针对空图的模式匹配功能 [英] Pattern match function against empty map

问题描述

我正在研究模式匹配，但我发现，针对空地图对方法的参数进行模式匹配并不容易。我认为应该是这样的：

I'm playing around with pattern match and I found out, that it's not quite easy to pattern match parameters of a method against an empty map. I thought it would go something like this:

defmodule PatternMatch do
  def modify(%{}) do
    %{}
  end

  def modify(map) do
    # expensive operation
    %{ modified: "map" }
  end
end

但似乎第一个函数子句匹配任意映射：

But it seems like the first function clause matches arbitrary maps:

iex> PatternMatch.modify(%{a: "map"})
==> %{}

还有另一种检查空地图的方法吗？

Is there another way to check for empty maps?

推荐答案

这种方式在设计上是可行的，但是乍一看，它可能会有些混乱。此功能使您可以使用模式匹配来解构地图，而不必指定所有键。例如：

It works this way by design, but admittedly it can be a bit confusing at first glance. This feature allows you to destructure maps using pattern matching, without having to specify all keys. For example:

iex> %{b: value} = %{a: 1, b: 2, c: 3}
%{a: 1, b: 2, c: 3}

iex> value
2

因此，％{} 将匹配任何地图。如果要匹配函数中的空映射，则必须使用保护子句：

Consequently, %{} will match any map. If you want to match an empty map in a function, you have to use a guard clause:

defmodule PatternMatch do
  def modify(map) when map == %{} do
    %{}
  end

  def modify(map) do
    # ...
  end
end

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