模式匹配 Enum.map 与 case 条件

Question

我有这张地图：

%{
  "a" => "1",
  "b" => "2",
  "c" => "3",
  "d" => "4",
  "e" => %{"f" => "5"}
}

我可以像这样迭代它的键值：

Enum.map(map, fn({k, v}) ->  
  v
end)

像元组一样使用 fn({k, v})，我得到了这样的东西：

["1", "2", "3", "4", %{"f" => "5"}]

但我不明白为什么这不起作用：

Enum.map(map, fn({k, v}) ->  
  case v do
    {key, value} -> "inner map"
    _ -> "something else"
end)
["something else", "something else", "something else", "something else",
 "something else"]

我可以使用 fn({k, v}) 对地图进行模式匹配，但我不能对 case 条件使用相同的模式匹配？

更新

这就是我正在尝试的

some = %{"a" => "1", "b" => "2", "c" => "3", "d" => "4", "e" => %{"f" => "5"}}
Enum.map(some, fn({k, v}) ->       
  case v do                        
    %{^condition => value} -> "inner map"
    _ "something else"
  end
end)
** (CompileError) iex:15: unknown variable ^condition. No variable "condition" has been defined before the current pattern
    (stdlib) lists.erl:1354: :lists.mapfoldl/3

Answer 1

{key, value} 不是一个ma​​p，它是一个tuple。你需要：

Enum.map(map, fn({k, v}) ->  
  case v do
    %{} = map -> "inner map: #{inspect map}"
    _ -> "something else"
  end
end)
#⇒ ["something else", "something else",
#   "something else", "something else",
#   "inner map: %{\"f\" => \"5\"}"]

---

无论是否证明了密钥的存在，都可以直接模式匹配它的值（注意固定key）：

key = "f"
Enum.map(map, fn({k, v}) ->  
  case v do
    %{^key => v} -> "inner map with value: #{inspect v}"
    _ -> "something else"
  end
end)
#⇒ ["something else", "something else",
#   "something else", "something else",
#⇒  "inner map with value: \"5\""]