函数定义的形式参数中的模式匹配
这是我在 erlang 代码中见过几次的东西，但是谷歌很难找到这个例子（下面链接中的第一个代码块）：
http://www.process-one.net/en/wiki/ejabberd_HTTP_request_handlers/
在process的函数定义的“head”中/2
process(_LocalPath = ["world"], _Request) ->
第一个参数/参数有模式匹配；
这是否类似于守卫，因此仅当传递给 process/2 的第一个参数是字符串“world”，或者“world”是某种默认参数时，才会执行以下子句？或者我完全误解/猜错了？
Here's something I've seen in erlang code a few times, but it's a tough thing to google and I can only find this example (the first code block in link below):
http://www.process-one.net/en/wiki/ejabberd_HTTP_request_handlers/
In the "head" of function definition of process/2
process(_LocalPath = ["world"], _Request) ->
there is a pattern match on first parameter / argument;
Does this act similarly like a guard, so the following clause will be executed only if the first argument passed to process/2 is string "world", or is "world" some kind of a default argument? Or i completely misunderstood/ mis-guessed?
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是的，这是一个模式匹配。如果第一个参数是具有单个元素的列表（该元素是字符串“world”），则将执行该子句。
Yes, this is a pattern match. The clause will be executed if the first argument is a list with a single element, the element being the string "world".
你是对的： _LocalPath = ["world"] 充当模式“守卫”。如果函数“process”的第一个参数不等于[“world”]，则模拟器将继续查找匹配项。
需要注意的一件事： _LocalPath 充当“装饰器”以增强可读性，因为标识符以下划线开头。
You are correct: _LocalPath = ["world"] acts as a pattern "guard". If the first parameter to the function "process" isn't equal to ["world"], then the emulator proceeds to find a match down.
One thing to note: _LocalPath serves as "decorator" to enhance readability since the identifier starts with an underscore.
模式中的
=用于别名，它基本上允许你鱼和熊掌兼得。它既进行正常的模式匹配，又将变量绑定到整个匹配的数据。如果您需要整个数据，那么它很实用，因为它使您无需重建数据。您可以在模式中的任何位置使用它。与守卫无关。以
_LocalPath中的_开头的变量也告诉编译器如果不使用该变量，不要抱怨。通常，如果您绑定变量但不使用它们，编译器会发出一些抱怨声。除此之外，名称以_开头的变量没有什么特别之处，您可以像使用任何变量一样使用它们。唯一真正特殊的变量是
_，即匿名变量。它始终匹配并且从不绑定，因此您可以将其用作匿名占位符。这就是它首先存在的原因。我个人很少使用以
_开头的变量，而更喜欢仅使用_。我还觉得用不必要的东西弄乱模式是一件坏事，所以我不会在这样的文档中使用别名。我会写：%% process(LocalPath, Request) -> ... .或者如果您愿意的话也可以是类型声明。我认为使代码更短、更清晰。
The
=in a pattern is used for an alias, it basically allows you to have your cake and eat it. It both does a normal pattern match and binds a variable to the whole matched data. It is practical if you need the whole data as it saves you having to reconstruct it. You can use it anywhere in a pattern. It has nothing to do with guards.Starting a variable with a
_as in_LocalPathis too tell the compiler not to complain if this variable is not used. Normally the compiler whines a bit if you bind variables and don't use them. Apart from this there is nothing special about variables whose names start with_, you can use them as you would any variable.The only really special variable is
_, the anonymous variable. It always matches and is never bound so you can use it as an anonymous place holder. Which is why it exists in the first place.I personally very rarely use variables starting with
_and prefer to use just_. I also feel that cluttering up patterns with unnecessary things is a Bad Thing so I wouldn't use aliases for documentation like that. I would write:%% process(LocalPath, Request) -> ... .or perhaps a type declaration if you prefer. Keeps the code shorter and more legible, I think.