gistfile1.ex

f = fn x -> {:ok, {x, [1,x,3]}} end
 
import M
f.(2) |> match!({:ok, {!x, _}})
#=> 2

f.(2) |> match!({:ok, {x, [1,!x,3]}})
#=> 2

f.(2) |> match!({:ok, {13, [1,!x,3]}})
#=> ** (MatchError) no match of right hand side value: {:ok, {2, [1, 2, 3]}}

match.exs

defmodule M do
  defmacro match(thing, pattern, selector) do
    quote do
      unquote(pattern) = unquote(thing)
      unquote(selector)
    end
  end

  defmacro match!(thing, pattern) do
    pattern = find_bang(pattern)
    selector = Process.get(:bang)
    quote do
      unquote(pattern) = unquote(thing)
      unquote(selector)
    end
  end

  defp find_bang({:!, _meta, [arg]}) do
    Process.put(:bang, arg)
    arg
  end

  defp find_bang({a, meta, b}) do
    {find_bang(a), meta, find_bang(b)}
  end

  defp find_bang({a, b}) do
    {find_bang(a), find_bang(b)}
  end

  defp find_bang(l) when is_list(l) do
    Enum.map(l, &find_bang/1)
  end

  defp find_bang(literal) do
    literal
  end
end