什么是模棱两可的上下文无关文法？

问题描述

我不太清楚上下文无关语法中的歧义概念。如果有人可以帮助我并解释这个概念或提供很好的资源，我将不胜感激。

解决方案

  T * U; 
  

这是指针声明还是乘法？在知道真正的 之前，您无法知道。 p>

因此，表达式的语法取决于表达式的语义（含义）。那不是上下文无关的-在上下文无关的语言中，那只能是一件事，而不是两件事。 （这就是为什么他们不让这样的表达式成为中的有效语句。）

另一个示例：

  T< U> V; 
  

是模板用法还是大于或小于操作？ （这就是为什么他们在-括号仅具有一种用途，而插入号具有另一种用途。）

I'm not really very clear on the concept of ambiguity in context free grammars. If anybody could help me out and explain the concept or provide a good resource I'd greatly appreciate it.

解决方案

T * U;

Is that a pointer declaration or a multiplication? You can't tell until you know what T and U actually are.

So the syntax of the expression depends on the semantics (meaning) of the expression. That's not context-free -- in a context-free language, that could only be one thing, not two. (This is why they didn't let expressions like that be valid statements in D.)

Another example:

T<U> V;

Is that a template usage or is that a greater-than and less-than operation? (This is why they changed the syntax to T!(U) V in D -- parentheses only have one use, whereas carets have another use.)

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