它们接受HTTP请求，解析url，把请求的处理转发给适当的动作。

They accept HTTP requests, parse the URL, and forward processing of the request to an appropriate action.

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默认情况下，函数体中的动作只作用于最终解析。

By default, the actions in a function body are only performed on a final parse.

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这就意味着，如果 yacc 在知道采取哪个动作之前，必须读取两个标记，那么它将无法解析该语法。

This means that if yacc would have to read two tokens before it knew which action to take, it would be unable to parse this grammar.

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