Here’s the internal representation of a simple expression:
| | iex(1)> quote do: 1 + 2 |
| | {:+, [context: Elixir, import: Kernel], [1, 2]} |
It’s just a three-element tuple. In this particular case, the first element is the function (or macro), the second is housekeeping metadata, and the third is the arguments.
We know we can evaluate this code fragment using eval_quoted, and we can save typing by leaving off the metadata:
| | iex> Code.eval_quoted {:+, [], [1,2]} |
| | {3,[]} |
And now we can start to see the promise (and danger) of a homoiconic language (a language in which the internal representation is expressed in the language itself). Because code is just tuples and because we can manipulate those tuples, we have the ability to rewrite the definitions of existing functions. We can create new code on the fly, and we can do it in a safe way because we can control the scope of both the changes and the access to variables.
Next we’ll look at protocols, a way of adding functionality to built-in code and of integrating our code into other people’s modules.