预期不到的类型

让我们想象一个模块实现Scaffold，该模块生成带有预定义自定义字段的结构，并使用将其注入到调用的模块中use Scaffold。调用时use Scaffold, fields: foo: [custom_type()], ... ，我们希望在Consumer模块中实现正确的类型（common_field在下面的示例中，它是在Scaffold外部或外部定义的）。

@type t :: %Consumer{
  common_field: [atom()],
  foo: [custom_type()],
  ...
}

如果我们可以准确地生成类型Consumer.t()以供将来使用并为新模块的用户创建适当的文档，那将很酷。

一个更复杂的示例如下所示：

defmodule Scaffold do
  defmacro __using__(opts) do
    quote do
      @fields unquote(opts[:fields])
      @type t :: %__MODULE__{
        version: atom()
        # magic
      }
      defstruct @fields
    end
  end
end

defmodule Consumer do
  use Scaffold, fields: [foo: integer(), bar: binary()]
end

并且，在编译之后：

defmodule Consumer do
  @type t :: %Consumer{
    version: atom(),
    foo: integer(),
    bar: binary()
  }
  defstruct ~w|version foo bar|a
end

看起来很简单，对吧？

天真的方法

让我们开始分析我们获得的AST Scaffold.__using__/1。

  defmacro __using__(opts) do
    IO.inspect(opts)
  end
#⇒ [fields: [foo: {:integer, [line: 2], []},
#            bar: {:binary, [line: 2], []}]]

优秀。看来我们离成功仅一步之遥。

  quote do
    custom_types = unquote(opts[:fields])
    ...
  end
#⇒ == Compilation error in file lib/consumer.ex ==
#  ** (CompileError) lib/consumer.ex:2: undefined function integer/0

Ba！正如在Privoz地区所说的那样，类型是特殊的。我们不能只是从任何地方从AST那里获取它们。也许它将unquote在本地工作？

      @type t :: %__MODULE__{
              unquote_splicing([{:version, atom()} | opts[:fields]])
            }
#⇒ == Compilation error in file lib/scaffold.ex ==
#  ** (CompileError) lib/scaffold.ex:11: undefined function atom/0

无论如何。类型很累；询问任何一个以Haskell为生的人（这是Haskell中吸烟者的类型；真实的（依赖于吸烟者的）类型有用100倍，但是困难200倍）。

, , AST , , .

AST

, , . , , , - . , . , , AST ( unquote binary() , CompileError.

, quote do, , quote,  — AST.

quote do
  Enum.map([:foo, :bar], & &1)
end
#⇒ {
#   {:., [], [{:__aliases__, [alias: false], [:Enum]}, :map]}, [],
#     [[:foo, :bar], {:&, [], [{:&, [], [1]}]}]}

? , AST, Enum, :map, . , AST quote . .

, AST AST, . ?  —  , .

defmacro __using__(opts) do
  fields = opts[:fields]
  keys = Keyword.keys(fields)
  type = ???

  quote location: :keep do
    @type t :: unquote(type)
    defstruct unquote(keys)
  end
end

, , — AST, . , ruby !

iex|1  quote do
...|1    %Foo{version: atom(), foo: binary()}
...|1  end
#⇒ {:%, [],
#   [
#     {:__aliases__, [alias: false], [:Foo]},
#     {:%{}, [], [version: {:atom, [], []}, foo: {:binary, [], []}]}
#   ]}

?

iex|2  quote do
...|2    %{__struct__: Foo, version: atom(), foo: binary()}
...|2  end
#⇒ {:%{}, [],
#   [
#     __struct__: {:__aliases__, [alias: false], [:Foo]},
#     version: {:atom, [], []},
#     foo: {:binary, [], []}
#   ]}

, , . .

defmacro __using__(opts) do
  fields = opts[:fields]
  keys = Keyword.keys(fields)
  type =
    {:%{}, [],
      [
        {:__struct__, {:__MODULE__, [], ruby}},
        {:version, {:atom, [], []}}
        | fields
      ]}

  quote location: :keep do
    @type t :: unquote(type)
    defstruct unquote(keys)
  end
end

, Scaffold, ( : Qqwy here). , , version: atom() quote .

defmacro __using__(opts) do
  fields = opts[:fields]
  keys = Keyword.keys(fields)
  fields_with_struct_name = [__struct__: __CALLER__.module] ++ fields

  quote location: :keep do
    @type t :: %{unquote_splicing(fields_with_struct)}
    defstruct unquote(keys)
  end
end

(mix docs):

: AST

, AST __using__/1 , ? , unquote quote? , , . , .

NB , atom(), , , GenServer.on_start() . .

, quote do, - atom() ( CompileError, ). , - :

keys = Keyword.keys(fields)
type =
  {:%{}, [],
    [
      {:__struct__, {:__MODULE__, [], ruby}},
      {:version, {:atom, [], []}}
      | Enum.zip(keys, Stream.cycle([{:atom, [], []}]))
    ]}

, @type? Quoted Fragment, :

defmodule Squares do
  Enum.each(1..42, fn i ->
    def unquote(:"squared_#{i}")(),
      do: unquote(i) * unquote(i)
  end)
end
Squares.squared_5
#⇒ 25

Quoted Fragments quote, (bind_quoted:). .

defmacro __using__(opts) do
  keys = Keyword.keys(opts[:fields])

  quote location: :keep, bind_quoted: [keys: keys] do
    type =
      {:%{}, [],
        [
          {:__struct__, {:__MODULE__, [], ruby}},
          {:version, {:atom, [], []}}
          | Enum.zip(keys, Stream.cycle([{:atom, [], []}]))
        ]}

    #          ⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓
    @type t :: unquote(type)
    defstruct keys
  end
end

unquote/1 , bind_quoted: quote/2.

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!