模式匹配的规则是，如果变量X出现在两个子模式中，例如{X，X}，或者{X，[X]}或类似的字符，则它们在两个位置必须具有相同的值，但每个子模式的匹配仍在相同的输入环境中完成-一侧的绑定不会转移到另一侧。相等性检查将在概念上完成，就像您在{X，X2}上进行了匹配并添加了保护X =：= X2一样。这意味着即使没有将元组中的Length字段设为最左侧的元素，也不能将其用作二进制模式的输入。

The rules for pattern matching are that if a variable X occurs in two subpatterns, as in {X, X}, or {X, [X]}, or similar, then they have to have the same value in both positions, but the matching of each subpattern is still done in the same input environment - bindings from one side do not carry over to the other. The equality check is conceptually done afterwards, as if you had matched on {X, X2} and added a guard X =:= X2. This means that your Length field in the tuple cannot be used as input to the binary pattern, not even if you make it the leftmost element.

但是，在是二进制模式，绑定在字段中的变量可以从左到右在其后的其他字段中使用。因此，以下工作（在二进制文件中使用前导32位大小的字段）：

However, within a binary pattern, variables bound in a field can be used in other fields following it, left-to-right. Therefore, the following works (using a leading 32-bit size field in the binary):

1> <<Length:32, A:Length/binary, Rest/binary>> = <<0,0,0,3,1,2,3,4,5>>.
<<0,0,0,3,1,2,3,4,5>>
2> A.
<<1,2,3>>
3> Rest.                                                               
<<4,5>>