带有绑定变量的字符串截断匹配表达式

发布于 2024-09-24 06:02:53 字数 333 浏览 2 评论 0原文

以下 shell 会话显示了我想了解的一些行为：

1> A = "Some text".
"Some text"
2> "Some " ++ R = A.
"Some text"
3> R.
"text"
4> B = "Some ".
"Some "
5> B ++ L = A.
* 1: illegal pattern

语句 2 和 5 在语法上肯定是相同的吗？我想使用这个习惯用法从字符串中提取一些文本，其中 B 正在从配置文件中读取。这可能吗？我应该使用什么语法来代替上面 5) 中显示的语法？

谢谢！

原文

The following shell sessions shows some behavior I would like to understand:

1> A = "Some text".
"Some text"
2> "Some " ++ R = A.
"Some text"
3> R.
"text"
4> B = "Some ".
"Some "
5> B ++ L = A.
* 1: illegal pattern

Surely statements 2 and 5 are syntacticially identical? I would like to use this idiom to extract some text from a string, where B is being read in from a configuration file. Is this possible, and what syntax should I use instead of that shown in 5) above?

Thanks!

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夜灵血窟げ 2024-10-01 06:02:54

LHS ++ RHS 模式在编译时扩展为 [ lhs0, lhs1, lhs2 | RHS] （其中 LHS =:= [lhs0, lhs1, lhs2]，并且编译器拒绝对除文字字符串/列表之外的任何内容执行此操作。理论上它可以对变量执行此操作，但现在

我认为您需要这样做：

Prefix = read_from_config(),
TestString = "Some test string",
case lists:prefix(Prefix, TestString) of
    true ->
        %% remove prefix from target string
        lists:nthtail(length(Prefix), TestString);
    false ->
        different_prefix
end.

The LHS ++ RHS pattern is expanded at compiletime to [ lhs0, lhs1, lhs2 | RHS] (where LHS =:= [lhs0, lhs1, lhs2], and the compiler refuses to do this for anything but literal strings/lists. In theory it could do this for variables, but it simply doesn't right now.

I think in your case you need to do:

Prefix = read_from_config(),
TestString = "Some test string",
case lists:prefix(Prefix, TestString) of
    true ->
        %% remove prefix from target string
        lists:nthtail(length(Prefix), TestString);
    false ->
        different_prefix
end.

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