string - 如何在 Erlang 中为字符串变量重新赋值？

Question

我是 Erlang 新手。我只想为字符串变量重新分配一个值:

get_alert_body(Packet) ->
    BodyElement = element(8,Packet),
    Body = "my text",
    Els = xmpp:get_els(Packet),
    lists:foreach(fun(El) ->
        ElementName = io_lib:format("~s",[xmpp:get_name(El)]),
        IsFile = string:equal(ElementName,"fileType"),
        if
            IsFile ->
                FileType = fxml:get_tag_cdata(El),
                IsPhoto = string:equal(FileType,"photo"),
                IsVideo = string:equal(FileType,"video"),
                if
                    IsPhoto ->
                        %% if it gets to this I would like to return "my photo"
                        Body = "my photo";
                    IsVideo ->
                        %% else if it gets to this I would like to return "my video"
                        Body = "my video";
                    true ->
                        %% otherwise I would like to return "my text"
                        ok
                end;
            true ->
                ok
        end
    end, Els),
    Body.

但我收到此错误:

error,{badmatch,"test2"}

即使我做了类似的事情:

A = "test1",
A = "test2",

我也遇到同样的错误。

非常感谢您的帮助。

Answer 1

你不能。 Erlang 有一个称为“单一赋值”的功能，这意味着在第一次赋值后就无法更改变量的值。如果您尝试这样做，它就会变成模式匹配，并且您会收到 badmatch 错误，就像您尝试执行 "test1"= "test2".

您的示例可以写为:

A =
  if
      Condition ->
          "test2";
      true ->
          "test1"
  end

参见this question及其有关单个作业的更多信息的答案。

<小时/>

在您的扩展示例中，您想要做的事情可以通过折叠来实现。也就是说，给定一个列表和一个称为“累加器”的附加值，遍历列表并为每个元素调用一个函数，并让该函数的返回值成为新的累加器 - 并最终返回累加器。

使用 lists:foldl/3 来实现，如下所示:

get_alert_body(Packet) ->
    BodyElement = element(8,Packet),
    DefaultBody = "my text",
    Els = xmpp:get_els(Packet),
    lists:foldl(fun(El, Body) ->
        ElementName = io_lib:format("~s",[xmpp:get_name(El)]),
        IsFile = string:equal(ElementName,"fileType"),
        if
            IsFile ->
                FileType = fxml:get_tag_cdata(El),
                IsPhoto = string:equal(FileType,"photo"),
                IsVideo = string:equal(FileType,"video"),
                if
                    IsPhoto ->
                        %% if it gets to this I would like to return "my photo"
                        "my photo";
                    IsVideo ->
                        %% else if it gets to this I would like to return "my video"
                        "my video";
                    true ->
                        %% otherwise return the existing value
                        Body
                end;
            true ->
                ok
        end
    end, DefaultBody, Els).