erlang“非法警卫表达”在Guards中使用功能时 [英] erlang "illegal guard expression" while using function in Guards

问题描述

我有以下代码。我正在检查3个条件。您可以看到的第一个条件是，我将 xml：get_tag_attr_s（...）的输出存储在变量中，然后在if块中使用了该变量。我的问题是，如果我尝试像在其他两个条件下一样在一行中执行上述过程，则会得到错误的非法守卫表达式。

I have the following code. I am checking 3 conditions. You can see for the first condition I stored the output of xml:get_tag_attr_s(...) in a variable and then used the variable within the if block. My problem is I get error illegal guard expression, if I try to do the above process in one line like I did for the other two conditions.

此外，我还从默认条件中获取了变量'_'是不受约束的。应该是同一回事。

Also, I am getting variable '_' is unbound from the default condition. It supposed to be the same thing.

有人可以解释这个问题吗？

Can somebody please explain the issue?

validate_xmpp(Packet) ->
      Type = xml:get_tag_attr_s(list_to_binary("type"), Packet),
      if
          (Type /= <<"chat">> ->
              {error, "Message type is not chat"};
          xml:get_path_s(Packet, [{elem, list_to_binary("body")}, cdata]) /= <<"">> ->
              {error, "No or empty body"};
          exml_query:path(Packet, [{element,<<"received">>},{attr,<<"xmlns">>}]) == <<"urn:xmpp:receipts">> ->
              {error, "delivery-receipts should be ignored"};
          _->
              {ok, xml:get_tag_attr_s(list_to_binary("from"), Packet)}
      end.

推荐答案

Erlang仅允许以下人员作为警卫：

Erlang allows only these to be guards:

• 原子 true


• 其他常量（项和绑定变量），都被视为错误


• 调用表中指定的BIF（内置函数）键入测试BIF


• 字词比较


• 算术表达式


• 布尔表达式


• 短路表达式（ 以及和 orelse ）

• The atom true
• Other constants (terms and bound variables), all regarded as false
• Calls to the BIFs (built-in functions) specified in table Type Test BIFs
• Term comparisons
• Arithmetic expressions
• Boolean expressions
• Short-circuit expressions (andalso and orelse)

有关更多信息，请查看 http：// www .erlang.org / doc / reference_manual / expressions.html＃id83606

For more info take a look http://www.erlang.org/doc/reference_manual/expressions.html#id83606

代替使用 _ true 。您不能在如果中使用 _ ，仅在 case 语句中使用，并查看文档。

Instead of _ use true. You cannot use _ in if, only in case statements, and also take a look at the docs.

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