmacro opposite({:+, _, [a, b]}) do
          quote do
               unquote(a) - unquote(b)
          end
     end
end

Think of unquote as string interpolation for macros

Usage

import Math.Opposite

opposite 1 + 1