如何在Map包含具有传入值的键的函数中进行模式匹配？ [英] How can I pattern match in a function where a Map has a key with the passed in value?

问题描述

因此，我正在创建IRC服务器，并且具有将用户从地图中删除的功能。这个想法是使用模式匹配，因此如果用户在地图上，则将调用一个函数的一个版本，否则将调用另一个函数。

So I am creating an IRC server, and I have a function that removes a user from a Map. The idea is to use pattern matching, so one version of a function gets called if the user is in the map and another function gets called otherwise.

我的第一个想法是请执行以下操作：

My first idea was to do the following:

remove_user_from_channel(User, Channel=#channel_details{users = UserMap=#{User := _}}) ->
  Channel#channel_details{users = maps:remove(User, UserMap)}.

但是，此操作无法编译，错误为变量'User'未绑定。

However, this fails to compile with the error variable 'User' is unbound.

有什么方法可以通过功能级别模式匹配来做到这一点？

Is there any way to accomplish this with function level pattern matching?

推荐答案

您不能在函数头中对映射键进行模式匹配，但可以在情况下进行：

You can't do pattern matching for map keys in a function head but you can do in case:

remove_user_from_channel(User, Map) ->
  case Map of
    Channel = #channel_details{users = UserMap = #{User := _}} ->
      Channel#channel_details{users = maps:remove(User, UserMap)};
    _ ->
      other
   end.

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