位語法預設值

在位語法上澄清 Erlang 文件 ：

4.4 預設值

[開頭省略：<< 3.14 >>甚至不是合法的語法。]

預設大小取決於型別。對於整數，它是 8.對於 float，它是 64.對於二進位制，它是指定二進位制檔案的實際大小：

    1> Bin = << 17/integer, 3.2/float, <<97, 98, 99>>/binary >>. 
    <<17,64,9,153,153,153,153,153,154,97,98,99>>
      ^ |<-------------------------->|<------>|
      |            float=64           binary=24
  integer=8


    2> `size(Bin)`. % Returns the number of bytes:
    12            % 8 bits + 64 bits + 3*8 bits = 96 bits => 96/8 = 12 bytes

在匹配時，只允許在模式的末尾使用沒有 Size 的二進位制段，預設的 Size 是匹配右側的二進位制的其餘部分：

25> Bin = <<97, 98, 99>>. 
<<"abc">>

26> << X/integer, Rest/binary >> = Bin. 
<<"abc">>

27> X. 
97

28> Rest. 
<<"bc">>

模式中具有二進位制型別的所有其他段必須指定大小：

12> Bin = <<97, 98, 99, 100>>.         
<<"abcd">>

13> << B:1/binary, X/integer, Rest/binary >> = Bin. %'unit' defaults to 8 for  
<<"abcd">>                    %binary type, total segment size is Size * unit  

14> B.
<<"a">>

15> X.
98

16> Rest.
<<"cd">>

17> << B2/binary, X2/integer, Rest2/binary >> = Bin. 
* 1: a binary field without size is only allowed at the end of a binary pattern