How can I pattern match in a function where a Map has a key with the passed in value?

问题

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.

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)}.

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

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

回答1:

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.

回答2:

remove_user_from_channel(User, Channel=#channel_details{users = UserMap}) ->
    case maps:is_key(User, UserMap) of
        true -> Channel#channel_details{users = maps:remove(User, UserMap)};
        false -> ok
    end.

I think you can't use match pattern in function level, but you can use is_key(Key, Map) -> boolean() to check User is in UserMap. Here is the link: http://erlang.org/doc/man/maps.html#is_key-2

来源：https://stackoverflow.com/questions/36416053/how-can-i-pattern-match-in-a-function-where-a-map-has-a-key-with-the-passed-in-v