Elixirのモジュール属性定義でパターンマッチがしたかったけどできなくて泣いちゃった

$ elixir -v
Erlang/OTP 23 [erts-11.1.3] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:1] [hipe] [dtrace]

Elixir 1.11.2 (compiled with Erlang/OTP 23)

defmodule MyModule do
  [@a_val, @z_val] 'az'
end

** (SyntaxError) practice7/caesar.exs:4:20: syntax error before: "az"
    (elixir 1.11.2) lib/kernel/parallel_compiler.ex:315: anonymous fn/4 in Kernel.ParallelCompiler.spawn_workers/7

defmodule MyModule do
  @a_val Enum.at('a', 0)
  @z_val Enum.at('z', 0)
end