elixir - 如何在宏创建的方法中创建数组的未绑定(bind)变量部分(方法参数)

Question

在phoenix应用程序中，我正在尝试像这样实现授权

#= Bouncers.Bouncer.check_entry(conn, %MyApp.User{})
#=> true

defmodule Bouncers.Bouncer do
  alias Bouncers.Areas

  def check_entry(conn, resource) do
    verify(resource, conn.path_info)
  end

  defp verify(resource, path) do
    try do
      Areas.verify(resource, path)
    rescue
      _ -> false
    end
  end

end

defmodule Bouncers.Doors do

  defmacro bouncer_for(model, allowed_routes) do
    for route <- allowed_routes do
      quote bind_quoted: [route: route, resource: model] do
        def verify(resource, route), do: true
      end
    end
  end
end

defmodule Bouncers.Areas do
  require Bouncers.Doors
  import  Bouncers.Doors

  bouncer_for %MyApp.User{}, [
    ["api", "users"]
  ]
end

bouncer_for 为 Struct 和 path_info 数组创建多个 verify 方法

bouncer_for %MyApp.User{},  [
      ["api", "users"],
      ["api", "posts"]
]

将创建

def verify(%MyApp.User{}, ["api", "users"], do: true
def verify(%MyApp.User{}, ["api", "posts"], do: true

它适用于这些路径，但我被困在格式为["api", "users", _ ]的路径上，这会抛出一个关于未绑定(bind)变量的错误。我至少想要这样的东西，

bouncer_for %MyApp.User{},  [
      ["api", "users", "*"],
]

并将宏生成的方法的参数中的"*"替换为_。有什么指点吗？

Answer 1

您需要像这样定义宏:

defmodule Bouncers.Doors do
  defmacro bouncer_for(model, allowed_routes) do
    for route <- allowed_routes do
      quote do
        def verify(unquote(model), unquote(route)), do: true
      end
    end
  end
end

然后它应该按预期工作。

原因是 bind_quoted 将尝试将绑定(bind)(即实际值)传递给宏，但是 ["api", "users", _] 本身不是有效值。它更像是一种模式，您可以使用 unquote 将其嵌入到动态生成的函数定义中。这就像您直接在定义中编写模式一样。为了说明这个问题，下面是一些大致相当于宏生成的定义的代码:

# using bind_quoted
@route ["api", "users", _]                    # invalid value, raises error
def verify(%MyApp.User{}, @route), do: true

# using unquote
def verify(%MyApp.User{}, ["api", "users", _]), do: true