[Elixir005] 查看指定数据的详细信息 i helper

elixir在1.2后增加了一个新的特性i helper. 在iex shell中使用i可以查看任意数据的数据类型和详细描述

#查看变量描述
iex(1)> i {:test, "That sounds great"}
Term
  {:test, "That sounds great"}
Data type
  Tuple
Reference modules
  Tuple
#查看Module描述(有点类似于Erlang的lists:module_info)
iex(2)> i List
Term
  List
Data type
  Atom
Module bytecode
  usr/local/Cellar/elixir/1.2.0/bin/../lib/elixir/ebin/Elixir.List.beam
Source
  private/tmp/elixir20160101-48495-1fg1arr/elixir-1.2.0/lib/elixir/lib/list.ex
Version
  [322093417650371381585336045669056703278]
Compile time
  2016-1-1 11:57:45
Compile options
  [:debug_info]
Description
  Use h(List) to access its documentation.
  Call List.module_info() to access metadata.
Raw representation
  :"Elixir.List"
Reference modules
  Module, Atom

来看看这么神奇的功能是怎么实现的吧~

https://github.com/elixir-lang/elixir/blob/master/lib/iex/lib/iex/helpers.ex#L420

 

  @doc """
  Prints information about the given data type.
  """
  def i(term) do
    info = ["Term": inspect(term)] ++ IEx.Info.info(term)

    for {subject, info} <- info do
      info = info |> to_string() |> String.strip() |> String.replace("\n", "\n  ")
      IO.puts IEx.color(:eval_result, to_string(subject))
      IO.puts IEx.color(:eval_info, "  #{info}")
    end

    dont_display_result
  end

 

可以看出它只是把IEx.info.info的结果打出来，我们再看看它发生了什么？

https://github.com/elixir-lang/elixir/blob/master/lib/iex/lib/iex/info.ex

defprotocol IEx.Info do
  @fallback_to_any true

  @spec info(term) :: [{atom, String.t}]
  def info(term)
end

defimpl IEx.Info, for: Tuple do
  def info(_tuple) do
    ["Data type": "Tuple",
     "Reference modules": "Tuple"]
  end
end

它是一个protocol，在这个文件中把elixir的基本类型都实现了一次，它会返回一个keyword list, 所以我们才能看到，那么如果我们试试自己定义？

iex(3)> defmodule User do
…(3)> defstruct name: "John", age: 25
…(3)> @type t :: %User{name: String.t, age: non_neg_integer}
…(3)> end

因为在info.ex中已处理struct的类型， 如果我们现在直接i的结果它是

iex(4)> i %User{}
Term
  %User{age: 25, name: "John"}
Data type
  User
Description
  This is a struct. Structs are maps with a __struct__ key.
Reference modules
  User, Map

接下来， 我们来自定义看看

iex(5)> defimpl IEx.Info, for: User do
…(5)> def info(item) do
…(5)>   ["Data type":  User, "Description": "The customer is god, pleasure they", "Reference": "blablabla..."]
…(5)> end
…(5)> end
iex(6)> i %User{}
Term
  %User{age: 25, name: "John"}
Data type
  Elixir.User
Description
  The customer is god, pleasure they
Reference
  blablabla...

 成功！

官方文档： http://elixir-lang.org/docs/stable/iex/IEx.Helpers.html#i/1

彩蛋：

有没有看到我们输入i得到的结果，只是把格式用有颜色的格式打印出来，但是确没有看到返回值被打印出来。。。

它的结果无论如何都打印不出来滴。因为它调用了

IEx.dont_display_result

在 evaluator.ex 里面：

 unless result == IEx.dont_display_result, do: io_inspect(result)

---

if my fingers were erlang processes