Elixir Access.filter用法及代码示例

@spec filter((term() -> boolean())) ::
  access_fun(data :: list(), current_value :: list())

返回一个函数，该函数访问列表中与提供的谓词匹配的所有元素。

返回的函数通常作为访问器传递给 Kernel.get_in/2 、 Kernel.get_and_update_in/3 和朋友。

例子

iex> list = [%{name: "john", salary: 10}, %{name: "francine", salary: 30}]
iex> get_in(list, [Access.filter(&(&1.salary > 20)), :name])
["francine"]
iex> get_and_update_in(list, [Access.filter(&(&1.salary <= 20)), :name], fn prev ->
...>   {prev, String.upcase(prev)}
...> end)
{["john"], [%{name: "JOHN", salary: 10}, %{name: "francine", salary: 30}]}

filter/1 也可用于从列表中弹出元素或列表中的键：

iex> list = [%{name: "john", salary: 10}, %{name: "francine", salary: 30}]
iex> pop_in(list, [Access.filter(&(&1.salary >= 20))])
{[%{name: "francine", salary: 30}], [%{name: "john", salary: 10}]}
iex> pop_in(list, [Access.filter(&(&1.salary >= 20)), :name])
{["francine"], [%{name: "john", salary: 10}, %{salary: 30}]}

如果没有找到匹配项，则返回一个空列表，并且永远不会调用更新函数

iex> list = [%{name: "john", salary: 10}, %{name: "francine", salary: 30}]
iex> get_in(list, [Access.filter(&(&1.salary >= 50)), :name])
[]
iex> get_and_update_in(list, [Access.filter(&(&1.salary >= 50)), :name], fn prev ->
...>   {prev, String.upcase(prev)}
...> end)
{[], [%{name: "john", salary: 10}, %{name: "francine", salary: 30}]}

如果谓词不是函数或元数不正确，则会引发错误：

iex> get_in([], [Access.filter(5)])
** (FunctionClauseError) no function clause matching in Access.filter/1

如果访问的结构不是列表，则会引发错误：

iex> get_in(%{}, [Access.filter(fn a -> a == 10 end)])
** (RuntimeError) Access.filter/1 expected a list, got: %{}