Curious case of 'in'. Add syntactic sugar while writing code.

in_macro.ex

defmodule Validator do
  defmacro having({:in, _, [lhs, rhs]}) when lhs |> is_binary and rhs |> is_binary,
    do: rhs |> String.contains?(lhs)

  defmacro having({:in, _, [_lhs, _rhs]}), do: false
end